PHOSPHORYLATION OF ELONGATION-FACTOR-1 AND RIBOSOMAL-PROTEIN-S6 BY MULTIPOTENTIAL-S6-KINASE AND INSULIN STIMULATION OF TRANSLATIONAL ELONGATION

Stimulation of protein synthesis in response to insulin is concomitant with increased phosphorylation of initiation factors 4B and 4G and ri bosomal protein S6 (Morley, S. J., and Traugh, J. A. (1993) Biochimie 75, 985-989) and is due at least in part to multipotential S6 kinase. When elongation factor 1 (EF-1) from rabbit reticulocytes was examined as substrate for multipotential S6 kinase, up to 1 mol/mol of phospha te was incorporated into the alpha, beta, and delta subunits. Phosphor ylation of EF-1 resulted in a 2-2.6-fold stimulation of EF-1 activity, as measured by poly(U)-directed polyphenylalanine synthesis. The rate of elongation was also stimulated by approximately a-fold with 80 S r ibosomes phosphorylated on S6 by multipotential S6 kinase. When the ra tes of elongation in extracts from serum-fed 3T3-L1 cells and cells se rum-deprived for 1.5 h were compared, a 40% decrease was observed upon serum deprivation. The addition of insulin to serum-deprived cells fo r 15 min stimulated elongation to a rate equivalent to that of serum-f ed cells. Similar results were obtained with partially purified EF-1, with both EF-1 and ribosomes contributing to stimulation of elongation . These data are consistent with a ribosomal transit time of 3.2 min f or serum-deprived cells and 1.6 min following the addition of insulin for 15 min. Taken together, the data suggest that insulin stimulation involves coordinate regulation of EF-1 and ribosomes through phosphory lation by multipotential S6 kinase.