We recently described a calcium- and calmodulin-dependent protein kinase in striatal synaptosomal cytosol. This kinase catalyzes the phosphorylation of several endogenous neuronal proteins. We are trying to further characterize this system by enzyme and substrate purification. Affinity chromatography using calmodulin-Sepharose results in a 20-fold purification of the enzyme. However, it was found that all of the major substrates for this enzyme are also retained by calmodulin-Sepharose. We are also studying the effects of endogenous proteolysis on this enzyme. Proteolysis results in a loss of sensitivity of the enzyme to calmodulin, and therefore may function in regulating the activity of this kinase.