Activation by cyclic GMP binding causes an apparent conformational change in cGMP-dependent protein kinase

Cyclic nucleotide binding activates cyclic nucleotide-dependent protein kinases, but the molecular mechanism is unknown. In the present studies, cGMP binding to type Ialpha or type Ibeta cGMP-dependent protein kinase (PKG) caused (i) a large electronegative charge shift of each enzyme on ion exchange chromatography, (ii) an increase in the Stokes radius (>3 A) of each enzyme, and (iii) a decreased mobility of type Ibeta PKG on native gel electrophoresis. These physical changes were not detected in the monomeric form of type Ibeta PKG upon activation by cGMP. However, the results of partial proteolysis of type Ialpha PKG revealed some degree of cGMP-induced conformational change within the PKG-monomer, since cGMP binding protects the PKG-monomer against chymotryptic cleavage. The altered sensitivity to proteolysis occurs at Met-200, which is located between the B and C alpha-helices in the high affinity site (site A), and implies that the cGMP-induced structural perturbations in this region may participate in activation of dimeric PKG. The cGMP-induced conformational effects observed using the physical separation methods are likely to reflect altered interactions within the dimeric PKG that are caused by structural alterations within the subunits.