Basic A1 Protein of the Myelin Membrane

Abstract

Abstract Twenty-seven tryptic peptides were isolated from the A1 protein from bovine spinal cord. These, together with 16 peptic peptides, were utilized to establish the complete amino acid sequence of the 170 residues of the A1 protein. Peptide T, derived from the A1 protein by cleavage of the carboxyl of the single tryptophan residue, was useful in positioning the peptides which comprise the COOH-terminal end. Seven chymotryptic and nine tryptic peptides were isolated from Peptide T. An unusual feature of the sequence is the methylated arginine residue at position 107 which is present as both the dimethyl and monomethyl derivatives. The methylated derivatives appear to be relatively resistant to tryptic hydrolysis. Located close to the methylated arginine residue is a Pro-Arg-Thr-Pro-Pro-Pro sequence, a structure which may induce a sharp bend in the molecule, suggesting a conformation more compatible with an open double chain structure than a random coil. The over-all sequence reveals no obvious periodicity but rather a general distribution of basic residues over the polypeptide chain, making the interaction with phospholipids within the myelin matrix highly probable. Several peptide segments of 9 residues exist, in which basic residues are missing; the somewhat nonpolar character of these regions suggests a possiblity for participation in nonpolar interactions.