Ca 2+ /Diacylglycerol‐Activated, Phospholipid‐Dependent Protein Kinase in the Hermissenda CNS

In mammalian systems, Ca 2+ /diacylglycerol‐activated phospholipid‐dependent protein kinase (C‐kinase) appears to play an important role in regulating physiological responses that outlast the transient rise in cytosolic Ca 2+ . Electrophysiological experiments in neurons of the nudibranch mollusc, Hermissenda crassicornis , have suggested a role for C‐kinase in the long‐lasting reductions in early and late K + currents that have been observed following associative learning. Accordingly, we have investigated the catalytic properties of C‐kinase in Hermissenda CNS. Following homogenization in Ca 2+ ‐free buffer, C‐kinase can be separated from Ca 2+ /calmodulin‐dependent protein kinase by centrifugation; C‐kinase activity is found in the supernatant whereas essentially all of the Ca 2+ /calmodulin‐dependent protein kinase is found in the membrane fraction. Addition of Ca 2+ , phosphatidylserine, and diacylglycerol to the cytosol results in phosphorylation of at least eight endogenous proteins. The Hermissenda CNS C‐kinase can also phosphorylate lysine‐rich histone, a substrate for mammalian C‐kinase. The molluscan enzyme exhibits phospholipid specificity in that phosphatidylserine is much more effective than phosphatidylethanolamine, phosphatidylcholine, phospha‐tidylinositol, and phosphatidic acid. Addition of diacylglycerol, in the presence of Ca 2+ and phosphatidylserine, increases the activity of the C‐kinase. The percentage of activation by diacylglycerol is larger at lower Ca 2+ concentrations. Enzyme activity is inhibited by trifluoperazine and polymixin B sulfate. These studies indicate that the Hermissenda C‐kinase is catalytically similar to mammalian C‐kinase.