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Bagaimanakah nukleus sel eukariotik berkembang?

Bagaimanakah nukleus sel eukariotik berkembang?


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Apakah teori/teori yang paling popular tentang bagaimana nukleus berkembang? Saya tahu mitokondria berasal daripada alpha-proteobacteria, kloroplas daripada cyanobacteria dan eukariota berevolusi terus daripada archaea, tetapi bagaimana pula dengan nukleus?


Saya berharap kertas kerja ini oleh Wilson dan Dawson (2011) dan Devos et al. (2014) akan membantu anda.

Secara ringkasnya, kedua-dua ulasan ini memberikan bukti untuk kenyataan berikut:

  1. Kompleks liang nuklear dipelihara dengan baik dengan beberapa kawasan yang berbeza.
  2. Lamina nuklear kelihatan agak berubah antara kumpulan super utama.
  3. Centrosom ialah struktur purba tetapi mempamerkan sejarah evolusi yang kompleks.
  4. Terdapat bukti untuk nenek moyang prokariotik beberapa komponen nuklear.
  5. Analisis organisma mencapah adalah penting untuk memahami sepenuhnya biologi nuklear dan asal-usulnya.

Evolusi fungsional struktur nuklear

Evolusi nukleus, ciri penentu sel eukariotik, telah lama diselubungi spekulasi dan misteri. Kini terdapat bukti kukuh bahawa kompleks liang nuklear (NPC) dan membran nuklear berkembang bersama dengan sistem endomembran, dan bahawa nenek moyang bersama eukariotik terakhir (LECA) mempunyai NPC berfungsi sepenuhnya. Kajian terbaru telah mengenal pasti banyak komponen sampul nuklear dalam Opisthokonts hidup, kumpulan super eukariotik yang merangkumi kulat dan haiwan metazoan. Komponen ini termasuk pelbagai protein membran pengikat kromatin, dan protein membran dengan domain lumenal pelekat yang mungkin telah menyumbang kepada evolusi seni bina membran nuklear. Penemuan lanjut mengenai nukleoskeleton menunjukkan bahawa evolusi struktur nuklear telah digabungkan dengan pembahagian genom semasa mitosis.

Pengenalan

Nukleus, petak terikat membran berganda yang mengandungi genom nuklear, adalah ciri morfologi dan fungsi utama eukariota (Wilson dan Berk, 2010). Selain kromatin, struktur nukleus yang paling menonjol ialah sampul nukleus (NE): dua membran bersempadan dengan kompleks liang nuklear besar (NPC) yang membolehkan molekul masuk dan keluar nukleus (Strambio-De-Castillia et al., 2010) . Satu lagi ciri yang jelas ialah nukleolus, yang merupakan tapak ekspresi gen rDNA dan perhimpunan ribosom (Németh dan Längst, 2011). Kurang jelas, oleh itu diiktiraf baru-baru ini, adalah seni bina dalaman yang dinamik dan kompleks nukleus, secara konsep disebut nukleoskeleton, yang merangkumi filamen perantaraan, aktin dan titin, dan juga berfungsi semasa mitosis (Simon dan Wilson, 2011). Bagaimanakah kerumitan struktur ini timbul?

Subunit kecil RNA (SSU) berasaskan filogenetik "pokok kehidupan" menunjuk kepada tiga domain-Bakteria, Archaea, dan Eucarya (Woese et al., 1990)-semuanya mempunyai asal-usul yang sangat mendalam (Pace, 2009), dan berkongsi set gen yang bertindih. Adakah ketiga-tiga keturunan ini timbul secara bebas daripada fasa praselular evolusi biologi (Pace, 2009), atau adakah prekursor eukariotik timbul dengan penggabungan sel bakteria dan archaeal? Kemungkinan terakhir adalah menarik memandangkan bukti genomik yang menarik untuk dua peristiwa simbiotik utama: endosimbiosis alphaproteobacterium yang akhirnya menimbulkan mitokondria, dan endosimbiosis cyanobacterium yang menimbulkan kloroplas (Margulis, 1970 Pace, 2009). Terdapat juga bukti filogenetik dan genomik yang kukuh untuk endosimbiosis sekunder dan tertier plastid dalam beberapa keturunan eukariotik (Palmer dan Delwiche, 1996). Walau bagaimanapun, berbeza dengan mitokondria, kloroplas, dan plastid, bukti penglibatan endosimbiosis dalam evolusi nukleus adalah jarang atau hilang.

Evolusi awal garis keturunan eukariotik kekal keruh, sebahagian besarnya kerana kepelbagaian genetik eukariota yang masih ada-terutamanya bersel tunggal masih tidak jelas (Dawson dan Pace, 2002). Malah, kepelbagaian genetik yang paling besar dilihat di kalangan eukariota mikrob (bersel tunggal) (Sogin dan Silberman, 1998). Walau bagaimanapun, seperti yang ditunjukkan oleh Pace (2009), perbandingan jujukan genom eukariota hidup memberikan "tiada bukti apa-apa" sama ada eukariota terawal sebenarnya mempunyai membran nuklear atau NPC sebagai ciri morfologi. Idea mudah ini, bahawa nenek moyang eukariotik yang pertama (FECA) tidak mempunyai morfologi nuklear, membebaskan seseorang untuk mempertimbangkan bagaimana jenis protein tertentu dalam FECA mungkin menyumbang kepada evolusi tambahan struktur nuklear seterusnya yang terdapat dalam nenek moyang eukariotik biasa yang terakhir (LECA Rajah). . 1). Seperti yang dibincangkan dalam kajian ini, bukti baru berdasarkan sifat nenek moyang protein endomembran menunjukkan bahawa sistem endomembran eukariotik berevolusi bersama, atau melahirkan, membran nuklear dan NPC, yang nampaknya telah berfungsi sepenuhnya dalam LECA (Neumann et al., 2010).

LECA kemudiannya menghasilkan enam kumpulan super eukariotik utama, setiap satunya termasuk eukariota mikrob: Opisthokonts (cth., kulat, haiwan, protista), Amoebozoa (cth., Dictyostelium), Ekskavasi (cth., Trypanosomes, Giardia), Kromoalveolat (cth., Plasmodium), Archaeplastids (cth., tumbuhan), dan Rhizaria (Hampl et al., 2009). Walaupun cawangan paling asas agak kontroversial (Rogozin et al., 2009 Parfrey et al., 2010), sistem klasifikasi ini membolehkan seseorang membandingkan genom pelbagai eukariota dalam setiap supergroup dan mencipta inventori gen yang mengekod protein struktur nuklear yang diketahui. Perbandingan antara kumpulan super kemudiannya boleh, secara teori, mengenal pasti gen teras yang disimpulkan sebagai terdapat dalam LECA (Keeling, 2007). Walau bagaimanapun, pendekatan ini kini dihadkan oleh kekurangan anotasi kebanyakan protein nukleoskeletal (Simon dan Wilson, 2011), dan oleh kekurangan pengetahuan tentang komponen protein membran nuklear dalam kebanyakan eukariota.

Kepelbagaian besar eukariota mikrob belum lagi dicerminkan dalam projek genom yang telah siap (Dawson dan Fritz-Laylin 2009), yang banyak menumpukan perhatian (lebih 80%) pada Opisthokont (terutamanya haiwan dan kulat, yang kekurangan banyak gen [genom yang dikurangkan secara kedua". ]) dan keturunan Archaeplastid (tumbuhan). Begitu juga, kebanyakan pengetahuan berfungsi tentang struktur nuklear datang daripada sistem model (haiwan, kulat, tumbuhan) yang mewakili hanya dua daripada enam kumpulan super eukariotik. Lebih banyak genom terjujukan, dan proteom sampul nuklear, daripada kumpulan super eukariotik lain akan menjadi penting untuk memahami bagaimana nukleus berkembang. Semua keturunan eukariotik dicirikan oleh kehilangan, keuntungan, pengembangan, dan kepelbagaian keluarga gen (Fritz-Laylin et al., 2010). Oleh itu, sejarah struktur nuklear selepas LECA sudah pasti mengikuti banyak laluan berbeza dalam enam garis keturunan eukariotik utama. Memahami perbezaan ini, dan ciri-ciri yang dikongsi, akan memberikan pandangan yang belum pernah terjadi sebelumnya tentang aspek paling asas struktur nuklear dan organisasi genom, dan mungkin juga mencadangkan sasaran molekul terapeutik dalam eukariota parasit.

