**3. Glycophorins of human RBC membranes**

Apart from the membrane proteins that Fairbanks et al. designated, glycophorins (GPs) exist as transmembrane glycoproteins that contain sialic acid. These sialo-oligosaccharide-rich glycoproteins are detectable on SDS-gels by staining with periodic acid-Schiff (PAS) reagent [3, 12]. **Figure 3** Lane 3 shows the nomenclature of human RBC membrane sialo-glycoproteins. These GPs are found in the RBC membranes of humans [13–16] and other mammals [17–20] and birds [21, 22]. In human RBC membranes, GP A (dimer) is observed below band 3 on SDS-gels (**Figure 3** Lane 3). GP A is a major component of red cell membrane glycoproteins. The electrophoretic migration of GP on SDS gels is relatively low when compared to other membrane proteins because GP is heavily glycosylated. Although the molecular mass of the other membrane proteins can be estimated by migration on SDS–PAGE, each GPs molecular mass cannot be estimated in this manner.

GPs C and D are thought to link to the band 4.1 protein and connect the cytoskeleton structure under the phospholipid bilayer [23–26]. These GPs C, D, and band 3 are associated with the cytoskeleton closely and contributed to the maintenance of the shape and mechanical properties of the RBC after passing through capillary vessels [27]. This information suggested that GPs C and D are anchored to the RBC membrane by the cytoskeleton. In contrast, it is believed that GPs A and B are not associated with the cytoskeleton, thus enabling them to be easily released from the RBC membrane [28].

While GP C and its shorter form, GP D, are antigenically distinct from GPs A and B. GP C carries several blood group antigens (Gerbich, Yus, Wb, Ana, Dha, and others) [29–31]. According to Podbielska et al. [32], O-linked oligosaccharides isolated from GP A carry the A, B, or H blood group antigen. Although these oligosaccharides reacted with ABH blood group antigens, the reaction was estimated at a relatively low level. Moreover, GP A-deficient RBCs did not clearly demonstrate the physiological role of GP A [33].
