Calcium ion stimulated endogenous protein kinase catalyzed phosphorylation of basic proteins in myelin subfractions and myelin-like membrane fraction from rat brain

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Abstract
Polypeptide composition and endogenous phosphorylation were investigated in the subfractions of rat brain myelin, isolated by discontinuous or continuous sucrose density gradient centrifugation of myelin. A myelin-like membrane fraction (SN4) was similarly studied. A Ca-stimulated protein kinase seemed present in a highly purified myelin preparation, which exclusively phosphorylated myelin basic proteins of the membrane preparation. cAMP stimulated kinase was considerably enriched in the myelin-like membrane fraction. Although [cAMP stimulated kinase] is capable of phosphorylating the basic proteins, its effect was at least 5 .times. weaker compared to the Ca-stimulated myelin protein kinase. Within the gradient subfractions a close relation appeared between the amount of basic proteins and their Ca-stimulated phosphorylation; a similar relationship was not obtained in the case of cAMP-dependent phosphorylation of myelin basic proteins. The former (i.e., Ca2+-stimulated phosphorylation) required a protein factor that functionally resembled calmodulin. Calmodulin-like proteins and a Ca-stimulated protein kinase may exist in adult myelin membrane from mammalian brain; both were previously unrecognized constituents of myelin membranes.