Calcium- and cyclic nucleotide-dependent protein kinases and their substrates in the Aplysia nervous system

Homogenates of the Aplysia nervous system contain protein kinase activities sensitive to cAMP, cGMP, and Ca2+/calmodulin. The cAMP- and cGMP-dependent activities are either soluble enzymes or are only loosely bound to membranes, since they can be detected only in crude but not in washed membrane fractions, and are present in 20,000 or 100,000 X g supernatants prepared from homogenates. In contrast there are both soluble and tightly membrane-bound Ca2+/calmodulin-dependent protein kinase activities. The three activities present in supernatant fractions can be separated by chromatography on DE-cellulose, indicating that they are due to distinct enzyme species. Substrates for these enzymes were analyzed by two-dimensional gel electrophoresis. Protein phosphorylation within the identified Aplysia neuron R15 in vivo was measured by the intracellular injection of [gamma-32P]ATP. cAMP stimulates the phosphorylation of nine proteins and decreases phosphorylation of two proteins in this cell. This in vivo pattern was compared with in vitro phosphorylation measured in homogenates of whole ganglion. Most of the phosphoproteins affected by cAMP in neuron R15 in vivo are also substrates for cAMP-dependent protein kinase in vitro. Thus, the in vitro system will be a useful tool for detailed biochemical analysis of phosphoproteins which have been identified as being physiologically relevant in vivo.