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Eukaryotic ribosome

Ribosomes are a large and complex molecular machine that catalyzes the synthesis of proteins, referred to as translation. The ribosome selects aminoacylated transfer RNAs (tRNAs) based on the sequence of a protein-encoding messenger RNA (mRNA) and covalently links the amino acids into a polypeptide chain. Ribosomes from all organisms share a highly conserved catalytic center. However, the ribosomes of eukaryotes (animals, plants, fungi, and large number unicellular organisms all with a nucleus) are much larger than prokaryotic (bacterial and archaeal) ribosomes and subject to more complex regulation and biogenesis pathways.[1][2]Eukaryotic ribosomes are also known as 80S ribosomes, referring to their sedimentation coefficients in Svedberg units, because they sediment faster than the prokaryotic (70S) ribosomes. Eukaryotic ribosomes have two unequal subunits, designated small subunit (40S) and large subunit (60S) according to their sedimentation coefficients. Both subunits contain dozens of ribosomal proteins arranged on a scaffold composed of ribosomal RNA (rRNA). The small subunit monitors the complementarity between tRNA anticodon and mRNA, while the large subunit catalyzes peptide bond formation.

Eukaryotic ribosome. The 40S subunit is on the left, the 60S subunit on the right. The ribosomal RNA (rRNA) core is represented as a grey tube, expansion segments are shown in red. Universally conserved proteins are shown in blue. These proteins have homologs in eukaryotes, archaea and bacteria. Proteins shared only between eukaryotes and archaea are shown in orange, and proteins specific to eukaryotes are shown in red. PDB identifiers 4a17, 4A19, 2XZM aligned to 3U5B, 3U5C, 3U5D, 3U5E

Composition Edit

Compared to their prokaryotic homologs, many of the eukaryotic ribosomal proteins are enlarged by insertions or extensions to the conserved core. Furthermore, several additional proteins are found in the small and large subunits of eukaryotic ribosomes, which do not have prokaryotic homologs. The 40S subunit contains a 18S ribosomal RNA (abbreviated 18S rRNA), which is homologous to the prokaryotic 16S rRNA. The 60S subunit contains a 28S rRNA that is homologous to the prokaryotic 23S ribosomal RNA. In addition, it contains a 5.8S rRNA that corresponds to the 5' end of the 23S rRNA, and a short 5S rRNA. Both 18S and 28S have multiple insertions to the core rRNA fold of their prokaryotic counterparts, which are called expansion segments. For a detailed list of proteins, including archaeal and bacterial homologs please refer to the separate articles on the 40S and 60S subunits. Recent research suggests heterogeneity in the ribosomal composition, i.e., that the stoichiometry among core ribosomal proteins in wild-type yeast cells and embryonic stem cells depends both on the growth conditions and on the number of ribosomes bound per mRNA.[3]

Eukaryotic[4] Bacterial[4]
Ribosome Sedimentation coefficient 80 S 70 S
Molecular mass ~3.2×106 Da ~2.0×106 Da
Diameter ~250–300 Å ~200 Å
Large subunit Sedimentation coefficient 60 S 50 S
Molecular mass ~2.0×106 Da ~1.3×106 Da
Proteins 46 33
rRNAs
  • 25/28 S rRNA (3354 nucleotides)
  • 5 S rRNA (120 nucleotides)
  • 5.8 S rRNA (154 nucleotides)
  • 23S rRNA (2839 nucleotides)
  • 5S rRNA (122 nucleotides)
Small subunit Sedimentation coefficient 40 S 30 S
Molecular mass ~1.2×106 Da ~0.7×106 Da
Proteins 33 20
rRNAs
  • 18S rRNA (1753 nucleotides)
  • 16S rRNA (1504 nucleotides)

