Band 3 (AE1, SLC4A1)-mediated transport of stilbenedisulfonates. II: Evidence for transmembrane allosteric interactions between the "primary" stilbenedisulfonate binding site and the stilbenedisulfonate efflux site

Mouse Ae1 E699Q mediates SO42-i/anion-o exchange with [SO42-]i-dependent reversal of wild-type pHo sensitivity

The SLC4A1/AE1 gene encodes the electroneutral Cl(-)/HCO(3)(-) exchanger of erythrocytes and renal type A intercalated cells. AE1 mutations cause familial spherocytic and stomatocytic anemias, ovalocytosis, and distal renal tubular acidosis. The mutant mouse Ae1 polypeptide E699Q expressed in Xenopus oocytes cannot mediate Cl(-)/HCO(3)(-) exchange or (36)Cl(-) efflux but exhibits enhanced dual sulfate efflux mechanisms: electroneutral exchange of intracellular sulfate for extracellular sulfate (SO(4)(2-)(i)/SO(4)(2-)(o) exchange), and electrogenic exchange of intracellular sulfate for extracellular chloride (SO(4)(2-)(i)/Cl(-)(o) exchange). Whereas wild-type AE1 mediates 1:1 H(+)/SO(4)(2-) cotransport in exchange for either Cl(-) or for the H(+)/SO(4)(2-) ion pair, mutant Ae1 E699Q transports sulfate without cotransport of protons, similar to human erythrocyte AE1 in which the corresponding E681 carboxylate has been chemically converted to the alcohol (hAE1 E681OH). We now show that in contrast to the normal cis-stimulation by protons of wild-type AE1-mediated SO(4)(2-) transport, both SO(4)(2-)(i)/Cl(-)(o) exchange and SO(4)(2-)(i)/SO(4)(2-)(o) exchange mediated by mutant Ae1 E699Q are inhibited by acidic pH(o) and activated by alkaline pH(o). hAE1 E681OH displays a similarly altered pH(o) dependence of SO(4)(2-)(i)/Cl(-)(o) exchange. Elevated [SO(4)(2-)](i) increases the K(1/2) of Ae1 E699Q for both extracellular Cl(-) and SO(4)(2-), while reducing inhibition of both exchange mechanisms by acid pH(o). The E699Q mutation also leads to increased potency of self-inhibition by extracellular SO(4)(2-). Study of the Ae1 E699Q mutation has revealed the existence of a novel pH-regulatory site of the Ae1 polypeptide and should continue to provide valuable paths toward understanding substrate selectivity and self-inhibition in SLC4 anion transporters.

Band 3 (AE1, SLC4A1)-mediated transport of stilbenedisulfonates. I: Functional identification of the proton-activated stilbenedisulfonate influx site

Stilbenedisulfonates (SD) bind to a "primary" SD (PSD) site on the outer membrane surface of band 3, and inhibit anion exchange (AE) allosterically. Yet, evidence [Membr. Biochem. 2 (1979) 297] suggests that SD can be transported by band 3, thus raising questions about the relative locations of the transport and SD binding sites. A "second" class of DBDS (4,4'-dibenzamido-2,2'-stilbenedisulfonate) binding sites has been discovered, which is activated by protons (pK approximately 5.0), and is located on the membrane domain of band 3 [Biochem. J. 388 (2005) 343]. Here we show that the "second" class of DBDS binding sites, not the PSD site, lies on the SD transport pathway. We compare the pH dependence of DBDS influx to DBDS binding using: (a) control cells, (b) cells selectively crosslinked at the PSD site by treatment with 300 microM BS3 (bis(sulfosuccinimidyl)suberate), and (c) cells with DIDS (4,4'-diisothiocyanato-2,2'-stilbenedisulfonate) bound covalently to the PSD site. DBDS binds to the "second" class of sites on band 3 in all three types of cells. DBDS does not bind to the PSD sites of BS3- or DIDS-modified cells. Proton-activated DBDS influx was observed using control and BS3-modified cells, but not when using DIDS-modified cells. The results with DIDS suggest that the PSD site and the transport site overlap. However, this interpretation is disproved by experiments with BS3-modified cells, where the PSD site is blocked, yet DBDS transport and binding to the "second" class of sites both take place.

Band 3 (AE1, SLC4A1)-mediated transport of stilbenedisulfonates. III: Role of solute and protein structure in proton-activated stilbenedisulfonate influx

DBDS (4,4'-dibenzamido-2,2'-stilbenedisulfonate) influx into magnesium resealed ghosts (MRSG) occurs over the anion/proton co-transport pH range (pK approximately 5.0). Here, factors are studied which may influence the pH dependence of DBDS transport. Accumulation of various stilbenedisulfonate (SD) molecules was studied and found to be correlated with the hydrophobicity of the R-groups (Hansch factor), not protonation of the sulfonates. The role of proton binding to glutamate 681 was found not to be part of the rate-limiting step in DBDS uptake by MRSG. Finally, the pH dependence of changes in quaternary structure/conformational state was investigated using an assay involving photo-crosslinking of band 3 subunits in the presence of DASD (4,4'-diazido-2,2'-stilbenedisulfonate). Lowering the pH promoted intersubunit crosslinking by DASD, with a pK value of 4.75+/-1.0. This value is comparable to the pK for DBDS binding to the "second" class of sites on control band 3 (pK = 5.01+/-0.16), and to DBDS influx into control MRSG (pK values between 4.57+/-0.15 and 4.7+/-0.1). Susceptibility to photo-crosslinking was reversed by raising the pH prior to initiation of the reaction. Significantly, no photo-crosslinking was observed between pH 6.0 and 8.0, where band 3 subunits are known to exist as stable dimers and tetramers. We conclude that intersubunit photo-crosslinking does not simply involve random collision between photo-activated DASD and band 3. Rather, proton binding to band 3 either alters the conformation at the interface between subunits of pre-existing tetramers, or it promotes self-association of stable dimers to a "novel" tetrameric conformational state.