Glycophorin C - Molecular Biology

Molecular Biology

After separation of red cell membranes by SDS-polyacrylamide gel electrophoresis and staining with periodic acid-Schiff staining (PAS) four glycophorins have been identified. These have been named glycophorin A, B, C and D in order of the quantity present in the membrane - glycophorin A being the most and glycophorin D the least common. A fifth (glycophorin E) has been identified within the human genome but cannot easily be detected on routine gel staining. In total the glycophorins constitute ~2% of the total erythrocyte membrane protein mass. Confusingly these proteins are also known under different nomenclatures but they are probably best known as the glycophorins.

Glycophorin C was first isolated in 1978 Glycophorin C and D are minor sialoglycoproteins contributing to 4% and 1% to the PAS-positive material and are present at about 2.0 and 0.5 x 105 copies/cell respectively. In polyacrylimide gels glycophorin C's apparent weight is 32 kilodaltons (32 kDa). Its structure is similar to that of other glycophorins: a highly glycoslated extracellular domain (residues 1-58), a transmembrane domain (residues 59-81) and an intracellular domain (residues 82-128). About 90% of the glycophorin C present in the erythrocyte is bound to the cytoskeleton and the remaining 10% moves freely within the membrane.

Glycophorin D's apparent molecular weight is 23kDa. On average this protein has 6 O linked oligosaccarides per molecule.

Within the erythrocyte it interacts with band 4.1 (an 80-kDa protein) and p55 (a palmitoylated peripheral membrane phosphoprotein and a member of the membrane-associated guanylate kinase family) to form a ternary complex that is critical for the shape and stability of erythrocytes. The major attachment sites between the erythrocyte spectrin-actin cytoskeleton and the lipid bilayer are glycophorin C and band 3. The interaction with band 4.1 and p55 is mediated by the N terminal 30 kD domain of band 4.1 binding to a 16 amino acid segment (residues 82-98: residues 61-77 of glycophorin D) within the cytoplasmic domain of glycophorin C and to a positively charged 39 amino acid motif in p55. The majority of protein 4.1 is bound to glycophorin C. The magnitude of the strength of the interaction between glycophorin C and band 4.1 has been estimated to be 6.9 microNewtons per meter, a figure typical of protein–protein interactions.

Glycophorin C normally shows oscillatory movement in the erythrocyte membrane. This is reduced in Southeast Asian ovalocytosis a disease of erythrocytes due to a mutation in band 3.