Structure determination Edit

Initial structures of eukaryotic ribosomes were determined by electron microscopy. First 3D structures were obtained at 30–40 Å resolution for yeast[5] and mammalian ribosomes.[6][7] Higher resolution structures of the yeast ribosome by cryo-electron microscopy allowed the identification of protein and RNA structural elements.[8] More recently structures at sub-nanometer resolution were obtained for complexes of ribosomes and factors involved in translation.[9][10][11] After the determination of the first bacterial[12][13][14] and archaeal[15] ribosome structures at atomic resolution in the 1990s, it took another decade until in 2011, high resolution structures of eukaryotic ribosome were obtained by X-ray crystallography, mainly because of the difficulties in obtaining crystals of sufficient quality.[16][17][18] The complete structure of a eukaryotic 40S ribosomal structure in Tetrahymena thermophila was published and described, as well as much about the 40S subunit's interaction with eIF1 during translation initiation.[16] The eukaryotic 60S subunit structure was also determined from T. thermophila in complex with eIF6.[17] The complete structure of the eukaryotic 80S ribosome from the yeast Saccharomyces cerevisiae was obtained by crystallography at 3.0 A resolution.[18] These structures reveal the precise architecture of eukaryote-specific elements, their interaction with the universally conserved core, and all eukaryote-specific bridges between the two ribosomal subunits.

Atomic coordinates (PDB files) and structure factors of the eukaryotic ribosome have been deposited in the Protein Data Bank (PDB) under the following accession codes:

Complex Source Organism Resolution PDB Identifier[19]
80S:Stm1 S. cerevisiae 3.0 Å
  • 3U5B on www.PDB.org
  • 3U5C on www.PDB.org
  • 3U5D on www.PDB.org
  • 3U5E on www.PDB.org
40S:eIF1 T. thermophila 3.9 Å
  • 2XZM on www.PDB.org
60S:eIF6 T. thermophila 3.5 Å
  • 4A17 on www.PDB.org
  • 4A19 on www.PDB.org

Architecture Edit

General features Edit

Some general architectural features of the ribosome are conserved across kingdoms:[20] The structure of the small subunit can be sub-divided into two large segments, the head and the body. Characteristic features of the body include the left and right feet, the shoulder and the platform. The head features a pointed protrusion reminiscent of a bird's beak. In the characteristic "crown view" of the large subunit, structural landmarks include the central protuberance, the L1-stalk and the P-stalk.[21][22] The majority of the eukaryote-specific RNA and protein elements are found on the solvent-exposed sides of the 40S [16] and 60S[17] subunits. The subunit interface, as well as important functional regions such as the peptidyl transferase center and the decoding site are mostly conserved, with some differences observed in the surrounding regions. In stark contrast to prokaryotic ribosomal proteins, which interact primarily with RNA, the eukaryote-specific protein segments engage in a multitude of protein-protein interactions. Long distance interactions are mediated by eukaryote-specific helical extensions of ribosomal proteins, and several eukaryotic ribosomal proteins jointly to form inter-protein beta-sheets.

The ribosomal RNA core is represented as a grey tube, expansion segments are shown in red. Universally conserved proteins are shown in blue. These proteins have homologs in eukaryotes, archaea and bacteria. Proteins Shared only between eukaryotes and archaea are shown in orange, and proteins specific to eukaryotes are shown in red.

Co-evolution of rRNA and proteins Edit

The structure of the 40S subunit revealed that the eukaryote-specific proteins (rpS7, rpS10, rpS12 and RACK1), as well as numerous eukaryote-specific extensions of proteins, are located on the solvent-exposed side of the small subunit.[16] Here, they participate in the stabilization of rRNA expansion segments. Moreover, the beak of the 40S subunit is remodeled, as rRNA has been replaced by proteins rpS10 and rpS12.[16] As observed for the 40S subunit, all eukaryote-specific proteins of the 60S subunit (RPL6, RPL22, RPL27, RPL28, RPL29 and RPL36) and many extensions are located at the solvent-exposed side, forming an intricate network of interactions with eukaryotic-specific RNA expansion segments. RPL6, RPL27 and RPL29 mediate contacts between the ES sets ES7–ES39, ES31–ES20–ES26 and ES9–ES12, respectively and RPL28 stabilized expansion segment ES7A.[17]

