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

摘要

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 ³⁶Cl-influx pathway, reminiscent of intact AE1. In the present study, we create a single construct, AE1Δ(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 AE1Δ(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 Lys539Ala at the covalent DIDS-reaction site of AE1 reduced the DIDS sensitivity, demonstrating that (1) the conductive pathway is intrinsic to AE1Δ(6: 7) and (2) the conductive pathway has some commonality with the electroneutral anion-exchange pathway. The conductance has an anion-permeability sequence: NO3⁻ ≈ I⁻ > NO2⁻ > Br⁻ > Cl⁻ > SO4²⁻ ≈ HCO3⁻ ≈ gluconate⁻ ≈ aspartate⁻ ≈ 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 AE1, it shares many properties with the anion-conductive pathways intrinsic to two other Cl–HCO3⁻ exchangers, trout AE1 and mammalian SLC26A7.