Basic Protein in Brain Myelin Is Phosphorylated by Endogenous Phospholipid‐Sensitive Ca2+‐Dependent Protein Kinase

Abstract

Phosphorylation of myelin basic protein (MBP) in rat or rabbit brain myelin was markedly stimulated by Ca²⁺, and this reaction was not essentially augmented by exogenous phosphatidylserine or calmodulin or both. Solubilization of myelin with 0.4% Triton X‐100 plus 4 mM EGTA, with or without further fractionation, showed that Ca²⁺‐dependent phosphorylation of MBP required phosphatidylserine, but not calmodulin. DEAE‐cellulose chromatography of solubilized myelin revealed a pronounced peak of protein kinase activity stimulated by a combination of Ca²⁺ and phosphatidylserine; a protein kinase stimulated by Ca²⁺ plus calmodulin was not detected. These findings clearly indicate an involvement of phospholipid‐sensitive Ca²⁺‐dependent protein kinase in phosphorylation of brain MBP, although a possible role for the calmodulin‐sensitive species of Ca²⁺‐dependent protein kinase in this reaction could not be excluded or established. Phosphorylation of MBP in solubilized rat myelin catalyzed by the phospholipid‐sensitive enzyme was inhibited by adriamycin, palmitoylcarnitine, trifluoperazine, melittin, polymyxin B, and N‐(6‐aminohexyl)‐5‐chloro‐l‐naphthalenesulfonamide (W–7).