Eukaryotic translation

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Eukaryotic translation is the biological process by which messenger RNA is translated into proteins in eukaryotes. It consists of four phases: initiation, elongation, termination, and recapping.

Contents

Initiation

The process of initiation of translation in eukaryotes. Eukaryotic Translation Initiation.png
The process of initiation of translation in eukaryotes.

Translation initiation is the process by which the ribosome and its associated factors bind to an mRNA and are assembled at the start codon. This process is defined as either cap-dependent, in which the ribosome binds initially at the 5' cap and then travels to the stop codon, or as cap-independent, where the ribosome does not initially bind the 5' cap.

Cap-dependent initiation

some of the protein complexes involved in initiation TransInit.png
some of the protein complexes involved in initiation

Initiation of translation usually involves the interaction of certain key proteins, the initiation factors, with a special tag bound to the 5'-end of an mRNA molecule, the 5' cap, as well as with the 5' UTR. These proteins bind the small (40S) ribosomal subunit and hold the mRNA in place. [1]

eIF3 is associated with the 40S ribosomal subunit and plays a role in keeping the large (60S) ribosomal subunit from prematurely binding. eIF3 also interacts with the eIF4F complex, which consists of three other initiation factors: eIF4A, eIF4E, and eIF4G. eIF4G is a scaffolding protein that directly associates with both eIF3 and the other two components. eIF4E is the cap-binding protein. Binding of the cap by eIF4E is often considered the rate-limiting step of cap-dependent initiation, and the concentration of eIF4E is a regulatory nexus of translational control. Certain viruses cleave a portion of eIF4G that binds eIF4E, thus preventing cap-dependent translation to hijack the host machinery in favor of the viral (cap-independent) messages. eIF4A is an ATP-dependent RNA helicase that aids the ribosome by resolving certain secondary structures formed along the mRNA transcript. [2] The poly(A)-binding protein (PABP) also associates with the eIF4F complex via eIF4G, and binds the poly-A tail of most eukaryotic mRNA molecules. This protein has been implicated in playing a role in circularization of the mRNA during translation. [3]

This 43S preinitiation complex (43S PIC) accompanied by the protein factors moves along the mRNA chain toward its 3'-end, in a process known as 'scanning', to reach the start codon (typically AUG). In eukaryotes and archaea, the amino acid encoded by the start codon is methionine. The Met-charged initiator tRNA (Met-tRNAiMet) is brought to the P-site of the small ribosomal subunit by eukaryotic initiation factor 2 (eIF2). It hydrolyzes GTP, and signals for the dissociation of several factors from the small ribosomal subunit, eventually leading to the association of the large subunit (or the 60S subunit). The complete ribosome (80S) then commences translation elongation.

Regulation of protein synthesis is partly influenced by phosphorylation of eIF2 (via the α subunit), which is a part of the eIF2-GTP-Met-tRNAiMet ternary complex (eIF2-TC). When large numbers of eIF2 are phosphorylated, protein synthesis is inhibited. This occurs under amino acid starvation or after viral infection. However, a small fraction of this initiation factor is naturally phosphorylated. Another regulator is 4EBP, which binds to the initiation factor eIF4E and inhibits its interactions with eIF4G, thus preventing cap-dependent initiation. To oppose the effects of 4EBP, growth factors phosphorylate 4EBP, reducing its affinity for eIF4E and permitting protein synthesis.[ citation needed ]

While protein synthesis is globally regulated by modulating the expression of key initiation factors as well as the number of ribosomes, individual mRNAs can have different translation rates due to the presence of regulatory sequence elements. This has been shown to be important in a variety of settings including yeast meiosis and ethylene response in plants. In addition, recent work in yeast and humans suggest that evolutionary divergence in cis-regulatory sequences can impact translation regulation. [4] Additionally, RNA helicases such as DHX29 and Ded1/DDX3 may participate in the process of translation initiation, especially for mRNAs with structured 5'UTRs. [5]

Cap-independent initiation

The best-studied example of cap-independent translation initiation in eukaryotes uses the internal ribosome entry site (IRES). Unlike cap-dependent translation, cap-independent translation does not require a 5' cap to initiate scanning from the 5' end of the mRNA until the start codon. The ribosome can localize to the start site by direct binding, initiation factors, and/or ITAFs (IRES trans-acting factors) bypassing the need to scan the entire 5' UTR. This method of translation is important in conditions that require the translation of specific mRNAs during cellular stress, when overall translation is reduced. Examples include factors responding to apoptosis and stress-induced responses. [6]

