TARGETED DISRUPTION OF P70(S6K) DEFINES ITS ROLE IN PROTEIN-SYNTHESISAND RAPAMYCIN SENSITIVITY

Here, we disrupted the p70 S6 kinase (p70(s6k)) gene in murine embryon ic stem cells to determine the role of this kinase in cell growth, pro tein synthesis, and rapamycin sensitivity. p70(s6k-/-) cells prolifera ted at a slower rate than parental cells, suggesting that p70(s6k) has positive influence on cell proliferation but is not essential. In add ition, rapamycin inhibited proliferation of p70(s6k-/-) cells, indicat ing that other events inhibited by the drug, independent of p70(s6k), also are important for both cell proliferation and the action of rapam ycin. In p70(s6k-/-) cells, which exhibited no ribosomal S6 phosphoryl ation, translation of mRNA encoding ribosomal proteins was not increas ed by serum nor specifically inhibited by rapamycin. In contrast, rapa mycin inhibited phosphorylation of initiation factor 4E-binding protei n 1 (4E-BP1), general mRNA translation, and overall protein synthesis in p70(s6k-/-) cells, indicating that these events proceed independent ly of p70(s6k) activity. This study localizes the function of p70(s6k) to ribosomal biogenesis by regulating ribosomal protein synthesis at the level of mRNA translation.