Correlation of Ca2+-and calmodulin-dependent protein kinase activity with secretion of insulin from islets of Langerhans

A Ca2+-activated and calmodulin-dependent protein kinase activity which phosphorylates predominantly two endogenous proteins of 57kDa and 54kDa was found in a microsomal fraction from islet cells. Half-maximal activation of the protein kinase occurs at approx. 1.9 microM-Ca2+ and 4 micrograms of calmodulin/ml (250 nM) for phosphorylation of both protein substrates. Similar phosphoprotein bands (57kDa and 54kDa) were identified in intact islets that had been labelled with [32P]Pi. Islets prelabelled with [32P]Pi and incubated with 28 mM-glucose secreted significantly more insulin and had greater incorporation of radioactivity into the 54 kDa protein than did islets incubated under basal conditions in the presence of 5 mM-glucose. Thus the potential importance of the phosphorylation of these proteins in the regulation of insulin secretion is indicated both by activation of the protein kinase activity by physiological concentrations of free Ca2+ and by correlation of the phosphorylation of the substrates with insulin secretion in intact islets. Experiments undertaken to identify the endogenous substrates indicated that this calmodulin-dependent protein kinase may phosphorylate the alpha- and β-subunits of tubulin. These findings suggest that Ca2+-stimulated phosphorylation of islet-cell tubulin via a membrane-bound calmodulin-dependent protein kinase may represent a critical step in the initiation of insulin secretion from the islets of Langerhans.