Elongation

The elongation and membrane targeting stages of eukaryotic translation. The ribosome is green and yellow, the tRNAs are dark-blue, and the other proteins involved are light-blue Protein translation.gif
The elongation and membrane targeting stages of eukaryotic translation. The ribosome is green and yellow, the tRNAs are dark-blue, and the other proteins involved are light-blue

Elongation depends on eukaryotic elongation factors. At the end of the initiation step, the mRNA is positioned so that the next codon can be translated during the elongation stage of protein synthesis. The initiator tRNA occupies the P site in the ribosome, and the A site is ready to receive an aminoacyl-tRNA. During chain elongation, each additional amino acid is added to the nascent polypeptide chain in a three-step microcycle. The steps in this microcycle are (1) positioning the correct aminoacyl-tRNA in the A site of the ribosome, which is brought into that site by eEF1, (2) forming the peptide bond, and (3) shifting the mRNA by one codon relative to the ribosome with the help of eEF2. Unlike bacteria, in which translation initiation occurs as soon as the 5' end of an mRNA is synthesized, in eukaryotes, such tight coupling between transcription and translation is not possible because transcription and translation are carried out in separate compartments of the cell (the nucleus and cytoplasm). Eukaryotic mRNA precursors must be processed in the nucleus (e.g., capping, polyadenylation, splicing) in ribosomes before they are exported to the cytoplasm for translation. Translation can also be affected by ribosomal pausing, which can trigger endonucleolytic attack of the tRNA, a process termed mRNA no-go decay. Ribosomal pausing also aids co-translational folding of the nascent polypeptide on the ribosome, and delays protein translation while it is encoding tRNA. This can trigger ribosomal frameshifting. [7]

Termination

Termination of elongation depends on eukaryotic release factors. The process is similar to that of bacterial termination, but unlike bacterial termination, there is a universal release factor, eRF1, that recognizes all three stop codons. Upon termination, the ribosome is disassembled and the completed polypeptide is released. eRF3 is a ribosome-dependent GTPase that helps eRF1 release the completed polypeptide. The human genome encodes a few genes whose mRNA stop codon are surprisingly leaky: In these genes, termination of translation is inefficient due to special RNA bases in the vicinity of the stop codon. Leaky termination in these genes leads to translational readthrough of up to 10% of the stop codons of these genes. Some of these genes encode functional protein domains in their readthrough extension so that new protein isoforms can arise. This process has been termed 'functional translational readthrough'. [8]

Regulation and modification of translation

Translation is one of the key energy consumers in cells, hence it is strictly regulated. Numerous mechanisms have evolved that control and regulate translation in eukaryotes as well as prokaryotes. Regulation of translation can impact the global rate of protein synthesis which is closely coupled to the metabolic and proliferative state of a cell. To delve deeper into this intricate process, scientists typically use a technique known as ribosome profiling. [9] This method enables researchers to take a snapshot of the translatome, showing which parts of the mRNA are being translated into proteins by ribosomes at a given time. Ribosome profiling provides valuable insights into translation dynamics, revealing the complex interplay between gene sequence, mRNA structure, and translation regulation. Expanding on this concept, a more recent development is single-cell ribosome profiling, a technique that allows us to study the translation process at the resolution of individual cells. [10] Single-cell ribosome profiling has the potential to shed light on the heterogeneous nature of cells, leading to a more nuanced understanding of how translation regulation can impact cell behavior, metabolic state, and responsiveness to various stimuli or conditions.

Amino acid substitution

In some cells certain amino acids can be depleted and thus affect translation efficiency. For instance, activated T cells secrete interferon-γ which triggers intracellular tryptophan shortage by upregulating the indoleamine 2,3-dioxygenase 1 (IDO1) enzyme. Surprisingly, despite tryptophan depletion, in-frame protein synthesis continues across tryptophan codons. This is achieved by incorporation of phenylalanine instead of tryptophan. The resulting peptides are called W>F "substitutants". Such W>F substitutants are abundant in certain cancer types and have been associated with increased IDO1 expression. Functionally, W>F substitutants can impair protein activity. [11]

See also

Related Research Articles

<span class="mw-page-title-main">Translation (biology)</span> Cellular process of protein synthesis

In biology, translation is the process in living cells in which proteins are produced using RNA molecules as templates. The generated protein is a sequence of amino acids. This sequence is determined by the sequence of nucleotides in the RNA. The nucleotides are considered three at a time. Each such triple results in addition of one specific amino acid to the protein being generated. The matching from nucleotide triple to amino acid is called the genetic code. The translation is performed by a large complex of functional RNA and proteins called ribosomes. The entire process is called gene expression.