Ubiquitin fusion proteins Edit

In eukaryotes, the small subunit protein RPS27A (or eS31) and the large subunit protein RPL40 (or eL40) are processed polypeptides, which are translated as fusion proteins carrying N-terminal ubiquitin domains. Both proteins are located next to important functional centers of the ribosome: the uncleaved ubiquitin domains of eS31) and eL40 would be positioned in the decoding site and near the translation factor binding site, respectively. These positions suggest that proteolytic cleavage is an essential step in the production of functional ribosomes.[16][17] Indeed, mutations of the linker between the core of eS31 and the ubiquitin domain are lethal in yeast.[23]

Active site Edit

Comparisons between bacterial, archaeal and eukaryotic ribosome structures reveal a very high degree of conservation in the active site—aka the peptidyl transferase center (PTC) -- region. None of the eukaryote-specific protein elements is close enough to directly participate in catalysis.[17] However, RPL29 projects to within 18Å of the active site in T. thermophila, and eukaryote-specific extensions interlink several proteins in the vicinity of the PTC of the 60S subunit,[17][21] while the corresponding 50S proteins are singular entities.[15]

Intersubunit bridges Edit

Contacts across the two ribosomal subunits are known as intersubunit bridges. In the eukaryotic ribosome, additional contacts are made by 60S expansion segments and proteins.[24] Specifically, the C-terminal extension of the 60S protein RPL19 interacts with ES6E of the 40S rRNA, and the C-terminal extension of the 60S protein RPL24 interacts with 40S rpS6 and rRNA helix h10. Moreover, the 60S expansion segments ES31 and ES41 interact with rpS3A(S1) and rpS8 of the 40S subunit, respectively, and the basic 25-amino-acid peptide RPL41 is positioned at the subunit interface in the 80S ribosome, interacting with rRNA elements of both subunits.[21][24]

Ribosomal proteins with roles in signaling Edit

Two 40S ribosomal proteins (RACK1 and RPS6 (or eS6)) have been implicated in cellular signaling: RACK1, first described as the receptor of activated protein kinase C (PKC), is an integral component of the eukaryotic ribosome and is located at the back of the head.[16] It may link signal-transduction pathways directly to the ribosome though it also has a role in multiple translational processes that appear unrelated (reviewed in [25]). Ribosomal protein eS6 is located at the right foot of the 40S subunit [16] and is phosphorylated in response to mammalian target of rapamycin (mTOR) signaling.[26]

Functional aspects Edit

Translation initiation Edit

Protein synthesis is primarily regulated at the stage of translation initiation. In eukaryotes, the canonical initiation pathway requires at least 12 protein initiation factors, some of which are themselves large complexes.[27] The structures of the 40S:eIF1 [16] and 60S:eIF6 [17] complexes provide first detailed insights into the atomic interactions between the eukaryotic ribosome and regulatory factors. eIF1 is involved in start codon selection, and eIF6 sterically precludes the joining of subunits. However, structural information on the eukaryotic initiation factors and their interactions with the ribosome is limited and largely derived from homology models or low-resolution analyses.[28] Elucidation of the interactions between the eukaryotic ribosome and initiation factors at an atomic level is essential for a mechanistic understanding of the regulatory processes, but represents a significant technical challenge, because of the inherent dynamics and flexibility of the initiation complexes. The first structure of the mammalian pre initiation complex was done by cryo-electron microscopy.[29] Other structures of initiation complexes followed soon, driven by cryo-EM technical improvements.[30][31] Those structures will help better understand the process of translation initiation in eukaryotes.

