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== Molecular biology ==
After separation of red cell membranes by [[SDS-polyacrylamide gel electrophoresis]] and staining with [[periodic-acid-Schiff stain]]ing (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 - gylycophorin 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 consitute ~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. Despite the similar names glycophorin C and D are unrelated to the other three glycophorins which encoded on [[chromosome 4]] at location 4q28-q31.
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 10<sup>5</sup> 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.
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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 [[palmitoylation|palmitoylated]] peripheral membrane phosphoprotein and a 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 12 amino acid segment 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.
== Molecular medicine ==
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