Allosteric effects in stilbenedisulfonate binding to band 3 protein (AE1)

Stilbenedisulfonates are potent competitive inhibitors of the anion exchange (AE) class of transporters. Although these molecules have been extensively used in studies of the anion exchange function, the actual mechanism by which stilbenedisulfonates compete with transported anions has been uncertain. Over the last several years, work in my laboratory has focused on understanding the mechanism of stilbenedisulfonate binding to human erythrocyte band 3 (AE1), with particular emphasis placed on deciding whether stilbenedisulfonates are pure competitive inhibitors, or whether they inhibit transport allosterically. I summarize our results suggesting that stilbenedisulfonates are allosteric inhibitors of band 3 anion exchange. I also summarize results which show that covalent binding of stilbenedisulfonates to one subunit produces allosteric effects which extend to the neighboring subunit in a band 3 dimer. Such allosteric subunit interactions have been observed: a) in divalent anion influx exchange experiments; b) in reversible stilbenedisulfonate binding studies, and c) in thermal unfolding studies of the membrane domain of band 3. In addition, two quaternary conformational states of the band 3 dimer, modulated by ligands of the stilbenedisulfonate site, have been identified in protein crosslinking studies. Finally, new evidence is discussed showing that Southeast Asian ovalocytic band 3 in a heterodimer composed of mutant and wild-type subunits, increases the 4,4'-diisothiocyanodihydro-2,2'-stilbenedisulfonate (H2DIDS) affinity of the wild-type subunit. Taken together, these results challenge the view that band 3 exists as structurally independent monomers. In addition, they suggest that subunit interactions may play a significant role in the transport function.