Evolusi bersama NPC dan endomembran: Hipotesis proto-coatomer

Dua dekad penyiasatan intensif telah menghasilkan banyak maklumat tentang NPC termasuk ~ 30 protein konstituennya (nukleoporin) dan kedudukan stoikiometri, biokimia, pemasangan, dan tiga dimensi dalam NPC (Doucet dan Hetzer, 2010 Fichtman et al., 2010 Wente dan Rout, 2010). Pengetahuan ini termasuk fungsi domain terlipat tertentu dalam setiap nukleoporin (Devos et al., 2006). Hebatnya, komponen dan struktur satu subkompleks NPC (kompleks Nup107-160 vertebrata) menyerupai lapisan protein lentur membran yang menjana vesikel dalam laluan rembesan dan endomembran (Rajah 2 Devos et al., 2004). Penemuan yang menakjubkan ini membawa kepada hipotesis proto-coatomer, yang menunjukkan bahawa kedua-dua struktur berkembang daripada protein lengkung membran leluhur (Rajah 2 Devos et al., 2004 Hsia et al., 2007 Debler et al., 2008 Leksa). dan Schwartz, 2010).

Untuk menguji hipotesis proto-coatomer, protein NPC telah disucikan daripada eukariota Ekskavate basal yang berbeza. Trypanosoma brucei, patogen utama manusia. Pengetahuan fungsional terperinci tentang domain polipeptida terlipat khusus adalah penting untuk mengenal pasti nukleoporin Trypanosome kerana gen Trypanosome yang sepadan tidak dapat dikenali oleh jujukan DNA dan perbandingan asid amino sahaja (DeGrasse et al., 2009). Proteome NPC Trypanosome mencadangkan pemuliharaan protein NPC dan seni bina NPC dalam LECA, dan menyokong hipotesis proto-coatomer (DeGrasse et al., 2009). Hipotesis ini telah diperluaskan dengan ketara oleh analisis 60 genom eukariotik yang mewakili lima kumpulan super, yang meletakkan sekurang-kurangnya 23 dan sebanyak 26 (daripada 30) nukleoporin dalam LECA (Neumann et al., 2010). Kesimpulan ini tidak dipengaruhi oleh kedudukan akar eukariotik. Antara lima nukleoporin transmembran yang diketahui, dua (gp210, Ndc1) telah dikenal pasti sebagai komponen utama yang menambat NPC ke membran dalam kesemua lima supergroup. Turut dipelihara dalam semua lima supergroup ialah nukleoporin "bakul" NPC Tpr dan Nup50 protein bakul ketiga, Nup153, yang dalam vertebrata mengikat lamin secara langsung (Smythe et al., 2000), telah dipelihara dalam empat daripada lima supergroup (Neumann et al. , 2010). Protein terpelihara ini, yang mungkin terdapat dalam LECA, mempunyai implikasi di luar struktur dan fungsi NPC seperti yang dibincangkan dalam bahagian seterusnya, Tpr dan Nup153 juga mempunyai fungsi yang berkaitan dengan ekspresi kromatin dan gen.

Protein struktur nuklear lain yang manakah terdapat dalam LECA?

Kehadiran NPC yang nampaknya berfungsi dalam LECA menimbulkan persoalan yang menarik: adakah nukleus nenek moyang ini mempunyai protein struktur nuklear lain, dan jika ya, yang mana? Untuk menjawab soalan ini seseorang memerlukan petunjuk tentang protein mana yang perlu dicari dalam genom yang pelbagai. Mujurlah, banyak protein yang mungkin menyumbang kepada evolusi struktur nuklear telah muncul daripada kajian fungsional dalam Opisthokonts dan tumbuhan. Protein yang diminati ini termasuk peningkatan bilangan protein membran nuklear, yang dibincangkan seterusnya, dan protein nukleoskeletal pelbagai fungsi termasuk aktin, motor molekul, protein ulangan spektrin, protein gegelung bergelung, dan filamen berpaut kompleks liang nuklear (Simon dan Wilson, 2011) , yang dibincangkan kemudian dalam ulasan ini.

Protein membran nuklear: Wilayah yang belum dipetakan

Mamalia mengekod repertoir besar (mungkin ratusan) protein transmembran sampul surat nuklear (NET) yang tidak dicirikan (Wilson dan Berk, 2010). Kerumitan yang tidak dijangka ini pertama kali didedahkan dalam kajian proteomik mercu tanda yang mengenal pasti lebih daripada 60 protein NET yang berbeza dalam sampul nuklear sel hati tikus yang telah dimurnikan (Schirmer et al., 2003), dan telah disahkan dan dilanjutkan oleh kajian dalam sel mamalia lain (Wilkie et al. , 2011). Kebanyakan protein membran nuklear dalam Opisthokonts sama ada tidak dicirikan, atau belum difahami pada tahap perincian struktur atau fungsian yang mungkin diperlukan untuk mengenal pasti gen ortolog dalam pelbagai eukariota. Untuk memintas masalah ini, sekurang-kurangnya sebahagiannya, seseorang boleh menentukan proteom BERSIH pelbagai eukariota, dan dengan itu mengenal pasti gen yang berkaitan khusus. Analisis lanjut terhadap protein NET yang dipelihara, walaupun dalam Opisthokonts lain, menghasilkan kejutan tentang struktur nuklear. Sebagai contoh, kulat Schizosaccharomyces pombe mengekod protein membran dalam nuklear bernama Ima1, yang dikenal pasti sebagai analog berpotensi mamalia NET5 (King et al., 2008). Kajian fungsional menunjukkan bahawa Ima1 melekat heterochromatin ke NE dan (melalui sambungan yang tidak diketahui) ke mikrotubulus (King et al., 2008). Protein Ima1 mengikat terus ke sentromer dan telomer, dan sifatnya menunjukkan bahawa heterochromatin menyediakan "kacang" mekanikal yang menguatkan NE terhadap daya yang dihasilkan oleh mikrotubulus (King et al., 2008). Ini selaras dengan bukti biomekanikal bahawa heterochromatin itu sendiri berfungsi sebagai struktur galas daya (Dahl et al., 2005). Protein nuklear yang sama ada kromatin diperkukuh secara mekanikal, atau melindungi kromatin daripada daya, mungkin telah mempengaruhi kapasiti sel bukan sahaja untuk bertahan dalam cabaran mekanikal luaran, tetapi juga untuk mengenakan daya ke atas dunia luar (Dahl et al., 2008). Oleh itu, pencirian masa depan protein NET yang dipelihara mempunyai potensi untuk mendedahkan aspek baru struktur nuklear dalam eukariota hidup, serta prinsip baru tentang bagaimana struktur ini berkembang.

Protein membran melekit membantu menstabilkan sampul surat.

Kami mencadangkan bahawa protein membran dengan domain ekstraselular "pelekat" menyumbang kepada evolusi struktur nuklear dengan menstabilkan organisasi selari membran plasma melengkung atau terlipat (Rajah 2). Idea ini adalah berdasarkan penemuan protein membran nuklear Opisthokont yang menjadi pengantara lekatan sama ada secara homotaip (antara dua atau lebih salinan satu protein) atau heterotipik (antara protein yang berbeza). Perhatikan bahawa domain lumenal protein membran menghadapi petak yang secara topologi bersamaan dengan bahagian luar sel. Domain lumenal protein transmembran yis Pom152 (ortolog vertebrata berpotensi: gp210) diramalkan untuk berinteraksi sendiri melalui lipatan kadherin (Devos et al., 2006). Lipatan ini adalah ciri domain ekstraselular bagi protein permukaan sel tertentu (contohnya, cadherin) dan mengantara lekatan homotip kepada sel jiran (Franke, 2009). Walaupun protein membran nuklear yang mengandungi lipatan kadherin setakat ini hanya dilihat dalam Opisthokonts, jenis domain pelekat lain dipelihara secara meluas. Sebagai contoh, Pom121, protein membran NPC yang dipelihara, direkrut untuk kromatin oleh nukleoporin ELYS yang mengikat DNA dan kompleks Nup107-160 (Lau et al., 2009). Interaksi ini entah bagaimana mencetuskan gabungan pelekat Pom121-pengantara membran selari untuk mencipta liang baru (Fichtman et al., 2010). Sebagai tambahan kepada bukti ini untuk lekatan homotip, terdapat bukti yang semakin meningkat bahawa protein membran nuklear tertentu mengantara lekatan secara heterotip.

