Translation Initiation: Molecular Mechanisms in Eukaryotes

Abstract

Translation initiation is a complex process in which initiator tRNA, 40S, and 60S ribosomal subunits are assembled by eukaryotic initiation factors (eIFs) into an 80S ribosome at the initiation codon of mRNA. The cap-binding complex eIF4F and the factors eIF4A and eIF4B are required for binding of 43S complexes (comprising a 40S subunit, eIF2GTPMet-tRNAi and eIF3) to the 5 end of capped mRNA but are not sufficient to promote ribosomal scanning to the initiation codon. eIF1A enhances the ability of eIF1 to dissociate aberrantly assembled complexes from mRNA, and these factors synergistically mediate 48S complex assembly at the initiation codon. Joining of 48S complexes to 60S subunits to form 80S ribosomes requires eIF5B, which has an essential ribosome-dependent GTPase activity and hydrolysis of eIF2-bound GTP induced by eIF5. Initiation on a few mRNAs is cap-independent and occurs instead by internal ribosomal entry. Encephalomyocarditis virus (EMCV) and hepatitis C virus epitomize distinct mechanisms of internal ribosomal entry site (IRES)-mediated initiation. The eIF4A and eIF4G subunits of eIF4F bind immediately upstream of the EMCV initiation codon and promote binding of 43S complexes. EMCV initiation does not involve scanning and does not require eIF1, eIF1A, and the eIF4E subunit of eIF4F. Initiation on some EMCV-like IRESs requires additional noncanonical initiation factors, which alter IRES conformation and promote binding of eIF4A4G. Initiation on the hepatitis C virus IRES is even simpler: 43S complexes containing only eIF2 and eIF3 bind directly to the initiation codon as a result of specific interaction of the IRES and the 40S subunit. T ranslation of mRNA into protein begins after assembly of initiator tRNA (Met-tRNA i), mRNA, and separated 40S and 60S ribosomal subunits into an 80S ribosome in which Met-tRNA i is positioned in the ribosomal P site at the initiation codon. The complex initiation process that leads to 80S ribosome formation consists of several linked stages that are mediated by eukaryotic initiation factors. These stages are: (i) Selection of initiator tRNA from the pool of elongator tRNAs by eukaryotic initiation factor (eIF)2 and binding of an eIF2GTPMet-tRNA i ternary complex and other eIFs to the 40S subunit to form a 43S preinitiation complex. (ii) Binding of the 43S complex to mRNA, which in most instances occurs by a mechanism that involves initial recognition of the m 7 G cap at the mRNA 5-terminus by the eIF4E (cap-binding) subunit of eIF4F. Ribosomes bind to a subset of cellular and viral mRNAs as a result of cap-and end-independent internal ribosomal entry. (iii) Movement of the mRNA-bound ribosomal complex along the 5 nontranslated region (5NTR) from its initial binding site to the initiation codon to form a 48S initiation complex in which the initiation codon is base paired to the anticodon of initiator tRNA. (iv) Displacement of factors from the 48S complex and joining of the 60S subunit to form an 80S ribosome, leaving Met-tRNA i in the ribosomal P site. Research in o...