Red-cell glycophorin A-band 3 interactions associated with the movement ofband 3 to the cell surface

We have examined the mechanism by which glycophorin A (GPA) Facilitates the movement of the human red-cell anion exchanger (band 3, AE1) to the cell s urface. GPA itself forms stable dimers in membranes and detergent solution. Four mutants of human CPA with impaired dimerization were prepared (L75I, I76A, G79L and G83L). All four GPA mutants enhanced band 3 translocation to the Xenopus oocyte plasma membrane in the same way as wild-type CPA, showi ng that the GPA monomer is sufficient to mediate this process. Cell-surface expression of the natural band 3 mutant G701D has an absolute requirement for GPA. GPA monomers also rescued the cell-surface expression of this muta nt band 3. Taking into account other evidence, we infer that the site of CP A interaction with band 3 is located outside the CPA dimerization interface but within the GPA transmembrane span. The results of examination of the c ell-surface expression of GPA and band 3 in different K562 erythroleukaemia cell clones stably transfected with band 3 are consistent with the movemen t of GPA and band 3 to the cell surface together. We discuss the pathways b y which band 3 moves to the cell surface in the presence and the absence of GPA, concluding that CPA has a role in enhancing the folding and maturatio n of band 3. We propose that GPA functions in erythroid cells to assist wit h the incorporation of large amounts of properly folded band 3 into the mem brane within a limited time span during erythroid maturation.