Sekurang-kurangnya tiga jenis protein membran NE diperlukan untuk organisasi selari membran nuklear dalam dan luar. Satu keluarga terdiri daripada polipeptida berkaitan lamina protein membran dalam 1 (LAP1) dan protein membran luar yang berkaitan, LULL1, yang berinteraksi melalui domain lumenal besar mereka bersama-sama dengan protein lumenal larut bernama torsinA dan torsinB (Nery et al., 2008 Vander Heyden et al., 2009 Kim et al., 2010). Oleh kerana LAP1 dan LULL1 adalah berkaitan, lekatannya mungkin dianggap homotip. Sebaliknya, dua keluarga pelekat lain, yang terdiri daripada protein domain KASH dan protein domain SUN, berinteraksi secara heterotip melalui domain lumenal mereka (Crisp et al., 2006 Starr dan Fridolfsson, 2010). Protein domain KASH dan SUN juga mempunyai domain tambahan yang menjadi pengantara sama ada interaksi kendiri atau pengikatan langsung kepada protein sitoskeletal tertentu, protein nukleoskeletal, atau protein membran NE (Wilson dan Berk, 2010). Walaupun gambar eksperimen masih jauh dari lengkap, bukti semasa menunjukkan domain KASH dan protein domain SUN membentuk pelbagai kompleks lekatan yang teguh secara mekanikal di NE, yang dipanggil kompleks LINC (menghubungkan nukleoskeleton dan sitoskeleton Crisp et al., 2006), beberapa daripadanya adalah penting untuk pasangan kromosom semasa meiosis dan penggabungan semula seksual (Fridkin et al., 2009 Hiraoka dan Dernburg, 2009). Dalam carian terhad kami, domain SUN telah dikesan dalam setiap kumpulan super yang diuji (Rajah 3, Jadual I). Ini sangat mencadangkan LECA mempunyai protein domain SUN. Sebaliknya, kami mendapati domain KASH hanya dalam Opisthokonts (Rajah 3, Jadual I). Potensi kehadiran protein domain KASH dalam LECA tidak boleh diketepikan pada masa ini. Walau bagaimanapun, jika protein domain SUN sememangnya lebih kuno, ia mungkin juga memainkan peranan purba (bebas KASH). Peranan ini mungkin melibatkan NPC (Liu et al., 2007) kerana SUN1 mempunyai peranan awal dalam perhimpunan NPC (Talamas dan Hetzer, 2011). Protein domain SUN juga berinteraksi dengan telomer meiotik, histon H2A.Z, dan pasangan nukleus ke pusat penganjur mikrotubule (Hiraoka dan Dernburg, 2009 Gardner et al., 2011). Penemuan ini menyokong idea bahawa protein membran, termasuk protein membran pelekat, mempengaruhi evolusi struktur nuklear.

Protein membran yang mengikat kromatin atau pasangan nukleoskeletal.

Evolusi struktur nuklear mungkin sangat dipengaruhi oleh protein membran yang mampu mengikat DNA atau protein kromatin (Wilson dan Foisner, 2010). Satu keluarga protein sedemikian dalam metazoans mempunyai ciri lipatan "domain LEM", pertama kali dikenal pasti dalam LAP2, emerin, dan MAN1 (Lin et al., 2000 Laguri et al., 2001 Wagner dan Krohne, 2007). Malah, yang pertama daripada dua domain LEM dalam LAP2 memberikan pengikatan langsung kepada dsDNA (Cai et al., 2001). Domain LEM lain yang diuji memberikan pengikatan kepada faktor penghalang-ke-autointegrasi (BAF), protein metazoan terpelihara yang juga mengikat terus kepada dsDNA, histon H3, dan lamin (Margalit et al., 2007 Montes de Oca et al., 2009), dan mempengaruhi pengubahsuaian pasca translasi histon (Montes de Oca et al., 2011). Protein domain LEM mempunyai domain lain yang mengikat satu atau lebih komponen nukleoskeleton, atau pelbagai isyarat atau protein pengawalseliaan gen (Wagner dan Krohne, 2007 Wilson dan Berk, 2010). Protein domain LEM emerin juga mengikat domain KASH dan protein domain SUN secara langsung (Simon dan Wilson, 2011), dan entah bagaimana pasangan kekuatan mekanikal kepada perubahan hiliran dalam ekspresi gen, fenomena yang dikenali sebagai mekanotransduksi (Lammerding et al., 2005). Nukleus vertebrata juga mengekspresikan protein domain LEM bukan membran khusus bernama LAP2α yang berinteraksi dengan dirinya sendiri (sebagai trimer), lamin A, kromatin, dan telomer, dan diperlukan untuk menyusun lamin jenis A di bahagian dalam nuklear (Snyers et al., 2007 Gotic dan Foisner, 2010 Dechat et al., 2011). Ragi, yang tidak mempunyai lamin dan BAF, bagaimanapun menyandikan domain LEM ("domain HEH") protein membran nuklear dalaman bernama Src1, yang mengaitkan dan menindas gen telomerik, subtelomerik, dan rDNA (Grund et al., 2008). Menariknya, Src1 juga berfungsi semasa sel mitosis yang kekurangan Src1 mempunyai anaphase yang lebih pendek dan telophase yang lebih panjang (Rodríguez-Navarro et al., 2002). Kedua-dua lipatan domain LEM (Cai et al., 2001) dan domain C-terminal "MSC" (MAN1-Src1p-C-terminal) yang dipelihara yang dikongsi oleh Src1 dan MAN1 manusia (Mans et al., 2004) dipelihara dalam bakteria dan boleh berfungsi untuk mengikat asid nukleik. Antara eukariota, carian terhad kami mengesan ORF berkaitan domain LEM hanya dalam Opisthokonts (Rajah 3, Jadual I). Walau bagaimanapun, penjajaran yang lebih luas sebelum ini menemui protein dengan kedua-dua ciri (domain MSC domain LEM / HEH) dalam semua kumpulan super eukariotik yang diuji, menunjukkan LECA mempunyai protein domain LEM (Mans et al., 2004).

Protein nurim mempunyai empat domain merangkumi membran (dan sedikit lagi). Protein ini menyetempat di membran dalam nuklear melalui mekanisme yang tidak diketahui kerana ia tidak menunjukkan pengikatan yang dapat dikesan pada NPC, lamin, atau komponen intranuklear lain (Rolls et al., 1999). Nurim dicadangkan untuk berfungsi dalam laluan yang menyusun protein membran nuklear yang baru disintesis melepasi NPC ke membran dalam (King et al., 2006 Braunagel et al., 2007). Carian kami mendedahkan ORF berkaitan nurim dalam dua kumpulan super eukariotik (Rajah 3). Mencadangkan asal usul kuno yang berpotensi, kajian terdahulu (Mans et al., 2004) mengumpulkan nurim dalam superfamili protein yang merangkumi protein membran nuklear dalaman mamalia LBR (sebuah sterol reduktase Holmer et al., 1998) dan enzim yang berkaitan dalam bakteria.

Protein LUMA (dikodkan oleh TMEM43) melintasi membran dalam nuklear empat kali, mempunyai domain lumen yang tidak dicirikan yang besar, bersekutu dengan SUN2, lamin, dan emerin, dan membentuk homo-oligomer (Bengtsson dan Otto, 2008 Liang et al., 2011). Oligomerisasi LUMA terganggu oleh mutasi yang menyebabkan distrofi otot Emery-Dreifuss (Liang et al., 2011). LUMA dipelihara dalam tiga kumpulan super eukariotik (Rajah 3) dan terutamanya dalam bakteria (Bengtsson dan Otto, 2008), mencadangkan LUMA mungkin terdapat dalam LECA. Idea bahawa LUMA, protein domain LEM, nurim, dan kemungkinan protein membran nuklear lain mempunyai peranan purba dalam struktur dan fungsi nuklear akan menjadikannya lebih menarik untuk menguraikan peranan mereka dalam eukariota hidup.

Aktin dan miosin: Komponen purba struktur nuklear?

