RAPAMYCIN SUPPRESSES 5'TOP MESSENGER-RNA TRANSLATION THROUGH INHIBITION OF P70(S6K)

Treatment of mammalian cells with the immunosuppressant rapamycin, a b acterial macrolide, selectively suppresses mitogen-induced translation of an essential class of mRNAs which contain an oligopyrimidine tract at their transcriptional start (5'TOP), most notably mRNAs encoding r ibosomal proteins and elongation factors. In parallel, rapamycin block s mitogen-induced p70 ribosomal protein S6 kinase (p70(s6k)) phosphory lation and activation. Utilizing chimeric mRNA constructs containing e ither a wildtype or disrupted 5'TOP, we demonstrate that an intact pol ypyrimidine tract is required for rapamycin to elicit an inhibitory ef fect on the translation of these transcripts. In turn, a dominant-inte rfering p70(s6k), which selectively prevents p70(s6k) activation by bl ocking phosphorylation of the rapamycin-sensitive sites, suppresses th e translation of the chimeric mRNA containing the wild-type but not th e disrupted 5'TOP. Conversion of the principal rapamycin-sensitive p70 (s6k) phosphorylation site, T389, to an acidic residue confers rapamyc in resistance on the kinase and negates the inhibitory effects of the macrolide on 5'TOP mRNA translation in cells expressing this mutant. T he results demonstrate that the rapamycin block of mitogen induced 5'T OP mRNA translation is mediated through inhibition of p70(s6k) activat ion.