Calmodulin-dependent protein kinase and associated substrates in Torpedo electric organ

Calcium plus calmodulin (Ca2+/CaM)-dependent protein kinase activity was demonstrated in subcellular fractions from Torpedo californica electric organ. A protein kinase activity dependent on Ca2+/CaM was purified about 200-fold from electric organ cytosol using DEAE-cellulose and CaM-affinity chromatography. The most effective exogenous substrates for this enzyme were the synapse-specific protein Synapsin I (Protein I) and histone f3. Phosphorylase b, skeletal muscle myosin light chains, casein, phosvitin, histone f2b, and G-substrate were relatively poorly phosphorylated by Torpedo CaM-protein kinase. Thus, the enzyme differs in its substrate specificity from known cyclic nucleotide-dependent protein kinases, myosin light chain kinase and phosphorylase kinase. The Km for ATP was 15-20 microM; for Synapsin I, 0.8 microM; and for CaM, 85 nM. Two major endogenous substrates (Mr = 62,000 and 54,000) for CaM-protein kinase co-purified with the enzyme through the CaM-affinity column step. These two substrates, as well as the enzyme, were present in other subcellular fractions in addition to the cytosol, including crude membranes and purified synaptic vesicles. A third major substrate (Mr = 39,000) could be separated from the enzyme during purification and appeared to be localized primarily in the cytosol. CaM-protein kinase increased the phosphorylation of both serine and threonine residues in endogenous substrates. In contrast to previous reports, no evidence for Ca2+/CaM-dependent phosphorylation of any subunit of the acetylcholine receptor was obtained.