Aktin eukariotik dan motor yang bergantung kepada aktin (myosin) adalah komponen sitoskeletal yang terkenal yang berkaitan dengan motilitas sel, dan kepentingan evolusi mereka hampir selalu dibincangkan secara eksklusif dalam istilah ini (Fritz-Laylin et al., 2010). Kurang dihargai ialah peranan asas dan berpotensi purba mereka dalam struktur nuklear dan fungsi genom. Aktin dan myosin boleh polimer terlibat dalam transkripsi oleh ketiga-tiga polimerase RNA yang bergantung kepada DNA, mengantara eksport RNA daripada nukleus, dan diperlukan untuk pergerakan jarak jauh lokus tertentu dalam nukleus (Gieni dan Hendzel, 2009 Hofmann, 2009 Mekhail dan Moazed, 2010 Skarp dan Vartiainen, 2010). Sekurang-kurangnya enam motor myosin berbeza (Pestic-Dragovich et al., 2000 Salamon et al., 2003 Hofmann et al., 2006, 2009 Vreugde et al., 2006 Cameron et al., 2007 Pranchevicius et al., 2008 Lindsay and McCaffrey , 2009) dan empat motor kinesin yang berbeza (Macho et al., 2002 Levesque et al., 2003 Mazumdar et al., 2004 Wu et al., 2008 Cross and Powers, 2011 Zhang et al., 2011) terdapat dalam nuklei haiwan , dengan peranan yang merangkumi transkripsi, pergerakan intranuklear kromatin, atau eksport di sepanjang rangkaian filamen berpaut liang yang menghubungkan nukleolus kepada NPC (Simon dan Wilson, 2011). Motor Myosin I dipelihara dalam pelbagai eukariota (Foth et al., 2006 Hofmann et al., 2009) termasuk Ekskavasi basal Naegleria gruberi (Goodson dan Dawson, 2006), yang mempunyai enam homolog myosin I (Fritz-Laylin et al., 2010). Yang (jika ada) Naegleria myosin sebenarnya berfungsi dalam nukleus tidak diketahui.

Protein nukleoskeletal lain termasuk radas mitosis nuklear (peranan antara fasa NuMA tidak jelas, tetapi dipasang sendiri ke dalam struktur pengisian ruang 3D Harborth et al., 1999 Radulescu dan Cleveland, 2010), spektrin nuklear (cth, perancah αII-spektrin kompleks pembaikan DNA Muda dan Kothary, 2005 Zhang et al., 2010), dan protein nuklear 4.1 (mengikat NuMA bersekutu dengan filamen berkaitan liang membantu mengatur nukleoskeleton dan beberapa protein membran NE Meyer et al., 2011 Simon dan Wilson, 2011). Malah, protein berulang spektrin nenek moyang dicadangkan berfungsi dalam nukleus (Young dan Kothary, 2005). Titin nuklear boleh mengikat terus kepada protein filamen perantaraan nuklear (Zastrow et al., 2006), dan penting untuk pemeluwapan kromosom semasa mitosis (Machado et al., 1998 Machado dan Andrew, 2000 Zhong et al., 2010). Protein seperti aktin, myosin, NuMA, spektrin, dan titin baru-baru ini diiktiraf sebagai mempunyai peranan asas dalam struktur nuklear dan fungsi genom dalam eukariota hidup (Simon dan Wilson, 2011), dan atas sebab sejarah peranan mereka dalam nukleoskeleton kekal sebahagian besarnya tidak. -beranotasi. Kinesins, yang mana LECA dicadangkan telah memiliki ~ 11 (Wickstead et al., 2010), juga terdapat dalam nukleus (Simon dan Wilson, 2011). Kinesin nuklear dikaitkan dengan kromatin, dan satu (Kif4A) terlibat dalam tindak balas terhadap kerosakan DNA (Wu et al., 2008) jika tidak, sedikit yang diketahui tentang fungsi nuklear mereka.

Protein nukleoskeletal gegelung bergelung

Protein nukleoskeletal gegelung bergelung pengikat DNA terdapat dalam dua keturunan multiselular, haiwan (Opisthokonts) dan tumbuhan (Archaeplastids), tetapi berkembang secara bebas. Dalam kes haiwan, protein (lamin) ini terdiri daripada filamen perantaraan nenek moyang (Prokocimer et al., 2009) dari mana filamen perantaraan sitoplasma kemudiannya berkembang. Filamen perantaraan nuklear (filamen lamin) adalah komponen struktur utama nukleoskeleton haiwan, dengan pautan genetik kepada pelbagai penyakit manusia (Dittmer dan Misteli, 2011). Lamin menyokong atau mempengaruhi hampir setiap aspek biologi genom, termasuk replikasi, transkripsi, isyarat, pembangunan, dan organisasi kromosom (Prokocimer et al., 2009 Dechat et al., 2010, 2011 Wilson dan Berk, 2010). Semua metazoan mempunyai satu atau dua gen yang mengekodkan lamin "jenis B", manakala haiwan kompleks (serangga, vertebrata) mempunyai gen tambahan yang mengekod rangkaian bebas filamen lamin "jenis A" yang diperlukan untuk fisiologi banyak jenis sel mekanosensitif yang indah seperti sebagai otot dan tulang (Dahl et al., 2008). Protein struktur gegelung bergelung lain yang diketahui dalam Opisthokonts termasuk protein bakul NPC yang dipelihara Tpr (Krull et al., 2004) dan Smc (penyelenggaraan struktur kromosom Wong, 2010). Dalam haiwan multiselular, di mana gen aktif boleh terletak jauh di dalam nukleus, Tpr dan protein yang berkaitan adalah komponen yang dicadangkan bagi filamen berkaitan liang nukleoskeletal yang menghubungkan NPC kepada gen aktif dan nukleolus, dan memudahkan eksport yang bergantung kepada aktin dan miosin daripada nukleus. (Simon dan Wilson, 2011). Dalam yis, yang mempunyai nukleus yang lebih kecil, gen aktif ditambat pada NPC melalui pengikatan langsung unsur promoter yang dipelihara (cth, "kod zip DNA" Ahmed et al., 2010) kepada nukleoporin tertentu (Casolari et al., 2004 Kalverda dan Fornerod , 2010). Sama ada kumpulan super eukariotik lain mempunyai rangkaian filamen berpaut liang tidak diketahui.

Tumbuhan yang lebih tinggi tidak mempunyai filamen perantaraan (Rose et al., 2005), tetapi mempunyai filamen analog yang berfungsi termasuk yang dibentuk oleh protein gegelung gegelung dsDNA yang mengikat MPF1 (Samaniego et al., 2006 Fiserova et al., 2009 Meier dan Brkljacic, 2009). Oleh itu, gen yang mengekodkan protein nukleoskeletal gegelung gegelung utama yang berbeza dicadangkan telah timbul secara bebas, selepas LECA, dalam keturunan Opisthokont dan Archaeplastid. Gen-gen ini mungkin telah mempengaruhi organisasi genom dan struktur nuklear kerana ia berkait rapat dengan kemunculan organisma multiselular.

Cadangan kesan penambatan genom pada membran sel

Evolusi jelas memihak kepada penstabilan membran melengkung positif dan melengkung negatif oleh protein proto-coatomer (Devos et al., 2004) dan protein ESCRT (Samson dan Bell, 2009), masing-masing. Walau bagaimanapun, molekul ini tidak mengambil kira sifat asas NPC yang berkaitan dengan genom, atau peranan intim mereka dalam mitosis dan pengasingan kromosom. Atas sebab ini, kami mencadangkan bahawa peralihan dari FECA ke LECA didorong, sebahagiannya, oleh beberapa jenis protein membran. Antara yang terawal, kami cadangkan, adalah yang mengikat DNA atau kromatin dan dengan itu menambat genom ke membran sel. Penambatan stabil kromatin yang agak padat akan mengenakan beban mekanikal pada membran sel (Rajah 2). Beban ini mungkin mempunyai sedikit kesan pada kelengkungan membran dalam sel dengan lampiran membran yang kuat pada dinding sel luar. Sebaliknya, dalam sel dengan tetulang luaran yang lebih lemah (cth, FECA dan mungkin prekursor bakteria planctomycetes, yang melampirkan genom mereka dalam struktur seperti nukleus Fuerst dan Sagulenko, 2011), membran yang dimuatkan kromatin mungkin telah terlipat secara meluas, mengancam struktur sel dan berpotensi mengganggu pengasingan kromosom atau pembahagian sel. Oleh itu, kami mencadangkan bahawa evolusi molekul tambahan sampul nuklear dan struktur nukleoskeletal digabungkan dengan evolusi pengasingan dan mitosis kromosom. Idea ini disokong oleh bukti daripada eukariota hidup bahawa banyak NPC dan protein nukleoskeletal adalah penting untuk pengasingan dan mitosis kromosom, seperti yang dibincangkan dalam bahagian seterusnya. Secara selari, kami mencadangkan terdapat pemilihan positif yang kuat, kedua-duanya untuk protein pelekat yang menstabilkan dan secara mekanikal mengukuhkan struktur membran infolded (Rajah 2) dan untuk protein fusogenik yang menghalang membran infolded daripada mengganggu mitosis.

