Phosphorylation of cAMP-dependent protein kinase subunits

The cAMP-dependent protein kinases comprise two enzyme forms designated as type I and type II. The type II enzyme can catalyze an autophosphorylation reaction whereby phosphate is transferred from ATP to one seryl residue on each regulatory subunit monomer. Since this reaction can occur in the absence of cAMP-induced enzyme dissociation, it has been used as a probe to identify one site of interaction between the catalytic subunit (C) and the type II regulatory subunit (R11). The type I cAMP-dependent protein kinase does not catalyze an analogous reaction; however, if cGMP-dependent protein kinase is substituted for C, the type I regulatory subunit (R1) becomes phosphorylated. The effects of cyclic nucleotides on this reaction, coupled with the ability of R1 to serve as an inhibitor of cGMP-dependent protein kinase suggest that this phosphorylation also occurs within an important functional domain on R1. A comparison of the autophosphorylation site on R11 with the cGMP-dependent protein kinase catalyzed phosphorylation site on R1 indicates that each modification takes place within a similar proteolytically sensitive region. On each subunit, this sensitive "hinge" region lies distal to the functional domain responsible for regulatory subunit dimerization and proximal to that responsible for cAMP binding. Phosphorylation of the "hinge" region decreases the affinity of each regulatory subunit for C, although the magnitude of this change appears greater for R1 than for R11. Phosphorylation of R1 also reduces the stoichiometry of cAMP binding from two to one mole of cAMP bound per mole of R1 monomer. These results suggest that the "hinge" regions of both R1 and R11 form part of the interaction site between the regulatory subunit and C; and, in the case of R1, it also forms a portion of one of two cAMP-binding sites. The amino acid sequence surrounding the phosphorylated serine of each regulatory subunit has been determined: R11: D-R-R-V-S(P)-V R1: R-R-R-R-G-A-I-S(P)-A It is thought that the number and position of the basic amino acid residues proximal to the modified serine may be responsible, in part, for determining the susceptibility of each site to phosphorylation by cAMP or cGMP-dependent protein kinase. Both R1 and R11 exist as phosphoproteins in vivo. Dephosphorylation of purified "native" phospho-R1 is without effect on the ability of R1 to interact with either C or cAMP. The site phosphorylated in vivo is therefore distinct from that modified in vitro by cGMP-dependent protein kinase. In addition to the autophosphorylation site, R11 possesses a second, less enzymatically reactive, phosphorylation site that is modified in vivo. Dephosphorylation of this site is also without apparent effect on the functional properties of R11. The kinases responsible for catalyzing the phosphorylation of R1 and the cryptic site on R11 and the role that these modifications play in modulating kinase activity are currently unknown but are under active investigation.