Regulatory roles of ribosomal proteins Edit

Recent genetic evidence has been interpreted to suggest that individual proteins of the eukaryotic ribosome directly contribute to the regulation of translation.[32][33][34] However, this interpretation is controversial and some researchers have proposed that genetic changes to ribosomal protein genes indirectly affect overall ribosome numbers or ribosome biogenesis processes.[35][36]

Protein translocation and targeting Edit

To exert their functions in the cell newly synthesized proteins must be targeted to the appropriate location in the cell, which is achieved by protein targeting and translocation systems.[37] The growing polypeptide leaves the ribosome through a narrow tunnel in the large subunit. The region around the exit tunnel of the 60S subunit is very similar to the bacterial and archaeal 50S subunits. Additional elements are restricted to the second tier of proteins around the tunnel exit, possibly by conserved interactions with components of the translocation machinery.[17] The targeting and translocation machinery is much more complex in eukaryotes.[38]

Ribosomal diseases and cancer Edit

Ribosomopathies are congenital human disorders resulting from defects in ribosomal protein or rRNA genes, or other genes whose products are implicated in ribosome biogenesis.[39] Examples include X-linked Dyskeratosis congenita (X-DC),[40] Diamond–Blackfan anemia,[41] Treacher Collins syndrome (TCS)[41][42] and Shwachman–Bodian–Diamond syndrome (SBDS).[39] SBDS is caused by mutations in the SBDS protein that affects its ability to couple GTP hydrolysis by the GTPase EFL1 to the release of eIF6 from the 60S subunit.[43]

Therapeutic opportunities Edit

The ribosome is a prominent drug target for antibacterials, which interfere with translation at different stages of the elongation cycle [44] Most clinically relevant translation compounds are inhibitors of bacterial translation, but inhibitors of eukaryotic translation may also hold therapeutic potential for application in cancer or antifungal chemotherapy.[45] Elongation inhibitors show antitumor activity 'in vivo' and 'in vitro'.[46][47][48] One toxic inhibitor of eukaryotic translation elongation is the glutarimide antibiotic cycloheximide (CHX), which has been co-crystallized with the eukaryotic 60S subunit [17] and binds in the ribosomal E site. The structural characterization of the eukaryotic ribosome [16][17][24] may enable the use of structure-based methods for the design of novel antibacterials, wherein differences between the eukaryotic and bacterial ribosomes can be exploited to improve the selectivity of drugs and therefore reduce adverse effects.

Formation mechanism Edit

Eukaryote ribosomes are produced and assembled in the nucleolus. Ribosomal proteins enter the nucleolus and combine with the four rRNA strands to create the two ribosomal subunits (one small and one large) that will make up the completed ribosome. The ribosome units leave the nucleus through the nuclear pores and unite once in the cytoplasm for the purpose of protein synthesis.

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Notes Edit

  • "EMDB-1067: Ribosomal 80S-eEF2-sordarin complex from S. cerevisiae - EM Navigator". emnavi.protein.osaka-u.ac.jp. Archived from the original on 2012-12-19. Retrieved 2009-08-06.
  • Giavalisco P, Wilson D, Kreitler T, et al. (March 2005). "High heterogeneity within the ribosomal proteins of the Arabidopsis thaliana 80S ribosome". Plant Mol. Biol. 57 (4): 577–591. doi:10.1007/s11103-005-0699-3. hdl:11858/00-001M-0000-0010-86C6-1. PMID 15821981. S2CID 14500573.
  • . www.cs.stedwards.edu. Archived from the original on 2009-03-20. Retrieved 2009-08-06.

