Autophosphorylation of type Ibeta cGMP-dependent protein kinase increases basal catalytic activity and enhances allosteric activation by cGMP or cAMP.

Autophosphorylation of purified bovine Ibeta isozyme of cGMP-dependent protein kinase (Ibeta cGK) in the presence of cGMP or cAMP increased basal kinase activity (-cGMP) as much as 4-fold and reduced the Ka for both cGMP and cAMP; maximum catalytic activity (+cGMP) was not altered. Autophosphorylation proceeded with at least two rate components. The faster rate correlated with phosphorylation of Ser-63. The slower rate, as well as the increase in basal kinase activity and decrease in Ka for cyclic nucleotides, correlated with phosphorylation of Ser-79. Autophosphorylation of either residue was an intramolecular reaction. Autophosphorylation of a proteolytically generated Ibeta cGK monomer lacking amino-terminal residues 1-64 increased basal activity (3-fold) and decreased Ka for cAMP (15-fold). This indicated that autophosphorylation of Ser-79 did not require dimeric cGK and that the phosphorylation of Ser-79 in the monomer was sufficient to alter enzymatic characteristics of Ibeta cGK. These studies suggested that increases in intracellular cGMP or cAMP could result in autophosphorylation of Ibeta cGK, which would increase basal kinase activity as well as the sensitivity of cGK to activation by cGMP or to cross-activation by cAMP. Autophosphorylation could also prolong the increased kinase activity after decline of the second messenger.