Differential phosphorylation of ribosomal protein S6 in isolated rat hepatocytes after incubation with insulin and glucagon.

Glucagon and insulin both stimulated the 32P-labelling of ribosomal protein S6 in rat hepatocytes that had been incubated with 32Pi. Glucagon selectively enhanced the labelling of the tryptic peptide phosphorylated by cyclic AMP-dependent protein kinase, demonstrating that 6 S is a physiological substrate for this enzyme. Insulin stimulated the phosphorylation of distinct tryptic peptides, at least one of which appears to be very close in the primary structure to the sites phosphorylated by cyclic AMP-dependent protein kinase.