The 5′ untranslated region is the region of a messenger RNA (mRNA) that is directly upstream from the initiation codon. This region is important for the regulation of translation of a transcript by differing mechanisms in viruses, prokaryotes and eukaryotes. While called untranslated, the 5′ UTR or a portion of it is sometimes translated into a protein product. This product can then regulate the translation of the main coding sequence of the mRNA. In many organisms, however, the 5′ UTR is completely untranslated, instead forming a complex secondary structure to regulate translation.

An internal ribosome entry site, abbreviated IRES, is an RNA element that allows for translation initiation in a cap-independent manner, as part of the greater process of protein synthesis. In eukaryotic translation, initiation typically occurs at the 5' end of mRNA molecules, since 5' cap recognition is required for the assembly of the initiation complex. The location for IRES elements is often in the 5'UTR, but can also occur elsewhere in mRNAs.

Bacterial translation is the process by which messenger RNA is translated into proteins in bacteria.

Initiation factors are proteins that bind to the small subunit of the ribosome during the initiation of translation, a part of protein biosynthesis.

Eukaryotic initiation factors (eIFs) are proteins or protein complexes involved in the initiation phase of eukaryotic translation. These proteins help stabilize the formation of ribosomal preinitiation complexes around the start codon and are an important input for post-transcription gene regulation. Several initiation factors form a complex with the small 40S ribosomal subunit and Met-tRNAiMet called the 43S preinitiation complex. Additional factors of the eIF4F complex recruit the 43S PIC to the five-prime cap structure of the mRNA, from which the 43S particle scans 5'-->3' along the mRNA to reach an AUG start codon. Recognition of the start codon by the Met-tRNAiMet promotes gated phosphate and eIF1 release to form the 48S preinitiation complex, followed by large 60S ribosomal subunit recruitment to form the 80S ribosome. There exist many more eukaryotic initiation factors than prokaryotic initiation factors, reflecting the greater biological complexity of eukaryotic translation. There are at least twelve eukaryotic initiation factors, composed of many more polypeptides, and these are described below.

A bacterial initiation factor (IF) is a protein that stabilizes the initiation complex for polypeptide translation.

<span class="mw-page-title-main">4EGI-1</span> Chemical compound

4EGI-1 is a synthetic chemical compound which has been found to interfere with the growth of certain types of cancer cells in vitro. Its mechanism of action involves interruption of the binding of cellular initiation factor proteins involved in the translation of transcribed mRNA at the ribosome. The inhibition of these initiation factors prevents the initiation and translation of many proteins whose functions are essential to the rapid growth and proliferation of cancer cells.

<span class="mw-page-title-main">Hepatitis A virus internal ribosome entry site (IRES)</span>

This family represents the internal ribosome entry site (IRES) of the hepatitis A virus. HAV IRES is a 450 nucleotide long sequence located in the 735 nt long 5’ UTR of Hepatitis A viral RNA genome. IRES elements allow cap and end-independent translation of mRNA in the host cell. The IRES achieves this by mediating the internal initiation of translation by recruiting a ribosomal 40S pre-initiation complex directly to the initiation codon and eliminates the requirement for eukaryotic initiation factor, eIF4F.

<span class="mw-page-title-main">Hepatitis C virus internal ribosome entry site</span>

The Hepatitis C virus internal ribosome entry site, or HCV IRES, is an RNA structure within the 5'UTR of the HCV genome that mediates cap-independent translation initiation.

