The major integral proteins of the human red cell.

The structures and functions of the major human red cell integral membrane proteins are summarized in this review. The proteins that are discussed are the anion transporter (band 3), the sialic acid-rich glycophorins and the glucose transporter. Band 3 (AE1) is a member of a family of anion transporters which carry out Cl-/HCO3- exchange. AE1 is largely restricted to red cells and functions in CO2 transport between the tissues and lungs. In addition to its transport function band 3 acts as an anchor site to the membrane of the red cell skeleton, and also binds a number of cytoplasmic red cell proteins. Variant forms of band 3 are known and some of these have an effect on red cell function and viability. The glycophorins comprise three major proteins, glycophorin A (GPA), glycophorin B (GPB) and glycophorin C (GPC). GPA and GPB (together with another putative gene product, GPE) are closely related products of highly homologous genes located in tandem on the human chromosome. The similarity between the genes gives rise to a number of genetic variants as a result of unequal crossover events. The gene products are erythroid specific. The function of the proteins is not clearly established, but GPA appears to have a role in facilitating the movement of band 3 to the cell surface during the biosynthesis of the latter. The GPC gene is not related to the GPA, GPB and GPE gene family. This gene gives rise to GPC and a form of GPC which is truncated at the N-terminus and is designated GPD. GPC functions in anchoring the red cell skeleton to the membrane, and absence of the protein is associated with red cell abnormalities. GPC transcripts are found in many other tissues, where they probably also have a role in cytoskeletal interactions. The red cell glucose transporter (GLUT1) is a member of the gene family of passive glucose transporters. GLUT1 is not erythroid specific but is also present in several other tissues.