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== Genomics ==
Despite the similar names glycophorin C and D are unrelated to the other three glycophorins which encoded on [[chromosome 4]] at location 4q28-q31. These latter proteins are closely related. Glycophorin A and B carry the [[blood group]] MN and Ss [[antigen]]s 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).
Originally it was thought that 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). The gene was first cloned in 1989 by High ''et al''.<ref name="pmid2818576">{{cite journal | author = High S, Tanner MJ, Macdonald EB, Anstee DJ | title = Rearrangements of the red-cell membrane glycophorin C (sialoglycoprotein beta) gene. A further study of alterations in the glycophorin C gene | journal = Biochem. J. | volume = 262 | issue = 1 | pages = 47–54 | year = 1989 | month = August | pmid = 2818576 | pmc = 1133227 | doi = | url = | issn = }}</ref> The GPC gene is organized in four [[exon]]s distributed over 13.5 kilobase pairs of [[DNA]]. Exon 1 encodes residues 1-16, exon 2 residues 17-35, exon 3 residues 36-63 and exon 4 residues 64-128. Exons 2 and 3 are highly homologous, with less than 5% nucleotide divergence. These exons also differ by a 9 amino acid instert at the 3' end of exon 3. The direct repeated segements containing these exons is 3.4 kilobase pairs long and may be derived from a recent duplication of a single ancestral domain. Exons 1, 2 and most of exon 3 encode the N-terminal extracellular domain while the remainder of exon 3 and exon 4 encode transmembrane and cytoplasmic domains.
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Two [[isoform]]s are known and the gene is expressed in a wide variety of [[biological tissue|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 [[glycosylation|glycosylated]]. In the erythrocyte glycophorin C makes up ~4% of the membrane [[sialoglycoproteins]]. The average number of O linked chains is 12 per molecule.
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. It is expressed early in developlment and before before the [[Kell antigen]]s, [[Rhesus-associated glycoprotein]], [[glycophorin A]], [[band 3]], the [[Rhesus antigen]] and [[glycophorin B]].<ref name="Daniels2000">Daniels G., Green C.(2000) Expression of red cell surface antigens during erythropoiesis. Vox Sang. 78 Suppl 2:149-153</ref>
Data on the regulation of glycophorin C is [http://srs6.bionet.nsc.ru/srs6bin/cgi-bin/wgetz?-e+TRRDGENES4-AC:A00322 here].
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