Structure of the Ss Blood Group Antigens, II. A Methionine/Threonine Polymorphism within the N-terminal Sequence of the Ss Glycoprotein

Abstract

The N-terminal amino acid sequence (residues 1-35) of the Ss sialoglycoprotein (or glycophorin B) from human erythrocyte membranes of defined Ss blood group activity was determined by manual sequencing methods, using N-terminal tryptic or chymotryptic glycopeptides and various secondary peptides. The proposed structure differs considerably from that suggested on the basis of work with glycopeptides of unknown Ss blood group activity. Only 1 different between glycopeptides from SS and ss erythrocytes was found, i.e., a methionine/threonine polymorphism at position 29. This amino acid heterogeneity probably represents the Ss polymorphism rather than the Ux or Uz polymorphisms, which are in strong genetic linkage disequilibrium with the Ss antigens. A part of the sequence (residues 9-30) of the major (MN) red cell membrane sialoglycoprotein (glycophorin A) was reinvestigated and revised at positions 11 and 17. As judged from the present data, the first 26 residues of the Ss and the blood group N-specific MN glycoprotein are identical. The sequence 27-35 of the Ss glycoprotein shows a homology with the residues 56-64 and 59-67 of the MN glycoprotein. Data on the partial N-terminal sequence of glycopeptides from a 3rd erythrocyte membrane sialoglycoprotein (component D or glycophorin C) indicate that its structure is different from those of the 2 other glycoproteins.