Conformational change of band 3 protein induced by diethyl pyrocarbonate modification in human erythrocyte ghosts

Substrate binding and inhibition of the anion exchanger 1 transporter

Anion exchanger 1 (AE1), a member of the solute carrier (SLC) family, is the primary bicarbonate transporter in erythrocytes, regulating pH levels and CO2 transport between lungs and tissues. Previous studies characterized its role in erythrocyte structure and provided insight into transport regulation. However, key questions remain regarding substrate binding and transport, mechanisms of drug inhibition and modulation by membrane components. Here we present seven cryo-EM structures in apo, bicarbonate-bound and inhibitor-bound states. These, combined with uptake and computational studies, reveal important molecular features of substrate recognition and transport, and illuminate sterol binding sites, to elucidate distinct inhibitory mechanisms of research chemicals and prescription drugs. We further probe the substrate binding site via structure-based ligand screening, identifying an AE1 inhibitor. Together, our findings provide insight into mechanisms of solute carrier transport and inhibition.

Agglutination of human erythrocytes by the interaction of Zn(2+)ion with histidine-651 on the extracellular domain of band 3

Clustering of band 3, chloride/bicarbonate exchanger, has been reported in Zn(2+)-treated human erythrocytes. However, the agglutination of human erythrocytes is also induced by the interaction of Zn(2+)ion with histidine on band 3. Identification of histidine that interacts with Zn(2+)ion remains to be determined. The Zn(2+)-induced agglutination of human erythrocytes was unaffected by chymotrypsin cleavage of the small loop region containing His-547 in the extracellular domain of band 3. On the other hand, papain digestion of the large loop region containing His-651 in band 3 inhibited such Zn(2+)-induced agglutination. Moreover, Zn(2+)-induced erythrocyte agglutination was inhibited by the peptide (ARGWVIHPLG) containing His-651, but not by the peptide such as ARGWVIRPLG, which His-651 was substituted by arginine. Among 10 kinds of animal erythrocytes tested, interestingly, no agglutination by Zn(2+)ions was observed in cow cells only that the forth amino acid in the upstream from His-669 on the large loop of cow band 3 is aspartate (Asp-665) instead of glycine. As expected, the agglutination of human erythrocytes by Zn(2+) ions was inhibited in the presence of aspartate. These data indicate that the interaction of Zn(2+) ion with His-651 residue of band 3 plays an important role in the Zn(2+)-induced agglutination of human erythrocytes.

Band 3, the human red cell chloride/bicarbonate anion exchanger (AE1, SLC4A1), in a structural context

The crystal structure of the dimeric membrane domain of human Band 3(1), the red cell chloride/bicarbonate anion exchanger 1 (AE1, SLC4A1), provides a structural context for over four decades of studies into this historic and important membrane glycoprotein. In this review, we highlight the key structural features responsible for anion binding and translocation and have integrated the following topological markers within the Band 3 structure: blood group antigens, N-glycosylation site, protease cleavage sites, inhibitor and chemical labeling sites, and the results of scanning cysteine and N-glycosylation mutagenesis. Locations of mutations linked to human disease, including those responsible for Southeast Asian ovalocytosis, hereditary stomatocytosis, hereditary spherocytosis, and distal renal tubular acidosis, provide molecular insights into their effect on Band 3 folding. Finally, molecular dynamics simulations of phosphatidylcholine self-assembled around Band 3 provide a view of this membrane protein within a lipid bilayer.

Membrane perturbations induced by the interactions of zinc ions with band 3 in human erythrocytes

Of group 12 metals, zinc is an essential element to maintain our life, but other metals such as cadmium and mercury are toxic in cellular activities. Interactions of these metals with biomembranes are important to understand their effects on our living cells. Here, we describe the membrane perturbations induced by these metals in human erythrocytes. Of these metals, Zn²⁺ ions only induced the erythrocyte agglutination. Histidine residues in extracellular domains of band 3 participated in Zn²⁺-induced agglutination. Interestingly, it was found that band 3-cytoskeleton interactions play an important role in Zn²⁺-induced agglutination. In contrast with Hg²⁺ and Cd²⁺ ions, Zn²⁺ ions greatly suppressed pressure-induced hemolysis by cell agglutination. Such a suppression was removed upon dissociation of agglutinated erythrocytes by washing, indicating the reversible interactions of Zn²⁺ ions with erythrocyte membranes. Excimer fluorescence of pyrene indicated that spectrin is denatured by a pressure of 200 MPa irrespective of hemolysis suppression. Taken together, these results suggest that the agglutination of erythrocytes due to the interactions of Zn²⁺ ions with band 3 is stable under pressure, but spectrin, cytoskeletal protein, is denatured by pressure

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Human erythrocytes show different responses to the group 12 metal ions.

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Interactions of Zn²⁺ with histidines of band 3 induce agglutination of erythrocytes.

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Interactions of band 3 with skeleton are important in Zn²⁺-induced cell agglutination.

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Pressure-induced hemolysis is suppressed in erythrocytes agglutinated by Zn²⁺ ions.

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Spectrin is denatured upon compression of erythrocytes agglutinated by Zn²⁺ ions.

