Related elF3 subunits TIF32 and HCR1 interact with an RNA recognition motif in PRT1 required for elF3 integrity and ribosome binding

eIF3 binds to 40S ribosomal subunits and stimulates recruitment of Met-tRNA (i)(Met) and mRNA to the preinitiation complex. Saccharomyces cerevisiae co ntains an ortholog of human eIF3 subunit p35, HCR1, whose interactions with yeast eIF3 are not well defined, We found that HCR1 has a dual function in translation initiation: it binds to, and stabilizes, the eIF3-eIF5-eIF1-eI F2 multifactor complex and is required for the normal level of 40S ribosome s, The RNA recognition motif (RRM) of eIF3 subunit PRT1 interacted simultan eously with HCR1 and with an internal domain of eIF3 subunit TIF32 that has sequence and functional similarity to HCR1, PRT1, HCR1 and TIF32 were also functionally linked by genetic suppressor analysis. We propose that HCR1, stabilizes or modulates interaction between TIF32 and the PRT1 RRM, Removal of the PRT1 RRM resulted in dissociation of TIF32, NIP1, HCR1 and eIF5 fro m eIF3 in vivo, and destroyed 40S ribosome binding by the residual PRT1-TIF 34-TLF35 subcomplex, Hence, the PRT1 RRM is crucial for the integrity and r ibosome-binding activity of eIF3.