A 110-kilodalton subunit of translation initiation factor eIF3 and an associated 135-kilodalton protein are encoded by the Saccharomyces cerevisiae TIF32 and TIF31 genes

Translation initiation factor eIF3 is a multisubunit protein complex requir ed for initiation of protein biosynthesis in eukaryotic cells. The complex promotes ribosome dissociation, the binding of the initiator methionyl-tRNA to the 40 S ribosomal subunit, and mRNA recruitment to the ribosome. In th e yeast Saccharomyces cerevisiae eIF3 comprises up to 8 subunits. Using par tial peptide sequences generated from proteins in purified eIF3, we cloned the TIF31 and TIF32 genes encoding 135- (p135) and 110-kDa (p110) proteins. Deletion/disruption of TIF31 results in no change in growth rate, whereas deletion of TIF32 is lethal. Depletion of p110 causes a severe reduction in cell growth and protein synthesis rates as well as runoff of ribosomes fro m polysomes, indicative of inhibition of the initiation phase. In addition, p110 depletion leads to p90 co-depletion, whereas other eIF3 subunit level s are not affected. Immunoprecipitation or nickel affinity chromatography f rom strains expressing (His)(6)-tagged p110 or p33 results in the co-purifi cation of the well characterized p39 and p90 subunits of eIF3 as well as p1 10 and p33, This establishes p110 as an authentic subunit of eIF3. In simil ar experiments, p135 and other eIF3 subunits sometimes, but not always, co- purify, making assignment of p135 as an eIF3 subunit uncertain. Far Western blotting and two-hybrid analyses detect a direct interaction of p110 with p90, p135 with p33, and p33 with eIF4B. Our results, together with those fr om other laboratories, complete the cloning and characterization of all of the yeast eIF3 subunits.