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

Cell Physiology and Molecular Mechanism of Anion Transport by Erythrocyte Band 3/AE1

The major transmembrane protein of the red blood cell, known as band 3, AE1, and SLC4A1, has two main functions: 1) catalysis of Cl^-/HCO₃^- exchange, one of the steps in CO₂ excretion; 2) anchoring the membrane skeleton. This review summarizes the 150 year history of research on red cell anion transport and band 3 as an experimental system for studying membrane protein structure and ion transport mechanisms. Important early findings were that red cell Cl^- transport is a tightly coupled 1:1 exchange and band 3 is labeled by stilbenesulfonate derivatives that inhibit anion transport. Biochemical studies showed that the protein is dimeric or tetrameric (paired dimers) and that there is one stilbenedisulfonate binding site per subunit of the dimer. Transport kinetics and inhibitor characteristics supported the idea that the transporter acts by an alternating access mechanism with intrinsic asymmetry. The sequence of band 3 cDNA provided a framework for detailed study of protein topology and amino acid residues important for transport. The identification of genetic variants produced insights into the roles of band 3 in red cell abnormalities and distal renal tubular acidosis. The publication of the membrane domain crystal structure made it possible to propose concrete molecular models of transport. Future research directions include improving our understanding of the transport mechanism at the molecular level and of the integrative relationships among band 3, hemoglobin, carbonic anhydrase, and gradients (both transmembrane and subcellular) of HCO₃^-, Cl^-, O₂, CO₂, pH, and NO metabolites during pulmonary and systemic capillary gas exchange.

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.

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

Results from the first paper in this series indicated that the "primary" stilbenedisulfonate (PSD) site was not located on the DBDS (4, 4'-dibenzamido-2, 2'-stilbenedisulfonate) transport pathway into magnesium resealed ghosts (MRSG). Rather, transport correlated with DBDS binding to the "second" class of proton-activated binding sites located on the membrane domain of band 3 [Biochem. J. 388 (2005) 343]. Here we report the discovery that reversible binding of extracellular H2DIDS (4, 4'-diisothiocyanatodihydro-2, 2'-stilbenedisulfonate) to the PSD site causes a greater than 5-fold acceleration in the rate of DBDS efflux from pre-loaded MRSG at physiological pH. Pre-labeling all of the PSD sites with H2DIDS inhibited the acceleration effect completely, thus confirming mediation by band 3. Acceleration of DBDS efflux could be mimicked by establishing an externally directed proton gradient (acidic inside, pH 7.4 outside). Under these conditions, addition of extracellular H2DIDS neither accelerated DBDS efflux further nor was proton-induced acceleration inhibited. The results of this paper support the view that the PSD binding site on band 3 is an allosteric regulatory site which is not located on the SD transport pathway. We propose a model where H2DIDS binding to the PSD site modulates activity at the "second" class of sites by raising the pK for transport of DBDS into the physiological pH range.