<span class="mw-page-title-main">Nuclear cap-binding protein complex</span> RNA-binding protein

Nuclear cap-binding protein complex is a RNA-binding protein which binds to the 5' cap of pre-mRNA. The cap and nuclear cap-binding protein have many functions in mRNA biogenesis including splicing, 3'-end formation by stabilizing the interaction of the 3'-end processing machinery, nuclear export and protection of the transcripts from nuclease degradation. During mRNA export, the nuclear cap-binding protein complex recruits ribosomes to begin the pioneer round of translation. When RNA is exported to the cytoplasm the nuclear cap-binding protein complex is replaced by cytoplasmic cap binding complex. The nuclear cap-binding complex is a functional heterodimer and composed of Cbc1/Cbc2 in yeast and CBP20/CBP80 in multicellular eukaryotes. Human nuclear cap-binding protein complex shows the large subunit, CBP80 consists of 757 amino acid residues. Its secondary structure contains approximately sixty percent of helical and one percent of beta sheet in the strand. The small subunit, CBP20 has 98 amino acid residues. Its secondary structure contains approximately twenty percent of helical and twenty-four percent of beta sheet in the strand. Human nuclear cap-binding protein complex plays important role in the maturation of pre-mRNA and in uracil-rich small nuclear RNA.

<span class="mw-page-title-main">EIF4A1</span> Protein coding gene in Humans

Eukaryotic initiation factor 4A-I is a 46 kDa cytosolic protein that, in humans, is encoded by the EIF4A1 gene, which is located on chromosome 17. It is the most prevalent member of the eIF4A family of ATP-dependant RNA helicases, and plays a critical role in the initiation of cap-dependent eukaryotic protein translation as a component of the eIF4F translation initiation complex. eIF4A1 unwinds the secondary structure of RNA within the 5'-UTR of mRNA, a critical step necessary for the recruitment of the 43S preinitiation complex, and thus the translation of protein in eukaryotes. It was first characterized in 1982 by Grifo, et al., who purified it from rabbit reticulocyte lysate.

Eukaryotic translation initiation factor 4 G (eIF4G) is a protein involved in eukaryotic translation initiation and is a component of the eIF4F cap-binding complex. Orthologs of eIF4G have been studied in multiple species, including humans, yeast, and wheat. However, eIF4G is exclusively found in domain Eukarya, and not in domains Bacteria or Archaea, which do not have capped mRNA. As such, eIF4G structure and function may vary between species, although the human EIF4G1 has been the focus of extensive studies.

Eukaryotic Initiation Factor 2 (eIF2) is an eukaryotic initiation factor. It is required for most forms of eukaryotic translation initiation. eIF2 mediates the binding of tRNAiMet to the ribosome in a GTP-dependent manner. eIF2 is a heterotrimer consisting of an alpha, a beta, and a gamma subunit.

The eukaryotic initiation factor-4A (eIF4A) family consists of 3 closely related proteins EIF4A1, EIF4A2, and EIF4A3. These factors are required for the binding of mRNA to 40S ribosomal subunits. In addition these proteins are helicases that function to unwind double-stranded RNA.

Translational regulation refers to the control of the levels of protein synthesized from its mRNA. This regulation is vastly important to the cellular response to stressors, growth cues, and differentiation. In comparison to transcriptional regulation, it results in much more immediate cellular adjustment through direct regulation of protein concentration. The corresponding mechanisms are primarily targeted on the control of ribosome recruitment on the initiation codon, but can also involve modulation of peptide elongation, termination of protein synthesis, or ribosome biogenesis. While these general concepts are widely conserved, some of the finer details in this sort of regulation have been proven to differ between prokaryotic and eukaryotic organisms.

The P-site is the second binding site for tRNA in the ribosome. The other two sites are the A-site (aminoacyl), which is the first binding site in the ribosome, and the E-site (exit), the third. During protein translation, the P-site holds the tRNA which is linked to the growing polypeptide chain. When a stop codon is reached, the peptidyl-tRNA bond of the tRNA located in the P-site is cleaved releasing the newly synthesized protein. During the translocation step of the elongation phase, the mRNA is advanced by one codon, coupled to movement of the tRNAs from the ribosomal A to P and P to E sites, catalyzed by elongation factor EF-G.

<span class="mw-page-title-main">Eukaryotic initiation factor 3</span> Multiprotein complex that functions during the initiation phase of eukaryotic translation

Eukaryotic initiation factor 3 (eIF3) is a multiprotein complex that functions during the initiation phase of eukaryotic translation. It is essential for most forms of cap-dependent and cap-independent translation initiation. In humans, eIF3 consists of 13 nonidentical subunits (eIF3a-m) with a combined molecular weight of ~800 kDa, making it the largest translation initiation factor. The eIF3 complex is broadly conserved across eukaryotes, but the conservation of individual subunits varies across organisms. For instance, while most mammalian eIF3 complexes are composed of 13 subunits, budding yeast's eIF3 has only six subunits.

