A new pathway of translational regulation mediated by eukaryotic initiation factor 3

We report a new pathway of translation regulation that may operate in inter feron-treated or virus-infected mammalian cells. This pathway is activated by P56, a protein whose synthesis is strongly induced by interferons or dou ble-stranded RNA. Using a yeast two-hybrid screen, we identified the P48 su bunit of the mammalian translation initiation factor eIF-3 as a protein tha t interacts with P56, The P56-P48 interaction was confirmed in human cells by co-immunoprecipitation assays and confocal microscopy, Gel filtration as says revealed that P56 binds to the large eIF-3 complex that contains P48. Purified recombinant P56 inhibited in vitro translation of reporter mRNAs i n a dose-dependent fashion, and that inhibition was reversed by the additio n of purified eIF-3. In vivo, expression of transfected P56 or induction of the endogenous P56 by interferon caused an inhibition of overall cellular protein synthesis and the synthesis of a transfected reporter protein. As e xpected, a P56 mutant that does not interact with P48 and eIF-3 failed to i nhibit protein synthesis in vitro and in vivo.