Differential kinetics of changes in the state of phosphorylation of ribosomal protein S6 and in the rate of protein synthesis in MPC 11 cells during tonicity shifts

Mouse myeloma (MPC 11) cells respond rapidly to hypertonic conditions by shutting down protein synthesis at the level of polypeptide chain initiation. Translational activity recovers equally quickly upon a return to isotonicity. Disaggregation and reformation of polysomes occur in parallel to the changes in protein synthesis. Ribosomal protein S6 becomes dephosphorylated under hypertonic conditions and rephosphorylated when isotonic conditions are restored. The kinetics with which these changes occur are, however, too slow to account for the changes in protein synthesis. Treatment of the cells with a low concentration of cycloheximide allows reformation of polysomes under hypertonic conditions; conversely, puromycin prevents the restoration of polysomes which otherwise occurs on return to isotonicity. Neither inhibitor prevents the changes in S6 phosphorylation resulting from the tonicity shifts. We conclude that the overall extent of phosphorylation of S6 neither regulates nor is determined by the rate of protein synthesis and is not obligatorily related to the proportion of ribosomes in polysomes.