eukaryotic, ribosome, ribosomes, large, complex, molecular, machine, that, catalyzes, synthesis, proteins, referred, translation, ribosome, selects, aminoacylated, transfer, rnas, trnas, based, sequence, protein, encoding, messenger, mrna, covalently, links, a. Ribosomes are a large and complex molecular machine that catalyzes the synthesis of proteins referred to as translation The ribosome selects aminoacylated transfer RNAs tRNAs based on the sequence of a protein encoding messenger RNA mRNA and covalently links the amino acids into a polypeptide chain Ribosomes from all organisms share a highly conserved catalytic center However the ribosomes of eukaryotes animals plants fungi and large number unicellular organisms all with a nucleus are much larger than prokaryotic bacterial and archaeal ribosomes and subject to more complex regulation and biogenesis pathways 1 2 Eukaryotic ribosomes are also known as 80S ribosomes referring to their sedimentation coefficients in Svedberg units because they sediment faster than the prokaryotic 70S ribosomes Eukaryotic ribosomes have two unequal subunits designated small subunit 40S and large subunit 60S according to their sedimentation coefficients Both subunits contain dozens of ribosomal proteins arranged on a scaffold composed of ribosomal RNA rRNA The small subunit monitors the complementarity between tRNA anticodon and mRNA while the large subunit catalyzes peptide bond formation Eukaryotic ribosome The 40S subunit is on the left the 60S subunit on the right The ribosomal RNA rRNA core is represented as a grey tube expansion segments are shown in red Universally conserved proteins are shown in blue These proteins have homologs in eukaryotes archaea and bacteria Proteins shared only between eukaryotes and archaea are shown in orange and proteins specific to eukaryotes are shown in red PDB identifiers 4a17 4A19 2XZM aligned to 3U5B 3U5C 3U5D 3U5E Contents 1 Composition 2 Structure determination 3 Architecture 3 1 General features 3 2 Co evolution of rRNA and proteins 3 3 Ubiquitin fusion proteins 3 4 Active site 3 5 Intersubunit bridges 3 6 Ribosomal proteins with roles in signaling 4 Functional aspects 4 1 Translation initiation 4 2 Regulatory roles of ribosomal proteins 4 3 Protein translocation and targeting 4 4 Ribosomal diseases and cancer 4 5 Therapeutic opportunities 5 Formation mechanism 6 References 7 NotesComposition EditCompared to their prokaryotic homologs many of the eukaryotic ribosomal proteins are enlarged by insertions or extensions to the conserved core Furthermore several additional proteins are found in the small and large subunits of eukaryotic ribosomes which do not have prokaryotic homologs The 40S subunit contains a 18S ribosomal RNA abbreviated 18S rRNA which is homologous to the prokaryotic 16S rRNA The 60S subunit contains a 28S rRNA that is homologous to the prokaryotic 23S ribosomal RNA In addition it contains a 5 8S rRNA that corresponds to the 5 end of the 23S rRNA and a short 5S rRNA Both 18S and 28S have multiple insertions to the core rRNA fold of their prokaryotic counterparts which are called expansion segments For a detailed list of proteins including archaeal and bacterial homologs please refer to the separate articles on the 40S and 60S subunits Recent research suggests heterogeneity in the ribosomal composition i e that the stoichiometry among core ribosomal proteins in wild type yeast cells and embryonic stem cells depends both on the growth conditions and on the number of ribosomes bound per mRNA 3 Eukaryotic 4 Bacterial 4 Ribosome Sedimentation coefficient 80 S 70 SMolecular mass 3 2 106 Da 2 0 106 DaDiameter 250 300 A 200 ALarge subunit Sedimentation coefficient 60 S 50 SMolecular mass 2 0 106 Da 1 3 106 DaProteins 46 33rRNAs 25 28 S rRNA 3354 nucleotides 5 S rRNA 120 nucleotides 5 8 S rRNA 154 nucleotides 23S rRNA 2839 nucleotides 5S rRNA 122 nucleotides Small subunit Sedimentation coefficient 40 S 30 SMolecular mass 1 2 106 Da 0 7 106 DaProteins 33 20rRNAs 18S rRNA 1753 nucleotides 16S rRNA 1504 nucleotides Structure determination EditInitial structures of eukaryotic ribosomes were determined by electron microscopy First 3D structures were obtained at 30 40 A resolution for yeast 5 and mammalian ribosomes 6 7 Higher resolution structures of the yeast ribosome by cryo