Guanosine 3':5'-monophosphate-dependent protein kinase from silkworm, properties of a catalytic fragment obtained by limited proteolysis.

Although guanosine 3':5'-monophosphate (cyclic GMP)-dependent protein kinase (protein kinase G) which was partially purified from silkworm pupae was not dissociated by cyclic GMP into catalytic and regulatory subunits as described for adenosine 3':5'-monophosphate-dependent protein kinase (protein kinase A) (Takai, Y., Nakaya, S., Inoue, M., Kishimoto, A., Nishiyama, K., Yamamura, H., and Nishizuka, Y. (1976) J. Biol. Chem. 251, 1481-1487), limited proteolysis with trypsin resulted in the formation of catalytic and cyclic GMP-binding fragments which showed molecular weights of approximately 3.4 X 10(4) and 3.6 X 10(4), respectively (the molecular weight of native protein kinase G was 1.4 X 10(5)). The catalytic fragment did not bind cyclic GMP and was fully active in the absence of the cyclic nucleotide. The fragment did not show an absolute requirement for a sulfhydryl compound and high concentrations of Mg2+ (50 to 100 mM), both of which were necessary for the maximal activation of native protein kinase G. The catalytic fragment was not inhibited by the cyclic GMP-binding fragment nor by the regulatory subunit of protein kinase A. Inversely, the cyclic GMP-binding fragment was unable to inhibit the catalytic subunit of protein kinase A. Protein inhibitor, which was described for protein kinase A, was inert for the catalytic fragment.