Structural polymorphism within the amino-terminal region of MM, NN, and MN glycoproteins (glycophorins) of the human erythrocyte membrane

MM, NN, and MN glycoproteins of human erythrocytes from single donors were cleaved by cyanogen bromide into three fragments-A, B, and C-which, upon gel electrophoresis, appeared to be common to the three antigens. Phenol/aqueous urea partitioning and gel filtration were used to separate the peptides quantitatively. Peptide C lacked carbohydrate and homoserine and represented the carboxyl-terminal portion of the glycoproteins. Peptides A and B contained one homoserine each and accounted for all the carbohydrate of the glycoproteins. The peptide portion of glycopeptide A from MM, NN, or MN antigens consisted of eight amino acid residues, of which six were homologous and two varied according to blood type. The variants were serine and glycine in glycopeptide A(MM), leucine and glutamic acid in A(NN), and half-residues of serine, glycine, leucine, and glutamic acid in A(MN). Serine was the amino-terminal residue in A(MM), leucine in A(NN), and one half residue of serine and leucine in A(MN). Each glycopeptide carried two tetrasaccharides (2 NANA, 1 Gal, 1 GalNAc) and one trisaccharide (NANA, Gal, GalNAc) linked O-glycosidically to one serine and two threonines as determined by beta-elimination and sulfite addition. The carbohydrate units were attached to serine and threonine located in the invariant region, because the amino-terminal serine residue could be oxidized by periodate. The M-N antigens are believed to be products of allelic genes which are expressed exclusively in homozygotes and equimolarly in heterozygotes.