Phosphorylation of Ribosomal Protein S6

The time course of S6 phosphorylation and several aspects of protein synthesis have been studied in suspension cultures of HeLa cells, following transfer to fresh medium and serum. The phosphorylation of S6 is not temporally correlated with changes in polypeptide initiation and elongation rates, as judged from polysome profiles. Phosphorylation of S6 can be maximal within 30 min after transfer; elongation and initiation rates increase coordinately and more slowly, becoming maximal about 6 h after transfer, a time at which the net phosphorylation of protein S6 is greatly reduced or negligible. Recruitment of messenger RNA into polysomes is another response to fresh medium and serum; this response occurs almost as rapidly as the phosphorylation of S6. We suggest that the phosphorylation of S6 may play a role in messenger RNA recruitment.