NIP1P ASSOCIATES WITH 40 S RIBOSOMES AND THE PRT1P SUBUNIT OF EUKARYOTIC INITIATION-FACTOR 3 AND IS REQUIRED FOR EFFICIENT TRANSLATION INITIATION

Nip1p is an essential Saccharomyces cerevisiae protein that was identi fied in a screen for temperature conditional (ts) mutants exhibiting d efects in nuclear transport. New results indicate that Nip1p has a pri mary role in translation initiation. Polysome profiles indicate that c ells depleted of Nip1p and nip1-1 cells are defective in translation i nitiation, a conclusion that is supported by a reduced rate of protein synthesis in Nip1p-depleted cells. Nip1p cosediments with free 40 S r ibosomal subunits and polysomal preinitiation complexes, but not with free or elongating 80 S ribosomes or 60 S subunits. Nip1p can be isola ted in an about 670-kDa complex containing polyhistidine-tagged Prt1p, a subunit of translation initiation factor 3, by binding to Ni2+-NTA- agarose beads in a manner completely dependent on the tagged form of P rt1p. The nip1-1 ts growth defect was suppressed by the deletion of th e ribosomal protein, RPL46. Also, nip1-1 mutant cells are hypersensiti ve to paromomycin. These results suggest that Nip1p is a subunit of eu karyotic initiation factor 3 required for efficient translation initia tion.