The role of sialic acid in the expression of human MN blood group antigens.

Human MN blood group activity has been restored to neuraminidase-treated erythrocytes by resialylation with different homogeneous sialyltransferases. The antigenicity of the resialylated cells always corresponded to that of the native cells from which they were prepared. In no case was a cell converted to an MN phenotype different from that of the blood donor. Either one of two sialyltransferases could restore the native MN phenotype to asialoerythrocytes, although some antisera appear to recognize selectively the products of only one of these enzymes. One sialyltransferase forms the structure, NeuAccw2 -+ 3Galpl-+ 3GalNAcal + OThr/Ser, and the other forms the structure, Galfil + 3meuAca2 + G]GalNAccul + 0-Thr/Ser. Both linkages are found in the Thr/Ser-linked tetrasaccharides of glycophorin, confirming the participation of alkali-labile oligosaccharides in MN blood group activity. A third sialyltransferase that forms the structure, NeuAca2 + 6Ga1/31+ 4GlcNAc, with asparagine-linked oligosaccharides did not restore MN blood group activity. Together these three sialyltransferases replace -60% of the sialic acid removed by neuraminidase. Radioactivity profiles obtained following gel electrophoresis of the 14C-labeled glycoproteins of ghosts from resialylated MM or NN cells were virtually identical. In each case, about 85 to 90% of the incorporated sialic acid was found associated with the major erythrocyte glycoprotein, glycophorin A. Since the modification of cell surface oligosaccharides by either of two sialyltransferases can restore both M and N activity, specific carbohydrate structures cannot be responsible for the difference between M and N antigens. These results are consistent with the proposal that the MN alleles specify the polypeptide sequence of glycophorin A. However, the selectivity of some antisera for erythrocytes modified by only one sialyltransferase shows that specific sialic acid structures may still be important for antibody recognition.