Molecular and Cellular Biology, December 2001, p. 8671-8683, Vol. 21, No. 24
0270-7306/01/$04.00+0 DOI: 10.1128/MCB.21.24.8671-8683.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Department of Biochemistry, The Hebrew University-Hadassah Medical School, Jerusalem 91120, Israel1; Cell Signaling Technology, Beverly, Massachusetts 019152; Department of Pathology, University of Virginia Medical School, Charlottesville, Virginia 229083; and Diabetes Unit, Medical Services, and Department of Molecular Biology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 021144
Received 20 July 2001/Returned for modification 10 September 2001/Accepted 24 September 2001
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 the activity of S6K1. We now show that the translation of TOP mRNAs is rapidly repressed by amino acid withdrawal and that this nutritional control depends strictly on the integrity of the 5'TOP motif. However, neither phosphorylation of ribosomal protein (rp) S6 nor activation of S6K1 per se is sufficient to relieve the translational repression of TOP mRNAs in amino acid-starved 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. Furthermore, TOP mRNAs were translationally regulated by amino acid sufficiency in embryonic stem cells lacking both alleles of the S6K1 gene. Inhibition of mTOR by rapamycin led to fast and complete repression of S6K1, as judged by rpS6 phosphorylation, but to only partial and delayed repression of translational activation of TOP mRNAs. In contrast, interference in the phosphatidylinositol 3-kinase (PI3-kinase)-mediated pathway by chemical or genetic manipulations blocked rapidly and completely the translational activation of TOP mRNAs. It appears, therefore, that translational regulation of TOP mRNAs, at least by amino acids, (i) is fully dependent on PI3-kinase, (ii) is partially sensitive to rapamycin, and (iii) requires neither S6K1 activity nor rpS6 phosphorylation.
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