Phosphorylation of the regulatory subunit of type I cyclic AMP-dependent protein kinase by its catalytic subunit.

The regulatory subunit (R) of Type I cAMP-dependent protein kinase from rabbit skeletal muscle can serve as a substrate for its catalytic subunit (C). The degree of phosphorylation depends on both the concentration of C and the the time of incubation. Moreover, the phosphorylation can be totally blocked by protein kinase inhibitors. In contrast, cAMP stimulates the phosphorylation of R using the holoenzyme. The purified holoenzyme isolated from rabbit skeletal muscle can be further fractionated into two fractions on DEAE Sephadex column. The first fraction eluted with low salt (50 mM NaCl) contains a much lower concentration of kinases than the second fraction eluted with high salt (100 mM NaCl), but the low salt kinase can be readily phosphorylated in the presence of MgATP. Our data thus implies that only a small fraction of Type I cAMP-dependent protein kinases in the skeletal muscle is present as the phosphorylatable species.