Calcium/Calmodulin-Dependent Protein Kinase II in Squid Synaptosomes

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Abstract
The Ca2+/calmodulin (CaM)-dependent protein kinase II system in squid nervous tissue was investigated. The Ca2+/CaM-dependent protein kianse II was found to be very active in the synaptosome preparation from optic lobe, where it was associated with the high-speed particulate fraction. Incubation of the synaptosomal homogenate with calcium, calmodulin, magnesium, and ATP resulted in partial and reversible conversion of the Ca2+/CaM-dependent protein kinase II from its calcium-dependent form to a calcium-independent species. The magnitude of this conversion reaction could be increased by inclusion of the protein phosphate inhibitor NaF or by substitution of adenosine 5''-O-(3-thiotriphosphate) for ATP. When [.gamma.-32P]ATP was used, proteins of 54 and 58 kilodaltons (kDa) as well as proteins > 100 kDa were rapidly 32P-labeled in a calcium-dependent manner. Major 125I-CaM binding proteins in the synaptosome membrane fraction were 38 and 54 kDa. The Ca2+/CaM-dependent protein kinase II was purified from the squid synaptosome and was shown to consist of 54- and 58-60-kDa subunits. The purified kinase, like Ca2+/CaM-dependent protein kinase II from rat brain, catalyzed autophosphorylation associated with formation of the calcium-independent form. These studies, characterizing the Ca2+/CaM-dependent protein kinase II in squid neural tissue, are supportive of the putative role of this kinase in regulating calcium-dependent synaptic functions.