Primary structure of human erythrocyte glycophorin A. Isolation and characterization of peptides and complete amino acid sequence

Abstract

Peptides of glycophorin AMN were prepared by cyanogen bromide cleavage and by chymotryptic and tryptic digestion. Cyanogen bromide cleavage produced 3 fragments which accounted for the entire polypeptide chain. Trypsin and chymotrypsin cleaved completely at several sites, but incompletely at sites within the glycosylated segment of the polypeptide chain. Some of the latter sites became accessible to proteolysis after desialation in addition to exposure of new sites for cleavage. The amino acid sequence of glycophorin AMN was determined by manual Edman degradation, using the direct Edman and the dansyl-Edman procedures simultaneously for determination of glycosylated amino acid residues. The automated procedure was used for sequence determination of a hydrophobic peptide. Glycophorin A is a polypeptide chain of 131 amino acid residues and contains 16 oligosaccharide units attached to the amino-terminal third of the molecule. Oligosaccharides [15] were linked O-glycosidically to threonine or serine residues and 1 complex oligosaccharide unit was attached N-glycosidically to an asparagine residue. Amino-terminal sequences were different for glycophorin AM and AN, the 2 forms of the glycophorin A molecule coded for by genes at the MN locus. The differences in sensitivity to proteases of various sites on glycophorin A seem due to heterogeneity in the carbohydrate components and not to differences in the primary structure of the polypeptide chains. This work contains a number of revisions and corrections of earlier preliminary reports.