Glycophorin C

Glycophorin C (GYPC; CD236/CD236R; glycoprotein beta; glycoconnectin; PAS-2') is an integral membrane protein of the erythrocyte and acts as the receptor for the Plasmodium falciparum protein PfEBP-2 (erythrocyte binding protein 2; baebl; EBA-140).

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Genomics

Glycophorin C (GPC) is a single polypeptide chain of 128 amino acids and is encoded by a gene on the long arm of Chromosome 2 (2q14-q21). Two isoforms are known and the gene is expressed in a wide variety of tissues including kidney, thymus, stomach, breast, adult liver and erythrocyte. In the non erythroid cell lines, expression is lower than in the erythrocyte and the protein is differentially glycosylated. In the erythrocyte glycophorin C makes up ~4% of the membrane sialoglycoproteins.

The GPC gene is organized in four exons distributed over 13.5 kilobase pairs of DNA and contains two directly repeated domains 3.4 kilobase pairs long which may be derived from a recent duplication of a single ancestral domain. The gene is expressed early in the development of the erythrocyte, specifically in the erythroid burst-forming unit and erythroid colony-forming unit. The mRNA from human erythroblasts is ~1.4 kilobases long and the transcription start site in erythroid cells has been mapped to 1050 base pairs 5' of the start codon. A second antigen, glycophorin D (GPD), is generated from the glycophorin C messenger RNA by leaky translation at an in frame AUG at codon 30: glycophorin D = glycophorin C residues 30 to 128. Glycophorin C shows very little homology with the major erythrocyte membrane glycophorins A and B. The latter two proteins are closely related and carry the blood group MN and Ss antigens respectively. There are ~225,000 molecules of GPC and GPD per erythrocyte. GPC appears to be synthesized in excess in the erythrocyte and that the membrane content is regulated by band 4.1 (protein 4.1).

Data on the regulation of glycophorin C is here.

Molecular biology

Glycophorin C possesses a single transmembrane domain (residues 49-88) and a cytoplasmic domain and in the erythrocyte interacts with band 4.1 (an 80-kDa protein) and p55 (a palmitoylated peripheral membrane phosphoprotein) 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 kiloDalton 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. About 90% of the glycophorin C present in the erythrocyte is bound to the cytoskeleton and the remaining 10% moves freely within the membrane. 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

Glycophorin C and D encode the Gerbich (Ge) antigens which were described in 1960 and are named after one of the three original patients. There are four allelles, Ge-1 to Ge-4. Three types of Ge antigen negativity are known: Ge-1,-2,-3 (Leach phenotype), Ge-2,-3 and Ge-2,+3. A 3.4 kilobase pair deletion within the gene, which probably arose because of unequal crossing over between the two repeated domains, is responsible for the formation of the Ge-2,-3 genotype. The breakpoints of the deletion are located within introns 2 and 3 and results in the deletion of exon 3. This mutant gene is transcribed as a messenger RNA with a continuous open reading frame extending over 300 nucleotides and is translated into the sialoglycoprotein found on Ge-2,-3 red cells. A second 3.4 kilobase pair deletion within the glycophorin C gene eliminates only exon 2 by a similar mechanism and generates the mutant gene encoding for the abnormal glycoprotein found on Ge-2,+3 erythrocytes.

The Yussef (Yus) phenotype is due to a 57 base pair deletion corresponding to exon 2. The rare Webb (Wb) antigen (~1/1000 donors), originally described in 1963 in Australia, is the result of an alteration in glycosylation of glycophorin C: an A to G transition at nucleotide 23 results in an asparagine residue instead of the normal serine residue with the resultant loss of glycoslation. The rare Duch (Dh) antigen - discovered in Aarhus, Denmark (1968) - is due to a C to T transition at nucleotide 40 resulting in the replacement of leucine by phenylalanine.

Antibodies to the Gerbich antigens have been associated with transfusion reactions and mild hemolytic disease of the newborn. The relatively rare Leach phenotype is due either to a deletion in exons 3 and 4 or to a frameshift mutation causing a premature stop codon in the glycophorin C gene, and persons with this phenotype are less susceptible (~60% of the control rate) to invasion by Plasmodium falciparum. Such individuals have a subtype of a condition called hereditary elliptocytosis.

Other antigens associated with this gene are Lewis II (Lsa; Ge-6) and Ahonen (Ana). Lsa has insert of 84 nucleotides into the ancestral GPC gene, an insert that corresponds to the entire sequence of exon 3. Two subtypes of Lsa are known: beta Lsa which carries the Ge3 epitope and gamma Lsa which carries both the Ge2 and Ge3 epitopes. Ana, a rare blood group antigen, is expressed on GPD positive cells only.

Glycophorin C mutations are rare in most of the Western world, but are more common in some places where malaria is endemic. In Melanesia a greater percentage of the population is Gerbich negative (46.5%) than in any other part of the world.

Naturally occurring anti-Ge antibodies have been found and appear to be of no clinical significance. Immunological tolerance towards Ge antigen has been suggested.