Serotonin alters the subcellular distribution of a Ca2+/calmodulin-binding protein in neurons of Aplysia.

We purified and radioiodinated calmodulin from Aplysia nervous tissue. Using blot analysis, we detected 14 calmodulin-binding proteins in extracts of the abdominal ganglion after gel electrophoresis in NaDodSO4. All 14 components are present in extracts from each of the major central Aplysia ganglia, but their relative proportions differed characteristically from one ganglion to another. When neural components are extracted with nonionic detergents in 2 M glycerol, we obtain a cytoplasmic and a membrane-cytoskeleton fraction. Of the 14 calmodulin-binding proteins, 2 are cytoplasmic, 10 are loosely associated, and 2 remain bound to the membrane-cytoskeleton complex. Treatment of isolated ganglia with serotonin, which has been shown to increase the content of cAMP within neurons, or with dibutyryl cAMP causes the partial dissociation of one of the loosely associated proteins, which has an apparent Mr of 55,000. In extracts prepared in the presence of glycerol and cAMP, this component also is dissociated from the membrane-cytoskeleton complex, but only if ATP is also present. These results suggest that a cAMP-dependent protein phosphorylation controls the subcellular distribution of the Mr 55,000 calmodulin-binding protein, whose state of phosphorylation itself, however, does not appear to be increased by cAMP.