A conductive pathway generated from fragments of the human red cell anion exchanger AE1

Human red cell anion exchanger AE1 (band 3) is an electroneutral Cl-HCO3- exchanger with 12-14 transmembrane spans (TMs). Previous work using Xenopus oocytes has shown that two co-expressed fragments of AE1 lacking TMs 6 and 7 are capable of forming a stilbene disulphonate-sensitive 16 Cl-influx pathway, reminiscent of intact AEI. In the present study, we create a single construct, AE1 Delta(6:7), representing the intact protein lacking TMs 6 and 7. We expressed this construct in Xenopus oocytes and evaluated it employing a combination of two-electrode voltage clamp and pH-sensitive microelectrodes. We found that, whereas AE1 Delta(6:7) has some electroneutral Cl-base exchange activity, the protein also forms a novel anion-conductive pathway that is blocked by DIDS. The mutation Ly(S539)Ala at the covalent DIDS-reaction site of AE1 reduced the DIDS sensitivity, demonstrating that (1) the conductive pathway is intrinsic to AEI Delta(6: 7) and (2) the conductive pathway has some commonality with the electroneutral anion-exchange pathway. The conductance has an anion-permeability sequence: NO3-approximate to I- > NO2- > Br- cl > so(4)(2-) approximate to HCO3 (-) approximate to gluconate(-)approximate to aspartate(-) approximate to cyclamate(-). It may also have a limited permeability to Na' and the zwitterion taurine. Although this conductive pathway is not a usual feature of intact mammalian AEI, it shares many properties with the anion-conductive pathways intrinsic to two other Cl-HCO3- exchangers, trout AE1 and mammalian SLC26A7.