Exogenous substrate stimulates autodephosphorylation of cyclic-AMP-dependent protein kinase II.

The autophosphorylated regulatory subunit (32P-RII) of cyclic-AMP-dependent protein kinase II was efficiently dephosphorylated by its C subunit in the absence of added ADP, provided that Mg/ATP and a standard protein kinase peptide substrate were present. This raises the possibility that autodephosphorylation could be significant in the intact cell. Only the cyclic-AMP-complexed free form of 32P-RII was efficiently dephosphorylated, indicating that the autodephosphorylation was intermolecular. Autodephosphorylation of 32P-RII in the presence of MgATP and kemptide occurred with formation of [gamma-32P]ATP, suggesting transfer of 32P of phospho-RII to a transient C*(MgADP) complex formed during the forward kinase reaction with peptide as substrate. Autodephosphorylation promoted by phosphorylation of exogenous substrates could operate also for other kinases conforming to a mechanism where MgADP remains bound to the active site after the other product (phosphorylated substrate) has left the catalytic complex.