electron microscopy allowed the identification of protein and RNA structural elements 8 More recently structures at sub nanometer resolution were obtained for complexes of ribosomes and factors involved in translation 9 10 11 After the determination of the first bacterial 12 13 14 and archaeal 15 ribosome structures at atomic resolution in the 1990s it took another decade until in 2011 high resolution structures of eukaryotic ribosome were obtained by X ray crystallography mainly because of the difficulties in obtaining crystals of sufficient quality 16 17 18 The complete structure of a eukaryotic 40S ribosomal structure in Tetrahymena thermophila was published and described as well as much about the 40S subunit s interaction with eIF1 during translation initiation 16 The eukaryotic 60S subunit structure was also determined from T thermophila in complex with eIF6 17 The complete structure of the eukaryotic 80S ribosome from the yeast Saccharomyces cerevisiae was obtained by crystallography at 3 0 A resolution 18 These structures reveal the precise architecture of eukaryote specific elements their interaction with the universally conserved core and all eukaryote specific bridges between the two ribosomal subunits Atomic coordinates PDB files and structure factors of the eukaryotic ribosome have been deposited in the Protein Data Bank PDB under the following accession codes Complex Source Organism Resolution PDB Identifier 19 80S Stm1 S cerevisiae 3 0 A 3U5B on www PDB org 3U5C on www PDB org 3U5D on www PDB org 3U5E on www PDB org40S eIF1 T thermophila 3 9 A 2XZM on www PDB org60S eIF6 T thermophila 3 5 A 4A17 on www PDB org 4A19 on www PDB orgArchitecture EditGeneral features Edit Some general architectural features of the ribosome are conserved across kingdoms 20 The structure of the small subunit can be sub divided into two large segments the head and the body Characteristic features of the body include the left and right feet the shoulder and the platform The head features a pointed protrusion reminiscent of a bird s beak In the characteristic crown view of the large subunit structural landmarks include the central protuberance the L1 stalk and the P stalk 21 22 The majority of the eukaryote specific RNA and protein elements are found on the solvent exposed sides of the 40S 16 and 60S 17 subunits The subunit interface as well as important functional regions such as the peptidyl transferase center and the decoding site are mostly conserved with some differences observed in the surrounding regions In stark contrast to prokaryotic ribosomal proteins which interact primarily with RNA the eukaryote specific protein segments engage in a multitude of protein protein interactions Long distance interactions are mediated by eukaryote specific helical extensions of ribosomal proteins and several eukaryotic ribosomal proteins jointly to form inter protein beta sheets Crystal structures of the eukaryotic ribosomal subunits from T thermophila nbsp 40S subunit viewed from the subunit interface side PDB identifier 2XZM nbsp 40S subunit viewed from the solvent exposed side PDB identifier 2XZM nbsp 60S subunit viewed from the subunit interface side PDB identifiers 4A17 4A19 nbsp 60S subunit viewed from the solvent exposed side PDB identifiers 4A17 4A19 The ribosomal RNA core is represented as a grey tube expansion segments are shown in red Universally conserved proteins are shown in blue These proteins have homologs in eukaryotes archaea and bacteria Proteins Shared only between eukaryotes and archaea are shown in orange and proteins specific to eukaryotes are shown in red Co evolution of rRNA and proteins Edit The structure of the 40S subunit revealed that the eukaryote specific proteins rpS7 rpS10 rpS12 and RACK1 as well as numerous eukaryote specific extensions of proteins are located on the solvent exposed side of the small subunit 16 Here they participate in the stabilization of rRNA expansion segments Moreover the beak of the 40S subunit is remodeled as rRNA has been replaced by proteins rpS10 and rpS12 16 As observed for the 40S subunit all eukaryote specific proteins of the 60S subunit RPL6 RPL22 RPL27 RPL28 RPL29 and RPL36 and many extensions are located at the solvent exposed side forming an intricate network of interactions with eukaryotic specific RNA