Translation initiation factor eIF3 is a multisubunit protein complex required 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 the yeast Saccharomyces cerevisiae eIF3 comprises up to 8 subunits. Using partial 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 from polysomes, indicative of inhibition of the initiation phase. In addition,
p110 depletion leads to
p90 co-depletion, whereas other eIF3 subunit levels are not affected. Immunoprecipitation or nickel affinity chromatography from strains expressing (His)6-tagged
p110 or
p33 results in the co-purification of the well characterized
p39 and
p90 subunits of eIF3 as well as
p110 and
p33. This establishes
p110 as an authentic subunit of eIF3. In similar 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 from other laboratories, complete the cloning and characterization of all of the yeast eIF3 subunits.