CONSERVATION AND DIVERSITY OF EUKARYOTIC TRANSLATION INITIATION-FACTOR EIF3

The largest of the mammalian translation initiation factors, eIF3, con sists of at least eight subunits ranging in mass from 35 to 170 kDa. e IF3 binds to the 40 S ribosome in an early step of translation initiat ion and promotes the binding of methionyl tRNA(i) and mRNA. We report the cloning and characterization of human cDNAs encoding two of its su bunits, p110 and p36. It was found that the second slowest band during polyacrylamide gel electrophresis of eIF3 subunits in sodium dodecyl sulfate contains two proteins: p110 and p116. Analysis of the cloned c DNA encoding p110 indicates that its amino acid sequence is 31% identi cal to that of the yeast protein, Nip1. The p116 cDNA was cloned and c haracterized as a human homolog of yeast Prt1, as described elsewhere (Methot, N., Rom, E., Olsen, H., and Sonenberg, N. (1997) J. Biol. Che m. 272, 1110-1116). p36 is a WD40 repeat protein, which is 46% identic al to the p39 subunit of yeast eIF3 and is identical to TRIP-1, a phos phorylation substrate of the TGF-beta type II receptor. The p116, p110 , and p36 subunits localize on 40 S ribosomes in cells active in trans lation and co-immunoprecipitate with affinity-purified antibodies agai nst the p170 subunit, showing that these proteins are integral compone nts of eIF3. Although p36 and p116 have homologous protein subunits in yeast eIF3, the p110 homolog, Nip1, is not detected in yeast eIF3 pre parations. The results indicate both conservation and diversity in eIF 3 between yeast and humans.