Studies on the phosphorylation of the type I cAMP-dependent protein kinase

The regulatory subunit of the type I cAMP-dependent protein kinase (RI) can be separated into multiple forms on isoelectric focusing gels. RI from bovine skeletal muscle gives rise to bands at pI = 5.57 and 5.45. Phosphate determinations indicate that the more acidic band contains protein-bound phosphate. RI from rat skeletal muscle can be separated into three bands of pI = 5.57, 5.45, and 5.35. The two acidic forms comigrate with labeled RI isolated from rat soleus muscles that were incubated with [32P]orthophosphate. RI from bovine muscle is isolated mainly in the unphosphorylated state while that from rat muscle is primarily phosphorylated. At lest 4 mol of phosphate can be incorporated into each RI dimer following extensive phosphorylation by cGMP-dependent protein kinase in vitro. Two phosphopeptides are observed on polyacrylamide gels following partial proteolysis of in vitro phosphorylated rat RI. One of these peptides is also observed following proteolysis of rat RI phosphorylated in intact soleus muscles.