Adakah evolusi NPC/nukleoskeleton digabungkan dengan pengasingan dan mitosis kromosom?

Terdapat bukti yang semakin meningkat bahawa nukleoporin menghubungi gen aktif, menyusun heterochromatin, dan sintesis mRNA pasangan kepada eksport nuklear (Strambio-De-Castillia et al., 2010 Liang dan Hetzer, 2011). Nukleoporin opisthokont juga muncul sebagai pemain utama dalam mitosis, terlibat dalam pemeluwapan kromosom dan kohesi kromatid kakak (Nakano et al., 2011), pemasangan kinetochore (Salina et al., 2003 Rasala et al., 2006 Roux dan Burke, 2006), pengawalan motor yang bergantung kepada mikrotubule (protein bakul Tpr Nakano et al., 2010), pengawalan pempolimeran mikrotubule pada kinetochores (Mishra et al., 2010), peraturan pusat pemeriksaan mitosis (De Souza dan Osmani, 2009 Lussi et al., 2010 Wozniak et al., 2010), dan pemasangan gelendong (Nakano et al., 2011). Penemuan ini sangat mencadangkan NPC berkembang bukan sahaja sebagai "portal", tetapi sebagai hab struktur yang ditambatkan membran untuk genom dan mitosis.

Begitu juga, gambar buku teks mitosis, di mana kromosom diasingkan terutamanya oleh mikrotubul gelendong, adalah tidak lengkap. Struktur pengisian ruang yang berasingan, "matriks" gelendong dengan sifat hidro-gel elastik yang dicadangkan, kini diketahui menyokong gelendong (Zheng, 2010 Johansen et al., 2011). Matriks ini termasuk kedua-dua nukleoporin yang disusun semula secara mitosis (cth., Tpr Ding et al., 2009 Lince-Faria et al., 2009) dan protein nukleoskeletal (cth., NuMA, lamin B-jenis Simon dan Wilson, 2011). Menariknya, pengasingan kromosom dalam oosit didorong oleh penguncupan rangkaian aktin nuklear (Lénárt et al., 2005). Begitu juga dalam yis, pengasingan kromosom boleh berlaku jika tiada mikrotubul gelendong melalui proses pembelahan nuklear yang memerlukan aktin (Castagnetti et al., 2010). Penemuan ini mendedahkan aktin dan komponen lain yang berpotensi dalam nukleoskeleton Opisthokont dalam cahaya baharu: sebagai protein pembahagian genom. Adakah pengasingan kromosom (mitosis) merupakan daya penggerak dalam evolusi nukleoskeleton?

Idea bahawa nukleoskeleton berkembang daripada protein pengasingan genom purba disokong oleh pemuliharaan banyak protein yang berkaitan sebagai komponen pembahagian genom ("par”) sistem dalam bakteria. Pemisahan bakteria paling baik difahami untuk plasmid, dan melibatkan tiga komponen mudah: urutan DNA berulang (DNA centromeric), protein pengikat sentromer, dan protein penjana kuasa (“motor”) yang berkaitan yang memisahkan dua sentromer dengan membentuk polimer ( Schumacher, 2008). Daripada komponen ini, hanya motor-empat jenis-yang dipelihara dengan ketara. Kebanyakan bakteria menggunakan motor dengan motif ATPase jenis Walker (jenis I par sistem) atau protein superfamili actin/hsp70 (jenis II Schumacher, 2008). Other bacteria use a tubulin/FtsZ GTPase superfamily protein (type III) or an unusual (type IV) protein that is predicted to form coiled-coil polymers and also has a predicted DNA-binding domain, potentially uniting both the centromere-binding and motor functions in a single polypeptide (Simpson et al., 2003 Schumacher, 2008). Eukaryotes express proteins related to potentially all four bacterial par motors. Actin is both a major component of the interphase nucleoskeleton (as discussed earlier) and essential for chromosome segregation (Castagnetti et al., 2010). The Smc family of Walker-type ATPases are conserved in all living cells (Hirano, 2005). Tubulin forms intranuclear microtubules in eukaryotes with “closed” mitosis, or “spindle matrix-associated” microtubules in eukaryotes with open mitosis, and is mitotically regulated by nucleoporins. Less clear is whether any nucleoskeletal protein(s) are related to coiled-coil (type IV par) proteins, but candidates include lamins, Tpr, and Smc. Interestingly, certain nuclear membrane proteins also appear to function during mitosis: Samp1, a nuclear inner membrane protein, colocalizes with the mitotic spindle (Buch et al., 2009).

Mengakhiri ucapan

The “conserved protein fold” strategy, coupled to purification of NPCs from diverse eukaryotes, yielded brilliant insight into the early coevolution of NPCs with endomembranes (DeGrasse et al., 2009). Further explorations of nuclear transmembrane and nucleoskeletal proteins purified from diverse eukaryotes may yield fascinating insights into the LECA nucleus, and the human cell nucleus. Current limitations include lack of knowledge about most Opisthokont nuclear membrane proteins, and a paucity of sequenced genomes from diverse eukaryotes. Genome analysis of the free-living predatory amoebo-flagellate Naegleria gruberi, which diverged from other eukaryotic lineages over a billion years ago, reveals a rich repertoire of proteins involved in cell structure, signaling, metabolism, and sexual recombination (Fritz-Laylin et al., 2010). This organism has a typical-appearing nucleus and can be cultured in the laboratory (Fulton et al., 1984 Fritz-Laylin et al., 2011). Naegleria, other diverse Excavates including Giardia dan Trypanosoma, and laboratory-friendly members of other eukaryotic supergroups including Amoeba (Dictyostelium) and Archaeplastids/Plants (Chlamydomonas) are all available to explore the evolution of nuclear structure. Additional clues to the early evolution of nuclear structure, whether independent or based on shared genes, may come from an unlikely source: planctomycetes bacteria, which enclose their genome within a double membrane, express a clathrin-related protein, and have an endocytosis-like pathway (Fuerst and Webb, 1991 Fuerst and Sagulenko, 2011). It will be exciting to understand how different kinds of proteins and possibly other types of molecules including noncoding RNAs (Pauli et al., 2011) and ADP-ribose chains (Chang et al., 2005) might have shaped the evolution of nuclear structure before the LECA, and to this day.


Eukaryotic origins

The origin of the eukaryotes is a fundamental scientific question that for over 30 years has generated a spirited debate between the competing Archaea (or three domains) tree and the eocyte tree. As eukaryotes ourselves, humans have a personal interest in our origins. Eukaryotes contain their defining organelle, the nucleus, after which they are named. They have a complex evolutionary history, over time acquiring multiple organelles, including mitochondria, chloroplasts, smooth and rough endoplasmic reticula, and other organelles all of which may hint at their origins. It is the evolutionary history of the nucleus and their other organelles that have intrigued molecular evolutionists, myself included, for the past 30 years and which continues to hold our interest as increasingly compelling evidence favours the eocyte tree. As with any orthodoxy, it takes time to embrace new concepts and techniques.

Kata kunci: dawn cell eocytes eukaryotes evolution nucleus origin.

Angka

This first ‘eocyte tree’ was…

This first ‘eocyte tree’ was reconstructed based on the presence and absence of…

The sister group relationship of…

The sister group relationship of the eocytes to the eukaryotes is illustrated by…


Endosymbiosis & The Origin of Eukaryotic Cells

This lecture will help Advanced Biology students understand how mitochondria and chloroplasts evolved. It includes the three domains of life, evidence for serial endosymbiotic theory, and advantages of multicellularity.

Nothing in molecular biology makes sense in light of the evolutionary history of organims in specific paleoenvironments

1960’s scientist Lynn Margulis studied cell structure
Thought mitochondria looked like baccteria- mito evolved from bacteria that lived in permanent symbiosis within the cells of animals and plants

Symbiotic events have a profound impact on the organization and complexity of many life forms

Living relics
Of the 5000 species of bacteria and archaea that have been described almost all were disc when isoalted from natural habitatsa snd grown under controlled conditions in the lab.

Taq polymerase- enzyme stable up to 95ºC- used to run PCR in research and commercial settings-
research- likely that first life forms lived at high temperatures and high in anoxic environments (no oxygen), PCR is necessary research tool that allows investigation in forensic crime scenes, genetic diseases, inheritance patterns, sex determination of embryos, drug discovery and detection of pathogens.

