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

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.