Calcium/calmodulin-dependent protein phosphorylation in the nervous system of Aplysia

An afterdischarge in the bag cell neurons of Aplysia was previously shown to be associated with calcium entry into these cells and with changes in the phosphorylation state of at least two bag cell proteins (BC-I and BC-II). We have now investigated the role of calcium plus calmodulin (Ca/CaM) in the control of phosphorylation of Aplysia nervous system proteins, including those of the bag cell neurons. In cell-free preparations of Aplysia CNS, we demonstrated Ca/CaM-stimulated protein phosphorylation that could be inhibited by the calmodulin-blocking drugs R24571 , trifluoperazine, chlorpromazine, and W7 . A number of substrate proteins for Ca/CaM-dependent protein phosphorylation with Mr values from 17,000 to 310,000 were consistently observed in homogenates of the Aplysia CNS. In the bag cells, we found that a major substrate for Ca/CaM-dependent protein phosphorylation was the bag cell-specific, Mr = 21,000 protein (BC-II). BC-I (Mr = 33,000), on the other hand, appeared not to be a substrate for a Ca/CaM-dependent protein kinase. We found that there are a minimum of two Ca/CaM-dependent protein kinases in the Aplysia nervous system. These enzymes were distinguished on the basis of their subcellular distribution and their ability to phosphorylate distinct sites on synapsin I, an exogenous neuronal protein from vertebrates. Phosphorylation by one of these kinases (calmodulin kinase I) was on a site recovered in an Mr = 10,000 proteolytic fragment of synapsin I, and phosphorylation by the other (calmodulin kinase II) was on a site recovered in an Mr = 30,000 fragment. The predominant enzyme in the Aplysia CNS, as in the mammalian nervous system, was calmodulin kinase II.(ABSTRACT TRUNCATED AT 250 WORDS)