expansion segments RPL6 RPL27 and RPL29 mediate contacts between the ES sets ES7 ES39 ES31 ES20 ES26 and ES9 ES12 respectively and RPL28 stabilized expansion segment ES7A 17 Ubiquitin fusion proteins Edit In eukaryotes the small subunit protein RPS27A or eS31 and the large subunit protein RPL40 or eL40 are processed polypeptides which are translated as fusion proteins carrying N terminal ubiquitin domains Both proteins are located next to important functional centers of the ribosome the uncleaved ubiquitin domains of eS31 and eL40 would be positioned in the decoding site and near the translation factor binding site respectively These positions suggest that proteolytic cleavage is an essential step in the production of functional ribosomes 16 17 Indeed mutations of the linker between the core of eS31 and the ubiquitin domain are lethal in yeast 23 Active site Edit Comparisons between bacterial archaeal and eukaryotic ribosome structures reveal a very high degree of conservation in the active site aka the peptidyl transferase center PTC region None of the eukaryote specific protein elements is close enough to directly participate in catalysis 17 However RPL29 projects to within 18A of the active site in T thermophila and eukaryote specific extensions interlink several proteins in the vicinity of the PTC of the 60S subunit 17 21 while the corresponding 50S proteins are singular entities 15 Intersubunit bridges Edit Contacts across the two ribosomal subunits are known as intersubunit bridges In the eukaryotic ribosome additional contacts are made by 60S expansion segments and proteins 24 Specifically the C terminal extension of the 60S protein RPL19 interacts with ES6E of the 40S rRNA and the C terminal extension of the 60S protein RPL24 interacts with 40S rpS6 and rRNA helix h10 Moreover the 60S expansion segments ES31 and ES41 interact with rpS3A S1 and rpS8 of the 40S subunit respectively and the basic 25 amino acid peptide RPL41 is positioned at the subunit interface in the 80S ribosome interacting with rRNA elements of both subunits 21 24 Ribosomal proteins with roles in signaling Edit Two 40S ribosomal proteins RACK1 and RPS6 or eS6 have been implicated in cellular signaling RACK1 first described as the receptor of activated protein kinase C PKC is an integral component of the eukaryotic ribosome and is located at the back of the head 16 It may link signal transduction pathways directly to the ribosome though it also has a role in multiple translational processes that appear unrelated reviewed in 25 Ribosomal protein eS6 is located at the right foot of the 40S subunit 16 and is phosphorylated in response to mammalian target of rapamycin mTOR signaling 26 Functional aspects EditTranslation initiation Edit Protein synthesis is primarily regulated at the stage of translation initiation In eukaryotes the canonical initiation pathway requires at least 12 protein initiation factors some of which are themselves large complexes 27 The structures of the 40S eIF1 16 and 60S eIF6 17 complexes provide first detailed insights into the atomic interactions between the eukaryotic ribosome and regulatory factors eIF1 is involved in start codon selection and eIF6 sterically precludes the joining of subunits However structural information on the eukaryotic initiation factors and their interactions with the ribosome is limited and largely derived from homology models or low resolution analyses 28 Elucidation of the interactions between the eukaryotic ribosome and initiation factors at an atomic level is essential for a mechanistic understanding of the regulatory processes but represents a significant technical challenge because of the inherent dynamics and flexibility of the initiation complexes The first structure of the mammalian pre initiation complex was done by cryo electron microscopy 29 Other structures of initiation complexes followed soon driven by cryo EM technical improvements 30 31 Those structures will help better understand the process of translation initiation in eukaryotes Regulatory roles of ribosomal proteins Edit Recent genetic evidence has been interpreted to suggest that individual proteins of the eukaryotic ribosome directly contribute to the regulation of translation 32 33 34 However this interpretation is controversial and some researchers have proposed