Glycophorin A facilitates the expression of human band 3-mediated anion transport in Xenopus oocytes.

The effects of human red cell glycophorin A (GPA) on the expression of the human erythrocyte anion transporter (band 3, AE1) has been examined in Xenopus oocytes. The coexpression of GPA with band 3 increased stilbene disulfonate-sensitive chloride transport into the oocytes. The effect of GPA was particularly noticeable at low band 3 concentrations and less marked at high band 3 cRNA concentrations. The enhancement of chloride transport was specific to GPA and was not observed when either glycophorin B or glycophorin C was coexpressed with band 3. Immunoprecipitations of whole oocyte homogenates showed the amount of band 3 synthesized was not affected by GPA at subsaturating cRNA concentrations. More band 3 was detected at the oocyte surface by immunoprecipitation when GPA was also expressed. Chymotrypsin treatment of intact oocytes was also used to assess surface band 3 and greater cleavage of band 3 by chymotrypsin was observed when GPA was present. Band 3 synthesis and assembly into canine pancreatic microsomes in the reticulocyte cell-free translation system was not altered by cotranslation of GPA. We suggest that GPA facilitates the translocation of band 3 to the plasma membrane at some point during band 3 biosynthesis in Xenopus oocytes. However, GPA is not essential for the expression of band 3 in red cells, since GPA-deficient individuals have apparently normal levels of band 3. Other GPA-independent mechanisms must also allow translocation of band 3 to the surface membrane in erythroid cells and oocytes. GPA may affect the rate of accumulation of band 3 at the cell surface, rather than the final level in the plasma membrane.