The functional importance of blood group-active molecules in human red blood cells

Antigens of 23 of the 30 human blood group systems are defined by the amino acid sequence of red cell membrane proteins. The antigens of DI, RH, RHAG, MNS, GE and CO systems are carried on blood group-active proteins (Band 3, D and CE polypeptides, RhAG, Glycophorins A and B, Glycophorins C and D and Aquaporin 1, respectively) which are expressed at high levels (> 200 000 copies/red cell). These major proteins contribute to essential red cell functions either directly as membrane transporters and by providing linkage to the underlying red cell skeleton or by facilitating the membrane assembly of the protein complexes involved in these processes. The proteins expressing antigens of the remaining 17 blood group systems are much less abundant (< 20 000 copies/red cell) and their functional importance for the circulating red cell is largely unknown. Human gene knock-outs (null phenotypes) have been described for many of these minor blood group-active proteins, but only absence of Kx glycoprotein has been clearly linked with pathology directly related to the function of circulating red cells. Recent evidence suggesting the normal quality control system for glycoprotein synthesis is altered during the latter stages of red cell production raises the possibility that many of these low abundance blood group-active proteins are vestigial. In sickle cell disease and polycythaemia vera, elevated Lutheran glycoprotein expression may contribute to pathology. Dyserythropoiesis with reduced antigen expression can result from mutations in the erythroid transcription factors GATA-1 and EKLF.