Amino acid-induced translation of TOP mRNAs is fully dependent on phosphatidylinositol 3-kinase-mediated signaling, is partially inhibited by rapamycin and is independent of S6K1 and rpS6 phosphorylation

Vertebrate TOP mRNAs contain an oligopyrimidine tract at their 5' termini ( 5'TOP) and encode components of the translational machinery. Previously it has been shown that they are subject to selective translational repression upon growth arrest and that their translational behavior correlates with th e activity of S6K1. We now show that the translation of TOP mRNAs is rapidl y repressed by amino acid withdrawal and that this nutritional control depe nds strictly on the integrity of the 5'TOP motif. However, neither phosphor ylation of ribosomal protein (rp) S6 nor activation of S6K1 per se is suffi cient to relieve the translational repression of TOP mRNAs in amino acid-st arved cells. Likewise, inhibition of S6K1 activity and rpS6 phosphorylation by overexpression of dominant-negative S6K1 mutants failed to suppress the translational activation of TOP mRNAs in amino acid-refed cells. Furthermo re, TOP mRNAs were translationally regulated by amino acid sufficiency in e mbryonic stem cells lacking both alleles of the S6K1 gene. Inhibition of mT OR by rapamycin led to fast and complete repression of S6K1, as judged by r pS6 phosphorylation, but to only partial and delayed repression of translat ional activation of TOP mRNAs. In contrast, interference in the phosphatidy linositol 3-kinase (P13-kinase) -mediated pathway by chemical or genetic ma nipulations blocked rapidly and completely the translational activation of TOP mRNAs. It appears, therefore, that translational regulation of TOP mRNA s, at least by amino acids, (i) is fully dependent on P13-kinase, (ii) is p artially sensitive to rapamycin, and (iii) requires neither S6K1 activity n or rpS6 phosphorylation.