Glycophorin, the major sialoglycoprotein of the human erythrocyte membrane, is composed of 131 amino acids and an average of 16 oligosaccharide chains. Fifteen oligosaccharides are linked to threonine/serine residues via O-glycosidic bonds, and one more complex unit is attached to asparagine. The location of each of these oligosaccharides and the complete amino-acid sequence of this molecular have been determined by Edman degradation techniques. Glycophorin appears to be organized into three distinct "domains" on the basis of the locations of glycosylated amino acids and the clustering of residues of similar type. These include (i) a glycosylated segment composed of approximately 64 residues from the NH2-terminus, (ii) a "hydrophobic" segment of approximately 32 nonpolar residues, and (iii) a COOH-terminal segment, composed of approximately 35 residues, which has an unusual concentration of hydrophilic amino acids. This unique structure is consistent with the earlier suggestions that glycophorin is one of the major "intrinsic" membrane proteins which has a transmembrane orientation.
Tomita et al. (Fri,) studied this question.