Calcium/Calmodulin‐Dependent Protein Kinase II in Squid Synaptosomes

Abstract: The Ca²⁺/calmodulin (CaM)‐dependent protein kinase II system in squid nervous tissue was investigated. The Ca²⁺/CaM‐dependent protein kinase 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 phosphatase inhibitor NaF or by substitution of adenosine 5′‐O‐(3‐thiotriphosphate) for ATP. When [γ‐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 auto‐phosphorylation 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.