Coupled release of eukaryotic translation initiation factors 5B and 1A from 80S ribosomes following subunit joining

The translation initiation GTPase eukaryotic translation initiation factor 513 (eIF5B) binds to the factor eIFIA and catalyzes ribosomal subunit joining in vitro. We show that rapid depletion of eIF5B in Saccharomyces cerevisiae results in the accumulation of eIFIA and mRNA on 40S subunits in vivo, consistent with a defect in subunit joining. Substituting Ala for the last five residues in eIF1A (eIFIA-5A) impairs eIF5B binding to eIF1A in cell extracts and to 40S complexes in vivo. Consistently, overexpression of eIF5B suppresses the growth and translation initiation defects in yeast expressing eIF1A-5A, indicating that eIF1A helps recruit eIF5B to the 40S subunit prior to subunit joining. The GTPase-deficient eIF5B-T439A mutant accumulated on 80S complexes in vivo and was retained along with eIF1A on 80S complexes formed in vitro. Likewise, eIF5B and eIFIA remained associated with 80S complexes formed in the presence of nonhydrolyzable GDPNP, whereas these factors were released from the 80S complexes in assays containing GTP. We propose that eIFIA facilitates the binding of eIF5B to the 40S subunit to promote subunit joining. Following 80S complex formation, GTP hydrolysis by eIF5B enables the release of both eIF5B and eIF1A, and the ribosome enters the elongation phase of protein synthesis.