Secondary structure of charge isomers of myelin basic protein before and after phosphorylation

Publisher Website
Google Scholar
Abstract
Human myelin basic protein (MBP) was fractionated into several of its charge isomers (components). Of these, the secondary structures of four isomers before and after phosphorylation have been studied by circular dichroism (CD). None of the four showed any .alpha.-helical structure. All of the components showed varying amounts of .beta.-structure, random structure, and turns. Component 1 (C-1), the most cationic of the components, showed 13%; component 2 (C-2) had 19%; C-3, 17%; and C-4, 24% of .beta.-structure. Each of the four components was phosphorylated with protein kinase C, from human brain. The extent of phosphorylation varied considerably from 2.8 .+-. 0.6 mol of PO4/mol of protein in C-1 to 5.2 .+-. 0.8 mol of PO4/mol of protein in C-4. The effect of phosphorylation on the secondary structure was to induce .beta.-structure in all the components. The largest change in .beta.-structure was in C-1 and the least in C-4. The surprising result is that although the components were phosphorylated to different extents, the amount of .beta.-structure in all four components increased to a final proportion of 35-40%. Treatment of phosphorylated C-1 with acid phosphatase removed 50% of the total radioactivity. Although the remainder represented approximately 1 mol of PO4/mol of protein, the proportion of .beta.-structure was unaltered. We concluded that a single phosphorylation site identified as residues 5-13 represented a critical size for stabilization of .beta.-structure of MBP in solution and that phosphorylation at the other sites had little infuence on secondary structure.