Initiation of translation usually involves the interaction of certain key proteins with a special tag bound to the 5'-end of an mRNA molecule, the 5' cap, as well as with the 5' UTR. The protein factors bind the small ribosomal subunit (also referred to as the 40S subunit), and these initiation factors hold the mRNA in place. The eukaryotic Initiation Factor 3 (eIF3) is associated with the small ribosomal subunit, and plays a role in keeping the large 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. It is the rate-limiting step of cap-dependent initiation, and is often cleaved from the complex by some viral proteases to limit the cell's ability to translate its own transcripts. This is a method of hijacking the host machinery in favor of the viral (cap-independent) messages. eIF4A is an ATP-dependent RNA helicase, which aids the ribosome in resolving certain secondary structures formed by the mRNA transcript.
There is another protein associated with the eIF4F complex called the Poly(A)-binding protein (PABP), which 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. This pre-initiation complex (43S subunit, or the 40S and tRNA) accompanied by the protein factors move along the mRNA chain toward its 3'-end, scanning for the 'start' codon (typically AUG) on the mRNA, which indicates where the mRNA begins coding for the protein. In eukaryotes and archaea, the amino acid encoded by the start codon is methionine. The initiator tRNA charged with Met forms part of the ribosomal complex and, thus, all proteins start with this amino acid (unless it is cleaved away by a protease in subsequent modifications). The Met-charged initiator tRNA 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, which results in the association of the large subunit (or the 60S subunit). The complete ribosome (80S) then commences translation elongation, during which the sequence between the 'start' and 'stop' codons is translated from mRNA into an amino acid sequence—thus, a protein is synthesized.
Regulation of protein synthesis depends on phosphorylation of initiation factor eIF2, which is a part of the met-tRNAi complex. When large numbers of eIF2 are phosphorylated, protein synthesis is inhibited. This would occur if there is amino acid starvation or there has been a virus infection. However, naturally a small percentage is of this initiation factor is phosphorylated. Another regulator is 4EBP, which binds to the initiation factor eIF4E found on the 5’ cap on mRNA stopping protein synthesis. To oppose the effects of the 4EBP, growth factors phosphorylate 4EBP, reducing its affinity for eIF4E and permitting protein synthesis.
While the global regulation of protein synthesis is achieved by modulating the expression of key initiation factors as well as the number of ribosomes, individual mRNAs can have different translation rates due to presence of regulatory sequence elements. This has been shown to be important in a variety of settings including yeast meiosis, ethylene response in plants. In addition, recent work in yeast and humans suggest that evolutionary divergence in cis-regulatory sequences can impact translation regulation.
The cap-independent initiation
The best-studied example of the cap-independent mode of translation initiation in eukaryotes is the Internal Ribosome Entry Site (IRES) approach. What differentiates cap-independent translation from cap-dependent translation is that cap-independent translation does not require the ribosome to start scanning from the 5' end of the mRNA cap until the start codon. The ribosome can be trafficked to the start site by ITAFs (IRES trans-acting factors) bypassing the need to scan from the 5' UTR. This method of translation has been recently discovered, and has found important in conditions that require the translation of specific mRNAs, despite cellular stress or the inability to translate most mRNAs. Examples include factors responding to apoptosis, stress-induced responses.
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, (2) forming the peptide bond and (3) shifting the mRNA by one codon relative to the ribosome.
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) before they are exported to the cytoplasm for translation.
Translation can also be affected by ribosomal pausing, which can trigger endonucleolytic attack of the mRNA, 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 mRNA. This can trigger ribosomal frameshifting.
- Eukaryotic initiation factor
- Eukaryotic elongation factors
- Eukaryotic release factors
- Rotavirus translation
- Prokaryotic translation
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