that genetic changes to ribosomal protein genes indirectly affect overall ribosome numbers or ribosome biogenesis processes 35 36 Protein translocation and targeting Edit To exert their functions in the cell newly synthesized proteins must be targeted to the appropriate location in the cell which is achieved by protein targeting and translocation systems 37 The growing polypeptide leaves the ribosome through a narrow tunnel in the large subunit The region around the exit tunnel of the 60S subunit is very similar to the bacterial and archaeal 50S subunits Additional elements are restricted to the second tier of proteins around the tunnel exit possibly by conserved interactions with components of the translocation machinery 17 The targeting and translocation machinery is much more complex in eukaryotes 38 Ribosomal diseases and cancer Edit Ribosomopathies are congenital human disorders resulting from defects in ribosomal protein or rRNA genes or other genes whose products are implicated in ribosome biogenesis 39 Examples include X linked Dyskeratosis congenita X DC 40 Diamond Blackfan anemia 41 Treacher Collins syndrome TCS 41 42 and Shwachman Bodian Diamond syndrome SBDS 39 SBDS is caused by mutations in the SBDS protein that affects its ability to couple GTP hydrolysis by the GTPase EFL1 to the release of eIF6 from the 60S subunit 43 Therapeutic opportunities Edit The ribosome is a prominent drug target for antibacterials which interfere with translation at different stages of the elongation cycle 44 Most clinically relevant translation compounds are inhibitors of bacterial translation but inhibitors of eukaryotic translation may also hold therapeutic potential for application in cancer or antifungal chemotherapy 45 Elongation inhibitors show antitumor activity in vivo and in vitro 46 47 48 One toxic inhibitor of eukaryotic translation elongation is the glutarimide antibiotic cycloheximide CHX which has been co crystallized with the eukaryotic 60S subunit 17 and binds in the ribosomal E site The structural characterization of the eukaryotic ribosome 16 17 24 may enable the use of structure based methods for the design of novel antibacterials wherein differences between the eukaryotic and bacterial ribosomes can be exploited to improve the selectivity of drugs and therefore reduce adverse effects Formation mechanism EditEukaryote ribosomes are produced and assembled in the nucleolus Ribosomal proteins enter the nucleolus and combine with the four rRNA strands to create the two ribosomal subunits one small and one large that will make up the completed ribosome The ribosome units leave the nucleus through the nuclear pores and unite once in the cytoplasm for the purpose of protein synthesis References Edit Difference Between 70S Ribosomes and 80S Ribosomes RNA Micromolecules www microbiologyprocedure com Archived from the original on 2008 09 05 Retrieved 2009 08 06 80S Ribosomes Eukaryotic Ribosomes Prokaryotic Ribosomes Nucleic Acids Sedimentation Coefficient www microbiologyprocedure com Archived from the original on 2009 06 23 Retrieved 2009 08 06 Slavov Nikolai Semrau Stefan Airoldi Edoardo Budnik Bogdan van Oudenaarden Alexander 2015 Differential Stoichiometry among 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eukaryotic translation elongation by cycloheximide and lactimidomycin Nat Chem Biol 6 3 209 217 doi 10 1038 nchembio 304 PMC 2831214 PMID 20118940 Dang Y et al 2011 Inhibition of eukaryotic translation elongation by the antitumor natural product Mycalamide B RNA 17 8 1578 1588 doi 10 1261 rna 2624511 PMC 3153980 PMID 21693620 Notes Edit EMDB 1067 Ribosomal 80S eEF2 sordarin complex from S cerevisiae EM Navigator emnavi protein osaka u ac jp Archived from the original on 2012 12 19 Retrieved 2009 08 06 Giavalisco P Wilson D Kreitler T et al March 2005 High heterogeneity within the ribosomal proteins of the Arabidopsis thaliana 80S ribosome Plant Mol Biol 57 4 577 591 doi 10 1007 s11103 005 0699 3 hdl 11858 00 001M 0000 0010 86C6 1 PMID 15821981 S2CID 14500573 Ribosomes www cs stedwards edu Archived from the original on 2009 03 20 Retrieved 2009 08 06 Retrieved from https en wikipedia org w index php title Eukaryotic ribosome amp oldid 1170990925, wikipedia, wiki, book, books, library,

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