Substrates induce conformational changes in human anion exchanger 1 (hAE1) as observed by fluorescence resonance energy transfer

The one-for-one exchange of Cl(-) and HCO(3)(-) ions is catalyzed by human erythrocyte anion exchanger 1 (hAE1) through a ping-pong mechanism whereby the protein exists in two main conformations, with the single anion-binding site exposed at either the cytoplasmic (inner) side (E(i)) or the extracellular side (E(o)), with interconversion between the two states being possible only after anion binding. Steady-state and time-resolved resonance energy transfer (FRET) techniques were used to determine the distance of the binding site for diTBA (bis-(1,3-diethylthiobarbituric acid)trimethine oxonol), a high affinity fluorescent oxonol inhibitor of hAE1, from a benchmark site (probably Lys-430) labeled by external fluorescein maleimide (FM). Using red cell ghost membranes, energy transfer distances were measured in media containing different anions between FM as the donor, covalently attached to one monomer, and diTBA as the acceptor, reversibly bound to the adjacent monomer of a hAE1 dimer. Energy transfer increased significantly in chloride or bicarbonate buffers relative to conditions where no transportable anions were present, that is, in citrate buffer. These differences in transfer efficiencies were interpreted in light of the conformational distributions of hAE1 in various buffers and the possible effects of diTBA itself on the distribution. The analysis indicates that the diTBA binding site comes closer to the FM site by approximately 7 A in chloride buffer as compared to that in citrate (or equivalent changes in diTBA orientation occur) because of the effects of anion binding. This provides the first direct physical evidence for structural changes in hAE1 induced by substrates.