2 DISTINCT ENZYMES CONTRIBUTE TO BIPHASIC S6 PHOSPHORYLATION IN SERUM-STIMULATED CHICKEN-EMBRYO FIBROBLASTS

Serum stimulation of quiescent chicken embryo fibroblasts resulted in a time-dependent, biphasic activation of S6 kinase activity. Chromatographic fractionation of serum-stimulated cell lysates resolved two distinct S6 kinase activities. Anti-Xenopus S6 kinase II antiserum immunoprecipitated a 90,000-M(r) S6 kinase but did not cross-react with a smaller, 65,000-M(r) S6 kinase. Phosphopeptide analysis confirmed that the 90,000- and 65,000-M(r) proteins were structurally unrelated and established that the 65,000-M(r) protein isolated by the current protocol was the same serum-stimulated chicken embryo fibroblast S6 kinase as that previously characterized (J. Blenis, C.J. Kuo, and R.L. Erikson, J. Biol. Chem. 262: 14373-14376, 1987). These results demonstrate the contribution of two distinct S6 kinases to total serum-stimulated ribosomal protein S6 phosphorylation.