Differential factor requirement to assemble translation initiation complexes at the alternative start codons of foot-and-mouth disease virus RNA

The foot-and-mouth disease virus (FMDV) RNA contains two in-frame AUG codons separated by 84 nt that direct translation initiation of the viral polyprotein. The mechanism of initiation at the IRES-proximal AUG codon (AUG1) has been previously analyzed, whereas no data on factor requirements for AUG2 have been reported. Here, using the method of 48S translation initiation complex reconstitution, we show that eIF1 is indispensable in forming the 48S initiation complex at AUG2. In contrast, it reduces the assembly of this complex at AUG1. Stabilization of a stem-loop between the initiation triplets induces a small decrease in the toeprint intensity at AUG2, accompanied by an increase in the AUG1/AUG2 ratio as well as a moderate reduction of protein synthesis initiated at AUG2 in transfected cells. PTB and ITAF45 exerted an additive positive effect on the 48S complex at AUG2, although a substantial reconstitution on both AUGs occurs on omission of either of these proteins. Relative to the beta-globin mRNA, the 48S complex formation at AUG1 and AUG2 is slow and occurs with the same kinetics as revealed by the "kinetic" toeprint assay. Mutation of AUG1 to AUA does not abrogate protein synthesis in transfected cells, and has no effect on the rate of the 48S complex formation at AUG2. We conclude that the AUG2 initiation region is selected independently of 48S complex formation at the upstream AUG1. The kinetic toeprint assay also shows that cap-dependent assembly of the 48S complex in vitro occurs faster than the FMDV IRES-mediated complex assembly.

A multifactor complex of eukaryotic initiation factors, eIF1, eIF2, eIF3, eIF5, and initiator tRNA(Met) is an important translation initiation intermediate in vivo

Translation initiation factor 2 (eIF2) bound to GTP transfers the initiator methionyl tRNA to the 40S ribosomal subunit. The eIF5 stimulates GTP hydrolysis by the eIF2/GTP/Met-tRNA(i)(Met) ternary complex on base-pairing between Met-tRNA(i)(Met) and the start codon. The eIF2, eIF5, and eIF1 all have been implicated in stringent selection of AUG as the start codon. The eIF3 binds to the 40S ribosome and promotes recruitment of the ternary complex; however, physical contact between eIF3 and eIF2 has not been observed. We show that yeast eIF5 can bridge interaction in vitro between eIF3 and eIF2 by binding simultaneously to the amino terminus of eIF3 subunit NIP1 and the amino-terminal half of eIF2beta, dependent on a conserved bipartite motif in the carboxyl terminus of eIF5. Additionally, the amino terminus of NIP1 can bind concurrently to eIF5 and eIF1. These findings suggest the occurrence of an eIF3/eIF1/eIF5/eIF2 multifactor complex, which was observed in cell extracts free of 40S ribosomes and found to contain stoichiometric amounts of tRNA(i)(Met). The multifactor complex was disrupted by the tif5-7A mutation in the bipartite motif of eIF5. Importantly, the tif5-7A mutant is temperature sensitive and displayed a substantial reduction in translation initiation at the restrictive temperature. We propose that the multifactor complex is an important intermediate in translation initiation in vivo.

GTP hydrolysis controls stringent selection of the AUG start codon during translation initiation in Saccharomyces cerevisiae

We have isolated and characterized two suppressor genes, SUI4 and SUI5, that can initiate translation in the absence of an AUG start codon at the HIS4 locus in Saccharomyces cerevisiae. Both suppressor genes are dominant in diploid cells and lethal in haploid cells. The SUI4 suppressor gene is identical to the GCD11 gene, which encodes the gamma subunit of the eIF-2 complex and contains a mutation in the G2 motif, one of the four signature motifs that characterizes this subunit to be a G-protein. The SUI5 suppressor gene is identical to the TIF5 gene that encodes eIF-5, a translation initiation factor known to stimulate the hydrolysis of GTP bound to eIF-2 as part of the 43S preinitiation complex. Purified mutant eIF-5 is more active in stimulating GTP hydrolysis in vitro than wild-type eIF-5, suggesting that an alteration of the hydrolysis rate of GTP bound to the 43S preinitiation complex during ribosomal scanning allows translation initiation at a non-AUG codon. Purified mutant eIF-2gamma complex is defective in ternary complex formation and this defect correlates with a higher rate of dissociation from charged initiator-tRNA in the absence of GTP hydrolysis. Biochemical characterization of SUI3 suppressor alleles that encode mutant forms of the beta subunit of eIF-2 revealed that these mutant eIF-2 complexes have a higher intrinsic rate of GTP hydrolysis, which is eIF-5 independent. All of these biochemical defects result in initiation at a UUG codon at the his4 gene in yeast. These studies in light of other analyses indicate that GTP hydrolysis that leads to dissociation of eIF-2 x GDP from the initiator-tRNA in the 43S preinitiation complex serves as a checkpoint for a 3-bp codon/anticodon interaction between the AUG start codon and the initiator-tRNA during the ribosomal scanning process.