The mechanism of eukaryotic translation initiation and principles of its regulation

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Abstract
Translation is a cyclical process in which ribosomal subunits that participate in initiation are derived by recycling of post-termination ribosomal complexes (post-TCs) that have completed the previous round of translation. Post-TCs first dissociate into free 60S subunits and 40S subunits bound to mRNA and with a P-site deacylated tRNA. Release of tRNA and mRNA from recycled 40S subunits requires eukaryotic initiation factor 3 (eIF3), eIF1 and eIF1A. Deacylated tRNA, which is removed from the P-site of the ribosomal 40S subunit, is replaced by Met-tRNAMeti, which binds as a ternary complex with eIF2-GTP. Initiation on most eukaryotic mRNAs involves scanning by ribosomal 43S preinitiation complexes on the 5′ untranslated region (UTR) from the 5′ cap-proximal point of initial attachment to the initiation codon. The mechanism of scanning is incompletely understood, but requires an 'open' conformation of the 40S subunit, which is induced by eIF1 and eIF1A, is coupled to the activities of the DEAD-box RNA helicase eIF4A, its cofactor eIF4B and eIF4G, and may involve additional DEAD box- or DExH box-containing proteins such as DExH box protein 29 (DHX29; higher eukaryotes) and DEAD box helicase 1 (Ded1; yeast). In a current model for initiation codon recognition, establishment of codon–anticodon base pairing is accompanied by displacement of eIF1 from the P-site, which switches the 40S subunit to a closed conformation that is locked onto mRNA and relieves repression of eIF5-induced hydrolysis of eIF2-bound GTP and Pi release. eIF5B then mediates the dissociation of eIF2–GDP, eIF1 and eIF1A from the interface surface of the 40S subunit and the joining to it of a 60S ribosomal subunit, to form an 80S ribosome with initiator tRNA in the P-site. Regulation of eIF activity by reversible phosphorylation affects most mRNAs that are translated by the canonical scanning-dependent mode of initiation. The best-characterized examples are phosphorylation of mammalian eIF2 by any of four stress-activated kinases (which leads to a reduction in the level of eIF2–GTP–Met-tRNAMeti ternary complexes) and of mammalian 4E-binding proteins, mainly by mTOR (in response to mitogens and growth factors), which releases eIF4E for assimilation into eIF4F and promotes translation. Sequence-specific RNA-binding proteins have the potential to selectively regulate specific mRNAs or classes of mRNA. 3′ UTR-binding proteins commonly repress translation by forming an inhibitory closed loop with a 5′ cap-binding protein (which may be eIF4E) and an intermediate bridging protein, whereas poly(A)-binding protein (PABP) bound to the 3′ poly(A) tail enhances initiation, possibly by tethering eIF4F so that it is available, despite dissociation from the 5′ cap, to promote further rounds of initiation on the same mRNA without having to be recruited de novo from solution. MicroRNAs have recently emerged as important regulators of translation that also act by binding to the 3′ UTR. The mechanism of repression has not yet been fully elucidated, but appears to have two components, both mediated by the carboxy-terminal domain of the protein GW182: true repression of translation and an accelerated rate of deadenylation-dependent mRNA degradation.