The effect of insulin and guanosine nucleotides on protein phosphorylations by sarcolemma membranes from skeletal muscle.

In a previous report we have shown that insulin increases the phosphorylation of an endogenous protein of mol. wt. 16 000 daltons in sarcolemma membranes. In the present work we have demonstrated that phosphorylations of exogenous histones by the sarcolemma membranes are also increased by insulin. These results indicate that insulin activates a cyclic-AMP-independent protein kinase in sarcolemma membranes. The stimulatory effect of insulin on protein phosphorylations is increased by GTP and its analogue GMP-P(NH)P. The insulin effect was increased 3--4-fold by micromolar concentrations of GTP. The effect by the analogue GMP-P(NH)P was somewhat less. In the absence of insulin guanosine nucleotides had no effect on phosphorylation of the proteins. The results suggest that GTP is a modulator in the activation of a sarcolemma membrane protein kinase by insulin.