Increase in vesicle permeability mediated by myelin basic protein: effect of phosphorylation of basic protein

The two most basic charge isomers of myelin basic protein (BP), components 1 and 2 (C1 and C2), which presumably differ in the degree of deamidation, were purified from bovine BP by cation-exchange chromatography. Two additional specific types of posttranslational modifications were introduced into the purified isomers: (1) C-terminal arginine deficient derivatives of C1 and C2 were prepared by incubating the isomers with a carboxypeptidase, and (2) phosphorylated derivatives of C1 (1.6 and 1.7 mol of phosphate/mol of protein) were prepared by incubating C1 with the protein kinase from rabbit muscle. The ability of these charge isomers to increase the permeability of multilamellar vesicles composed of phosphatidylserine/phosphatidylcholine (1:11.5 w/w) and sphingomyelin/cholesterol/phosphatidic acid (1:1:0.2 w/w/w) was measured by monitoring the release of a water-soluble spin-label (tempocholine chloride) from the vesicles. The increase in vesicle permeability caused by BP was taken as a measure of the degree of perturbation of the bilayer by the protein, most likely by penetration partly into the bilayer. All classes of charge isomers (naturally occurring or generated in vitro) were more effective at increasing vesicle permeability than was poly(L-lysine), a polycation that only interacts electrostatically with the bilayer. Although C1 and C2 and their C-terminal-deficient derivatives did not differ in the amount of marker released, the phosphorylated derivative of C1 caused a smaller increase in vesicle permeability than did the other isomers, suggesting that phosphorylation had altered the ability of the protein to perturb the bilayer.(ABSTRACT TRUNCATED AT 250 WORDS)