Calcium-dependent protein kinase from apple fruit membranes is calmodulin-independent but has calmodulin-like properties.

Crude Ca(2+)-activated protein kinase from membranes of apple (Malus domestica L. Borkh., Cox's Orange Pippin) fruit can be partially purified to yield a Ca(2+)-dependent protein kinase whose activity is apparently not regulated by calmodulin. The autophosphorylating catalytic subunit of this protein kinase shows a Ca(2+)-dependent mobility shift of approx. 10 kilodaltons (kDa) on sodium dodecyl sulphate-polyacrylamide gel electrophoresis; in the absence of added Ca(2+) or ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) its apparent molecular mass is approx. 50 kDa. The Ca(2+)-dependent protein kinase is inhibited by the calmodulin antagonists N-(6-aminohexyl)-5-chloro-1-naphthalenesulphonamide and trifluoperazine with IC50 values of approx. 45 μM and 15 μM, respectively. These similarities between the protein kinase and calmodulin indicate that the kinase may be a calmodulin-like protein.