Woese's work on Archaea is also significant in its implications for the search for life on other planets. Before the discovery by Woese and Fox, scientists thought that Archaea were extreme organisms that evolved from the organisms more familiar to us. Now, most believe they are ancient, and may have robust evolutionary connections to the first organisms on Earth.[28] Organisms similar to those archaea that exist in extreme environments may have developed on other planets, some of which harbor conditions conducive to extremophile life.[29]

In addition to a nucleus, eukaryotic cells contain a variety of membrane-enclosed organelles within their cytoplasm. These organelles provide compartments in which different metabolic activities are localized. Eukaryotic cells are generally much larger than prokaryotic cells, frequently having a cell volume at least a thousandfold greater.

The compartmentalization provided by cytoplasmic organelles is what allows eukaryotic cells to function efficiently. Two of these organelles, mitochondria and chloroplasts, play critical roles in energy metabolism. Mitochondria, which are found in almost all eukaryotic cells, are the sites of oxidative metabolism and are thus responsible for generating most of the ATP derived from the breakdown of organic molecules. Chloroplasts are the sites of photosynthesis and are found only in the cells of plants and green

Despite their many similarities, mitochondria (and chloroplasts) aren't free-living bacteria anymore. The first eukaryotic cell evolved more than a billion years ago. Since then, these organelles have become completely dependent on their host cells. For example, many of the key proteins needed by the mitochondrion are imported from the rest of the cell. Sometime during their long-standing relationship, the genes that code for these proteins were transferred from the mitochondrion to its host's genome. Scientists consider this mixing of genomes to be the irreversible step at which the two independent organisms become a single individual.

Single eukaryotic cells became living in close association- colonies

Volvox
Somatic cells- swim and keep it near the light to PS, cannot divide – colonies of up to 50,000 individuals, cannot live alone
characteristics of a single individual. Multicellularity has arisen many times among the eukaryotes. Practically every organism big enough to see with the unaided eye is multicellular, including all animals and plants. The great advantage of multicellularity is that it fosters specialization some cells devote all of their energies to one task, other cells to another. Few innovations have had as great an impact on the history of life asspecialization made possible by multicellularity

Multicellular organisms need specialised organ systems, whereas all the life processes in a unicellular organism take place in that one cell. Multicellular organisms need organ systems to carry out functions such as:
Communication between cells, eg the nervous system and circulatory system
Supplying the cells with nutrients, eg the digestive system
Controlling exchanges with the environment, eg the respiratory system and excretory system


Poxviruses and the Origin of the Eukaryotic Nucleus

A number of molecular forms of DNA polymerases have been reported to be involved in eukaryotic nuclear DNA replication, with contributions from α-, δ-, and ε-polymerases. It has been reported that δ-polymerase possessed a central role in DNA replication in archaea, whose ancestry are thought to be closely related to the ancestor of eukaryotes. Indeed, in vitro experiment shown here suggests that δ-polymerase has the potential ability to start DNA synthesis immediately after RNA primer synthesis. Therefore, the question arises, where did the α-polymerase come from? Phylogenetic analysis based on the nucleotide sequence of several conserved regions reveals that two poxviruses, vaccinia and variola viruses, have polymerases similar to eukaryotic α-polymerase rather than δ-polymerase, while adenovirus, herpes family viruses, and archaeotes have eukaryotic δ-like polymerases, suggesting that the eukaryotic α-polymerase gene is derived from a poxvirus-like organism, which had some eukaryote-like characteristics. Furthermore, the poxvirus's proliferation independent from the host-cell nucleus suggests the possibility that this virus could infect non-nucleated cells, such as ancestral eukaryotes. I wish to propose here a new hypothesis for the origin of the eukaryotic nucleus, posing symbiotic contact of an orthopoxvirus ancestor with an archaebacterium, whose genome already had a δ-like polymerase gene.

Ini ialah pratonton kandungan langganan, akses melalui institusi anda.


In ancient giant viruses lies the truth behind evolution of nucleus in eukaryotic cells

DNA exchange between ancient giant viruses and ancient biological cells might have been the key to the evolution of nuclei in eukaryotic cells Credit: Tokyo University of Science

Perhaps as far back as the history of research and philosophy goes, people have attempted to unearth how life on earth came to be. In the recent decades, with exponential advancement in the fields of genomics, molecular biology, and virology, several scientists on this quest have taken to looking into the evolutionary twists and turns that have resulted in eukaryotic cells, the type of cell that makes up most life forms today.

The most widely accepted theories that have emerged state that the eukaryotic cell is the evolutionary product of the intracellular evolution of proto-eukaryotic cells, which were the first complex cells, and symbiotic relationships between proto-eukaryotic cells and other unicellular and simpler organisms such as bacteria and archaea. But according to Professor Masaharu Takemura of the Tokyo University of Science, Japan, "These hypotheses account for and explain the driving force and evolutionary pressures. But they fail to portray the precise process underlying eukaryotic nucleus evolution."

Prof Takemura cites this as his motivation behind his recent article published in Frontiers in Microbiology, where he looks into the recent theories that, in addition to his own body of research, have built up his current hypothesis on the subject.

In a way, Prof Takemura's hypothesis has its roots in 2001 when, along with PJ Bell, he made the revolutionary proposal that large DNA viruses, like the poxvirus, had something to do with the rise of the eukaryotic cell nucleus. Prof Takemura further explains the reasons for his inquiry into the nucleus of the eukaryotic cell as such: "Although the structure, function, and various biological functions of the cell nucleus have been intensively investigated, the evolutionary origin of the cell nucleus, a milestone of eukaryotic evolution, remains unclear."

The origin of the eukaryotic nucleus must indeed be a milestone in the development of the cell itself, considering that it is the defining factor that sets eukaryotic cells apart from the other broad category of cells—the prokaryotic cell. The eukaryotic cell is neatly compartmentalized into membrane-bound organelles that perform various functions. Among them, the nucleus houses the genetic material. The other organelles float in what is called the cytoplasm. Prokaryotic cells do not contain such compartmentalization. Bacteria and archaea are prokaryotic cells.

The 2001 hypothesis by Prof Takemura and PJ Bell is based on striking similarities between the eukaryotic cell nucleus and poxviruses: in particular, the property of keeping the genome separate in a compartment. Further similarities were uncovered after the discovery and characterization of a type of large DNA virus called "giant virus," which can be up to 2.5 μm in diameter and contain DNA "encoding" information for the production of more than 400 proteins. Independent phylogenetic analyses suggested that genes had been transferred between these viruses and eukaryotic cells as they interacted at various points down the evolutionary road, in a process called "lateral gene transfer."

Viruses are "packets" of DNA or RNA and cannot survive on their own. They must enter a "host" cell and use that cell's machinery to replicate its genetic material, and therefore multiply. As evolution progressed, it appears, viral genetic material became integrated with host genetic material and the properties of both altered.

In 2019, Prof Takemura and his colleagues made another breakthrough discovery: the medusavirus. The medusavirus got its name because, like the mythical monster, it causes encystment in its host that is, it gives its host cell a 'hard' covering.

Via experiments involving the infection of an amoeba, Prof Takemura and his colleagues found that the medusavirus harbors a full set of histones, which resemble histones in eukaryotes. Histones are proteins that keep DNA strands curled up and packed into the cell nucleus. It also holds a DNA polymerase gene and major capsid protein gene very similar to those of the amoeba. Further, unlike other viruses, it does not construct its own enclosed 'viral factory' in the cytoplasm of the cell within which to replicate its DNA and contains none of the genes required to carry out the replication process. Instead, it occupies the entirety of the host nucleus and uses the host nuclear machinery to replicate.

These features, Prof Takemura argues, indicate that the ancestral medusavirus and its corresponding host proto-eukaryotic cells were involved in lateral gene transfer the virus acquired DNA synthesis (DNA polymerase) and condensation (histones) genes from its host and the host acquired structural protein (major capsid protein) genes from the virus. Based on additional research evidence, Prof Takemura extends this new hypothesis to several other giant viruses as well.

Thus, Prof Takemura connects the dots between his findings in 2019 and his original hypothesis in 2001, linking them through his and others' work in the two decades that come in between. All of it taken together, it becomes clear how the medusavirus is prime evidence of the viral origin of the eukaryotic nucleus.

Takemura says, "This new updated hypothesis can profoundly impact the study of eukaryotic cell origins and provide a basis for further discussion on the involvement of viruses in the evolution of the eukaryotic nucleus." Indeed, his work may have unlocked several new possibilities for future research in the field.