<span class="mw-page-title-main">Eukaryotic initiation factor 4F</span> Multiprotein complex used in gene expression

Eukaryotic initiation factor 4F (eIF4F) is a heterotrimeric protein complex that binds the 5' cap of messenger RNAs (mRNAs) to promote eukaryotic translation initiation. The eIF4F complex is composed of three non-identical subunits: the DEAD-box RNA helicase eIF4A, the cap-binding protein eIF4E, and the large "scaffold" protein eIF4G. The mammalian eIF4F complex was first described in 1983, and has been a major area of study into the molecular mechanisms of cap-dependent translation initiation ever since.

Archaeal initiation factors are proteins that are used during the translation step of protein synthesis in archaea. The principal functions these proteins perform include ribosome RNA/mRNA recognition, delivery of the initiator Met-tRNAiMet, methionine bound tRNAi, to the 40s ribosome, and proofreading of the initiation complex.

References

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  2. ↑ Hellen CU, Sarnow P (July 2001). "Internal ribosome entry sites in eukaryotic mRNA molecules". Genes & Development. 15 (13): 1593–612. doi: 10.1101/gad.891101 . PMID   11445534.
  3. ↑ Wells SE, Hillner PE, Vale RD, Sachs AB (July 1998). "Circularization of mRNA by eukaryotic translation initiation factors". Molecular Cell. 2 (1): 135–40. doi: 10.1016/S1097-2765(00)80122-7 . PMID   9702200.
  4. ↑ Cenik C, Cenik ES, Byeon GW, Grubert F, Candille SI, Spacek D, Alsallakh B, Tilgner H, Araya CL, Tang H, Ricci E, Snyder MP (November 2015). "Integrative analysis of RNA, translation, and protein levels reveals distinct regulatory variation across humans". Genome Research. 25 (11): 1610–21. doi:10.1101/gr.193342.115. PMC   4617958 . PMID   26297486.
  5. ↑ Pisareva VP, Pisarev AV, Komar AA, Hellen CU, Pestova TV (December 2008). "Translation initiation on mammalian mRNAs with structured 5'UTRs requires DExH-box protein DHX29". Cell. 135 (7): 1237–50. doi:10.1016/j.cell.2008.10.037. PMC   2948571 . PMID   19109895.
  6. ↑ López-Lastra M, Rivas A, Barría MI (2005). "Protein synthesis in eukaryotes: the growing biological relevance of cap-independent translation initiation". Biological Research. 38 (2–3): 121–46. doi: 10.4067/s0716-97602005000200003 . PMID   16238092.
  7. ↑ Buchan JR, Stansfield I (September 2007). "Halting a cellular production line: responses to ribosomal pausing during translation". Biology of the Cell. 99 (9): 475–87. doi: 10.1042/BC20070037 . PMID   17696878.
  8. ↑ Schueren F, Thoms S (August 2016). "Functional Translational Readthrough: A Systems Biology Perspective". PLOS Genetics. 12 (8): e1006196. doi: 10.1371/JOURNAL.PGEN.1006196 . PMC   4973966 . PMID   27490485.
  9. ↑ Ingolia NT, Ghaemmaghami S, Newman JR, Weissman JS (April 2009). "Genome-wide analysis in vivo of translation with nucleotide resolution using ribosome profiling". Science. 324 (5924): 218–23. Bibcode:2009Sci...324..218I. doi:10.1126/science.1168978. PMC   2746483 . PMID   19213877.
  10. ↑ Ozadam H, Tonn T, Han CM, Segura A, Hoskins I, Rao S; et al. (2023). "Single-cell quantification of ribosome occupancy in early mouse development". Nature. 618 (7967): 1057–1064. doi:10.1038/s41586-023-06228-9. PMC   10307641 . PMID   37344592.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  11. ↑ Pataskar, Abhijeet; Champagne, Julien; Nagel, Remco; Kenski, Juliana; Laos, Maarja; Michaux, Justine; Pak, Hui Song; Bleijerveld, Onno B.; Mordente, Kelly; Navarro, Jasmine Montenegro; Blommaert, Naomi (2022-03-24). "Tryptophan depletion results in tryptophan-to-phenylalanine substitutants". Nature. 603 (7902): 721–727. doi:10.1038/s41586-022-04499-2. ISSN   0028-0836. PMC   8942854 . PMID   35264796.
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