Protein Kinase Modulator from Lobster Tail Muscle

A heat-stable protein which modulates the activity of cyclic nucleotide-dependent protein kinases was purified from lobster tail muscle, and its properties were studied. The modulator is similar in several respects to the inhibitory factor reported by others (Appleman et al. (1969) Arch. Biochem. Biophys. 116, 39; Walsh et al. (1971) J. Biol. Chem. 246, 1977) to be present in rat and rabbit skeletal muscle. Polyacrylamide gel electrophoresis suggested the presence of aggregate forms of the modulator, each of which possessed both stimulatory and inhibitory activities. A minimum molecular weight of 34,000 for the modulator was estimated by using either sodium dodecyl sulfate-polyacrylamide gel electrophoresis or amino acid analysis. The protein kinase modulator was shown to be a highly charged, acidic protein containing a low level of aromatic amino acid residues. p-Chloromercuribenzoate-treatment of the protein kinase modulator from lobster tail muscle caused a dramatic change in its actions on adenosine 3',5'-monophosphate (cyclic AMP)-dependent but not on guanosine 3',5'-monophosphate (cyclic GMP)-dependent protein kinase. Modulator activity was demonstrated in preparations from a wide variety of sources. However, the various preparations demonstrated considerable differences in the ratio of stimulatory to inhibitory activity. The modulator acts to alter the substrate specificity both of cyclic GMP-dependent and cyclic AMP-dependent protein kinases, increasing the phosphorylation of some protein substrates and decreasing that of others. It is possible that the modulator may function in vivo by regulating the activity of cyclic GMP-dependent and cyclic AMP-dependent protein kinases through modification of their substrate specificity.