Lynn Margulis

Editor kami akan menyemak perkara yang telah anda serahkan dan menentukan sama ada hendak menyemak semula artikel tersebut.

Lynn Margulis, (born March 5, 1938, Chicago, Illinois, U.S.—died November 22, 2011, Amherst, Massachusetts), American biologist whose serial endosymbiotic theory of eukaryotic cell development revolutionized the modern concept of how life arose on Earth.

Margulis was raised in Chicago. Intellectually precocious, she graduated with a bachelor’s degree from the University of Chicago in 1957. Soon after, she married American astronomer Carl Sagan, with whom she had two children one, Dorion, would become her frequent collaborator. The couple divorced in 1964. Margulis earned a master’s degree in zoology and genetics from the University of Wisconsin at Madison in 1960 and a Ph.D. in genetics from the University of California, Berkeley, in 1965. She joined the biology department of Boston University in 1966 and taught there until 1988, when she was named distinguished university professor in the department of botany at the University of Massachusetts at Amherst. She retained that title when her affiliation at the university changed to the department of biology in 1993 and then to the department of geosciences in 1997.

Throughout most of her career, Margulis was considered a radical by peers who pursued traditional Darwinian “survival of the fittest” approaches to biology. Her ideas, which focused on symbiosis—a living arrangement of two different organisms in an association that can be either beneficial or unfavourable—were frequently greeted with skepticism and even hostility. Among her most important work was the development of the serial endosymbiotic theory (SET) of the origin of cells, which posits that eukaryotic cells (cells with nuclei) evolved from the symbiotic merger of nonnucleated bacteria that had previously existed independently. In this theory, mitochondria and chloroplasts, two major organelles of eukaryotic cells, are descendants of once free-living bacterial species. She explained the concept in her first book, Asal-usul Sel Eukariotik (1970). At the time, her theory was regarded as far-fetched, but it has since been widely accepted. She elaborated in her 1981 classic, Symbiosis in Cell Evolution, proposing that another symbiotic merger of cells with bacteria—this time spirochetes, a type of bacterium that undulates rapidly—developed into the internal transportation system of the nucleated cell. Margulis further postulated that eukaryotic cilia were also originally spirochetes and that cytoplasm evolved from a symbiotic relationship between eubacteria and archaebacteria (lihat archaea).

Her 1982 book Five Kingdoms, written with American biologist Karlene V. Schwartz, articulates a five- kingdom system of classifying life on Earth—animals, plants, bacteria (prokaryotes), fungi, and protoctists. The protist kingdom, which comprises most unicellular organisms (and multicellular algae) in other systems, is rejected as too general. Many of the organisms usually categorized as protists are placed in one of the other four kingdoms protoctists make up the remaining organisms, which are all aquatic, and include algae and slime molds. Margulis edited portions of the compendium Handbook of Protoctista (1990).

Another area of interest for Margulis was her long collaboration with British scientist James Lovelock on the controversial Gaia hypothesis. This proposes that the Earth can be viewed as a single self-regulating organism—that is, a complex entity whose living and inorganic elements are interdependent and whose life-forms actively modify the environment to maintain hospitable conditions.

In addition to Margulis’s scholarly publications, she wrote numerous books interpreting scientific concepts and quandaries for a popular audience. Among them were Mystery Dance: On the Evolution of Human Sexuality (1991), Apa itu hidup? (1995), What Is Sex? (1997), and Dazzle Gradually: Reflections on Nature in Nature (2007), all cowritten with her son. She also wrote a book of stories, Luminous Fish (2007). Her later efforts were published under the Sciencewriters Books imprint of Chelsea Green Publishing, which she cofounded with Dorion in 2006.


General Overviews

For reviews of eukaryogenesis, refer to Martin, et al. 2015 and Baum 2015. Martin 2005 provides an older, but still useful review, whereas Harold 2014 is an accessible, book-length exploration of cell evolution. Gould, et al. 2008 focuses on the acquisition of plastids and subsequent additional endosymbiotic events. Koonin, et al. 2010 and Lombard, et al. 2012 discuss protein regulatory networks and membrane chemistry across the three domains of life and their implications for eukaryogenesis. Eme, et al. 2014 links molecular data, which drive much of the field, to fossil evidence. Williams, et al. 2013 summarizes phylogenetic arguments for the phylogenetic model that dominates current thinking, namely that archaea/eukarya and bacteria represent the two primary domains of life. Eme, et al. 2017 builds upon the relationship between archaea and models of eukaryogenesis.

Baum, D. A. 2015. A comparison of autogenous theories for the origin of eukaryotic cells. Jurnal Botani Amerika 102:1954–1965.

Compares different autogenous theories for the origin of the nucleus and eukaryogenesis in general, framed in the context of cellular topology, consilience with modern cell biology and the timing of mitochondrial acquisition.

Eme, L., S. C. Sharpe, M. W. Brown, et al. 2014. On the age of eukaryotes: Evaluating evidence from fossils and molecular clocks. Cold Spring Harbor Perspectives in Biology 6:a016139.

Reviews the phylogenetic and fossil evidence on the age of eukaryotes.

Eme, L., A. Spang, J. Lombard, C. W. Stairs, and T. J. G. Ettema. 2017. Archaea and the origin of eukaryotes. Nature Reviews Microbiology 15:711–723.

Discusses models for eukaryogenesis in the light of newly discovered and characterized archaeal lineages.

Gould, S. B., R. F. Waller, and G. I. McFadden. 2008. Plastid evolution. Annual Review of Plant Biology 59:491–517.

Provides a comprehensive overview of plastid evolution, encompassing primary and secondary endosymbioses, protein targeting to plastids and plastid metabolism.

Harold, F. M. 2014. In search of cell history: The evolution of life’s building blocks. Chicago: Chicago Univ. Tekan.

Provides a synthetic overview of the origin and evolution of cells, with a major focus on the origin of eukaryotes.

Koonin, E. V., J. Dacks, W. Doolittle, et al. 2010. The origin and early evolution of eukaryotes in the light of phylogenomics. Genome Biology 11:209.

Analyzes the origins of key eukaryotic protein regulatory modules using comparative genomics.

Lombard, J., P. López-García, and D. Moreira. 2012. The early evolution of lipid membranes and the three domains of life. Nature Reviews Microbiology 10:507–515.

Reviews the molecular composition of archaeal and bacterial phospholipid membranes and consequences for models of eukaryogenesis.

Martin, W. F. 2005. Archaebacteria (Archaea) and the origin of the eukaryotic nucleus. Pendapat Semasa dalam Mikrobiologi 8:630–637.

Summarizes the diversity of models for the origin of the nuclear compartment, arguing against nuclear endosymbiotic models.

Martin, W. F., S. Garg, and V. Zimorski. 2015. Endosymbiotic theories for eukaryote origin. Philosophical Transactions of the Royal Society B: Biological Science 370.1678: 20140318.

Surveys models of eukaryogenesis with a historical slant, focusing on origins of the nuclear and mitochondrial compartment as well as metabolic considerations.

Williams, T. A., P. G. Foster, C. J. Cox, et al. 2013. An archaeal origin of eukaryotes supports only two primary domains of life. alam semula jadi 504:231–236.

Summarizes support for having only two primary domains of life, with eukaryotes being embedded within a paraphyletic Archaea.

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The eukaryotic ancestor shapes up

Laura Eme is in the Department of Cell and Molecular Biology, Science for Life Laboratory, Uppsala University, 75123 Uppsala, Sweden.

Anda juga boleh mencari pengarang ini dalam PubMed Google Scholar

Thijs J. G. Ettema is in the Department of Cell and Molecular Biology, Science for Life Laboratory, Uppsala University, 75123 Uppsala, Sweden.

Anda juga boleh mencari pengarang ini dalam PubMed Google Scholar

Eukaryotic cells, which carry their DNA in a nucleus, are thought to have evolved from a merger between two other organisms — an archaeal host cell 1 – 3 and a bacterium from which eukaryotic organelles called mitochondria emerged 4 . Some insights into the biological properties of the host have come from the closest known archaeal relatives of eukaryotes, the Asgard superphylum 5 , 6 . The genomes of organisms belonging to this archaeal group encode a suite of proteins typically involved in functions or processes thought to be eukaryote-specific. The functions of these ‘eukaryotic genes’ in Asgard archaea have been elusive, but in a paper in alam semula jadi, Akıl and Robinson 7 provide evidence that some of them encode proteins that are structurally and functionally similar to their eukaryotic counterparts.

