Abstract

Treatment of mammalian cells with the immunosuppressant rapamycin, a bacterial 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 ribosomal proteins and elongation factors. In parallel, rapamycin blocks mitogen-induced p70 ribosomal protein S6 kinase (p70s6k) phosphorylation and activation. Utilizing chimeric mRNA constructs containing either a wild-type or disrupted 5'TOP, we demonstrate that an intact polypyrimidine tract is required for rapamycin to elicit an inhibitory effect on the translation of these transcripts. In turn, a dominant-interfering p70s6k, which selectively prevents p70s6k activation by blocking phosphorylation of the rapamycin-sensitive sites, suppresses the translation of the chimeric mRNA containing the wild-type but not the disrupted 5'TOP. Conversion of the principal rapamycin-sensitive p70s6k phosphorylation site, T389, to an acidic residue confers rapamycin resistance on the kinase and negates the inhibitory effects of the macrolide on 5'TOP mRNA translation in cells expressing this mutant. The results demonstrate that the rapamycin block of mitogen-induced 5'TOP mRNA translation is mediated through inhibition of p70s6k activation.