Glycophorin A as an erythroid marker in normal and malignant hematopoiesis.

Glyeophorin A (GpA), which is the major sialoglyeoprotein on human red eells, is one of the best eharaeterized mammalian integral membrane proteins (Marehesi et al. 1972; Tomita and Marehesi 1975). Its amino acid sequenee is known. The protein moleeule eontains three distinet domains. A large hydrophilie portion, earrying the NHrterminal, is loeated on the external surfaee of the red eell, and the COOH-terminal is loeated in the eytoplasm (Bretseher 1975) and probably interaets with peripheral proteins on the inner aspeet of the membrane. These two hydrophilie sequenees are eonneeted by a hydrophobie segment of 23 amino aeids which must be embedded within the lipid bilayer. An unusually large proportion (about 60%) of the GpA is made up of earbohydrates. The moleeule eontains 15 O-glyeosidie oligosaeharides with the strueture of N -acetyl neuraminyl a(2-3)-galactosyl ß(1-3) IN-acetyl neuraminyl a(2-6)1 N-acetyl galactosamine (Thomas and Winzler 1969) andoneN-glycosidicoligosaccharide located at asn-26. The carbohydrate is located outside the lipid bilayer in the NH2-terminal portion of the molecule (Fig. 1). Glycophorin A carries the MN blood group activity of red cells (Marchesi et al. 1972; Tomita and Marchesi 1975). This is most probably due to an interaction between amino acids and O-glycosidic oligosaccharides, since either treatment with neuraminidase (Mähelä and Cantell 1958) or modifications of amino acids (Lisowska and Duk 1975) abolish the activity. GpAs from M and N cells also show different amino acid sequences in the NH2-terminal portions (Dahr et al. 1977; Tomita and Marchesi 1975).