Read the paper: Genomes of Asgard archaea encode profilins that regulate actin

Apart from their nucleus and energy-producing mitochondria, eukaryotic cells are characterized by a complex internal system of membrane-bound compartments (the endomembrane system), and by a dynamic network of proteins such as actin, called the cytoskeleton. The latter gives the cells their shape and structure, but is also involved in a variety of cellular processes specific to eukaryotes 8 . These features are thought to have been present in the last common ancestor of all eukaryotes, which lived about 1.8 billion years ago 9 , but no life forms have been found that represent an intermediate between eukaryotes and their bacterial and archaeal ancestors. The seemingly sudden emergence of cellular complexity in the eukaryotic lineage is a conundrum for evolutionary biologists.

Several of the proteins produced by Asgard archaea are evolutionarily related to proteins that in eukaryotes modulate complex cellular processes 5 , 6 . The identification of these proteins raised the question of whether Asgard archaea have some primitive versions of certain eukaryotic properties. If they do, it would suggest that the last archaeal ancestor of eukaryotes already displayed a certain — albeit probably limited — degree of cellular complexity reminiscent of eukaryotes.

Experiments to support such ideas are complicated by the fact that evidence for the existence of the four known Asgard lineages (Lokiarchaeota, Odinarchaeota, Thorarchaeota and Heimdallarchaeota) 5 , 6 is based solely on metagenomics analyses. The cells have yet to be observed under a microscope, and have not been cultured dalam vitro. Nevertheless, Akıl and Robinson were determined to gain insight into the properties of Asgard proteins related to the eukaryotic proteins actin and profilin. In eukaryotes, profilin regulates the polymerization of actin into filaments of the cytoskeleton. These filaments have pivotal roles in processes that include vesicle and organelle movement, cell-shape formation and phagocytosis 8 , in which cells ingest foreign particles or other cells.

To produce Asgard profilins, Akıl and Robinson expressed these proteins in the bacterium Escherichia coli using a circular DNA molecule called a plasmid that harboured the profilin-encoding genes. They then purified the proteins and studied their structures using X-ray crystallography. Asgard profilins share limited amino-acid sequence identity with their eukaryotic counterparts. Nonetheless, the authors found that the structure of lokiarchaeal profilin is topologically similar to that of human profilin, although some structural divergences could be observed. This confirms that Asgard and eukaryotic profilins are indeed evolutionarily related, albeit distantly.

Next, the researchers set out to investigate whether Asgard profilins could interact with Asgard actins. Unfortunately, despite considerable efforts, they were unable to produce functional Asgard actin. As an alternative, they therefore carried out dalam vitro and co-crystallization experiments to test whether Asgard profilins could interact with eukaryotic actins. Remarkably, despite being separated by 2 billion to 3 billion years of evolution 9 , several of the Asgard profilins bound to mammalian actin and regulated its polymerization kinetics. Asgard and mammalian profilins seem to have similar effects on mammalian actin, although the Asgard proteins act less efficiently. These results suggest that Asgard archaea harbour a profilin-regulated actin cytoskeleton — a cellular feature generally regarded as a defining characteristic of eukaryotic cells (Fig. 1).

Rajah 1 | Cellular complexity along the tree of life. The Eukarya (organisms whose cells harbour DNA in a nucleus) are thought to have arisen from a merger between their last archaeal ancestor and a bacterium. In addition to a nucleus, eukaryotes have several characteristics that are thought to separate them from archaea, including: a complex internal system of membranes called endomembranes a structural feature called the actin cytoskeleton, the dynamics of which are regulated by the protein profilin and energy-producing organelles called mitochondria, which arose from the bacterial partner. But Akıl and Robinson 7 provide evidence that members of the Asgard superphylum — an extant group of archaea thought to be related to eukaryotes — harbour a primitive profilin-regulated actin cytoskeleton. If the last archaeal ancestor of eukaryotes had this feature, it might have enabled the cell to wrap around its presumed bacterial partner. In addition, it is possible that Asgard archaea and the last archaeal ancestor of eukaryotes carry primitive endomembrane systems. (Cells and cellular features are not drawn to scale.)

The inference of a primitive dynamic actin cytoskeleton in Asgard archaea sheds light on the biological properties of the ancestor of eukaryotes. In eukaryotic cells, the energy required to dynamically regulate actin is mainly provided by mitochondria 10 . Although the energetic and metabolic properties of Asgard archaea are currently unknown, they certainly lack the firepower that mitochondria provide. A profilin-regulated actin cytoskeleton in the archaeal ancestor of eukaryotes is therefore unlikely to sustain energy-consuming processes such as phagocytosis.

But was the energy provided by mitochondria necessarily the ultimate driving force for the emergence of complex cellular features in eukaryotes? Archaea such as Ignicoccus hospitalis, along with several types of bacterium, have independently evolved endomembrane systems 11 . Because these lineages lack mitochondria, energetic constraints can be ruled out as a limiting factor in the emergence of such a system. It is therefore feasible that Asgard archaeal cells produce sufficient energy to harbour both a primitive endomembrane system and undergo actin-driven membrane and cell-shape deformation. Perhaps the latter ability could have facilitated the symbiotic interaction between the Asgard-related host cell and the bacterial ancestor of mitochondria, for example by optimizing the membrane surface area for metabolic exchange between the two cells. Once mitochondria became an intrinsic part of eukaryotic cells, their capacity for energy production could have conferred selective advantages on their host. However, the exact contribution of these organelles to the emergence of the complex features of eukaryotic cells remains unresolved.

Future efforts to elucidate the biological and physiological properties of Asgard archaea will be essential to increase our understanding of the emergence of eukaryotes. Although biochemical and structural studies of individual Asgard proteins, such as those by Akıl and Robinson, are likely to provide piecemeal insights, it is the ability to grow Asgard archaeal lineages dalam vitro that will ultimately unravel their obscure biology.

alam semula jadi 562, 352-353 (2018)


How did prokaryotic cells evolve into eukaryotic cells?

Is there any fossil or biochemical evidence showing how or when prokaryotic cells evolved into eukaryotic cells? I know mitochondria and chloroplasts for example are the result of symbiotic relationships but do we know when or how this happened?

Also, are archaea linked to this transition or do they have eukaryotic features due to convergent evolution?

One idea is that eukaryotes evolved from the union of archaea and bacteria into one single organism.

This would manifest itself as the overall cell housing the descendants of bacteria in the form of mitochondria and chloroplasts. There is some genetic evidence supporting the idea and often phylogenetic trees position eukaryotes and archaea closer together than bacteria to reflect this idea.

However, if true, the tree of life would need to be modified to allow the convergence of branches into one singular organism.

Edit: There is quite of bit of evidence suggesting that mitochondria and chloroplasts are the descendants of bacteria. However, the idea that the the larger cell which engulfed those early bacteria was an archaea and not another bacteria is not as well established.

The way I learned it at university is this:

At first there was an ancient procaryotic cell with ring-like DNA and without any endomembranes. Then the cytomembrane (with some attached ribosomes, but thats not important yet) startet to fold in and began to surround the DNA molecule. At one point, the membrane divided from the outer membrane and suddenly you had a DNA with a surrounding membrane system, the nucleus. This was the first, ancient eucaryotic cell.

The problem that remained was, that this cell was anaerobic and therefore couldn't really do many highly energy consuming reactions. The theory of endosymbiosis says that such an anaerobic ancient pre-eucaryotic cell engulfed an aerobic procaryotic cell by phagocytosis and somehow got into a symbiotic relationship with it - meaning it would supply the procaryote with raw material (i.e. glucose, or better the catabole pyruvate) and the aerobic procaryote supplies the cell with the energy (in form of ATP) it generated from this pyruvate. Over time, the procaryote transferred most of its genes into the eucaryotic DNA, and thats about the point where you can speak of an early aerobic eucaryotic cell. The procaryote had become a mitochondrium/chloroplast.

Whats interesting is, that not every eucaryotic cell today is aerobic, there are still some anaerobic eucaryotes left (some protozoes, for example chlamydia). Scientists are not sure whether they lost their procaryotic endosymbionts or whether they never had them in the first place. And it is proven that chloroplasts are derived from cyanobacteria. When you compare chloroplastide DNA with the DNA from modern cyanobacteria, they are really really similar. Source: Alberts et al.: Molecular Biology of the Cell


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