Microheterogeneity and Phosphoamino Acids in the Carboxy‐terminal Half of Myelin Basic Protein

Abstract —Three chromatographically distinct peptic peptides (F80‐1, F80‐2 and F80‐3) derived from the C‐terminal half of the bovine and guinea‐pig myelin basic proteins were characterized. The three peptides of each animal species had the same N‐terminal residue (phenylalanine) and essentially the same amino acid composition, but they differed in electrophoretic mobility at alkaline pH. The least basic peptide (F80‐3) differed from the others in showing a deficit of C‐terminal arginine residues and in containing phosphorus, 0·37 and 0·46 g‐atom/mol of bovine and guinea‐pig peptides, respectively. Other peptic peptides. derived from the N‐terminal half of the basic protein, were essentially phosphorus‐frcc. Analyses of partial acid hydrolyzates of peptide F80‐3 by high voltage electrophoresis showed the presence of both phosphoserine and phosphothreonine. After incubation with E. coli alkaline phosphatase (EC 3.1.3.1). 34 and 40% of the bovine and guinea‐pig F80‐3 peptides. respectively, were converted to peptide F80‐1. This reaction involved the loss of 2 net negative charges, and its extent corresponded to loss of essentially all of the phosphate originally present in the peptide. This result indicated that the phosphorylated species of peptide F80‐3 contained one phosphate group per molecule.