Importance of His192 in the human thyroid hormone transporter MCT8 for substrate recognition

Monocarboxylate transporter 8 (MCT8) facilitates cellular uptake and efflux of thyroid hormone (TH). So far, functional domains within MCT8 are not well defined. Mutations in MCT8 result in severe psychomotor retardation due to impaired neuronal differentiation. One such mutation concerns His192 (H192R), located at the border of transmembrane domain (TMD) 1 and extracellular loop (ECL) 1, suggesting that this His residue is important for efficient TH transport. Here, we studied the role of different His residues, predicted within TMDs or ECLs of MCT8, in substrate recognition and translocation. Therefore, we analyzed the effects of the His-modifying reagent diethylpyrocarbonate (DEPC) and of site-directed mutagenesis of several His residues on TH transport by MCT8. Reaction of MCT8 with DEPC inhibited subsequent uptake of T(3) and T(4), whereas T(3) and T(4) efflux were not inhibited. The inhibitory effect of DEPC on TH uptake was prevented in the presence of T(3) or T(4), suggesting that TH blocks access to DEPC-sensitive residues. Three putative DEPC target His residues were replaced by Ala: H192A, H260A, and H450A. The H260A and H450A mutants showed similar TH transport and DEPC sensitivity as wild-type MCT8. However, the H192A mutant showed a significant reduction in TH uptake and was insensitive to DEPC. Taken together, these results indicate that His192 is sensitive to modification by DEPC and may be located close to a putative substrate recognition site within the MCT8 protein, important for efficient TH uptake.

The role of band 3 protein in oxygen delivery by red blood cells

The synergistic effects of hemoglobin, carbonic anhydrase and the band 3 protein make red blood cells the ideal vehicle for oxygen delivering to the tissues. As long as oxygen is supplied by these ideal vehicles, oxygen intoxication of the tissues is precluded. Band 3 protein mediates the "Chloride-Shift", i.e., the anion exchange of Cl(-)/HCO(3) (-). Because of the Chloride-Shift, red blood cells are able to recognize metabolically active tissues and to supply the minimum amount of oxygen to the tissues. Investigation into the molecular mechanisms of the anion exchange mediated by the band 3 protein was introduced.

Structure of the membrane domain of human erythrocyte anion exchanger 1 revealed by electron crystallography

The membrane domain of human erythrocyte anion exchanger 1 (AE1) works as a Cl(-)/HCO(3)(-) antiporter. This exchange is a key step for CO(2)/O(2) circulation in the blood. In spite of their importance, structural information about AE1 and the AE (anion exchanger) family are still very limited. We used electron microscopy to solve the three-dimensional structure of the AE1 membrane domain, fixed in an outward-open conformation by cross-linking, at 7.5-A resolution. A dimer of AE1 membrane domains packed in two-dimensional array showed a projection map similar to that of the prokaryotic homolog of the ClC chloride channel, a Cl(-)/H(+) antiporter. In a three-dimensional map, there are V-shaped densities near the center of the dimer and slightly narrower V-shaped clusters at a greater distance from the center of the dimer. These appear to be inserted into the membrane from opposite sides. The structural motifs, two homologous pairs of helices in internal repeats of the ClC transporter (helices B+C and J+K), are well fitted to those AE1 densities after simple domain movement.

Transmembrane domain histidines contribute to regulation of AE2-mediated anion exchange by pH

Activity of the AE2/SLC4A2 anion exchanger is modulated acutely by pH, influencing the transporter's role in regulation of intracellular pH (pHi) and epithelial solute transport. In Xenopus oocytes, heterologous AE2-mediated Cl−/Cl−and Cl−/HCO3−exchange are inhibited by acid pHior extracellular pH (pHo). We have investigated the importance to pH sensitivity of the eight histidine (His) residues within the AE2 COOH-terminal transmembrane domain (TMD). Wild-type mouse AE2-mediated Cl−/Cl−exchange, measured as DIDS-sensitive36Cl−efflux from Xenopus oocytes, was experimentally altered by varying pHiat constant pHoor varying pHo. Pretreatment of oocytes with the His modifier diethylpyrocarbonate (DEPC) reduced basal36Cl−efflux at pHo7.4 and acid shifted the pHovs. activity profile of wild-type AE2, suggesting that His residues might be involved in pH sensing. Single His mutants of AE2 were generated and expressed in oocytes. Although mutation of H1029 to Ala severely reduced transport and surface expression, other individual His mutants exhibited wild-type or near-wild-type levels of Cl−transport activity with retention of pHosensitivity. In contrast to the effects of DEPC on wild-type AE2, pHosensitivity was significantly alkaline shifted for mutants H1144Y and H1145A and the triple mutants H846/H849/H1145A and H846/H849/H1160A. Although all functional mutants retained sensitivity to pHi, pHisensitivity was enhanced for AE2 H1145A. The simultaneous mutation of five or more His residues, however, greatly decreased basal AE2 activity, consistent with the inhibitory effects of DEPC modification. The results show that multiple TMD His residues contribute to basal AE2 activity and its sensitivity to pHiand pHo.

The Functional Role of Arginine 901 at the C-Terminus of the Human Anion Transporter Band 3 Protein

To determine which arginine residues are responsible for band 3-mediated anion transport, we analyzed hydroxyphenylglyoxal (HPG)-modified band 3 protein in native erythrocyte membranes. HPG-modification leads to inhibition of the transport of phosphoenolpyruvate, a substrate for band 3-mediated transport. We analyzed the HPG-modified membranes by reverse phase-HPLC, and determined that arginine 901 was modified by HPG. To determine the role of Arg 901 in the conformational change induced by anion exchange, we analyzed HPG-modification of the membranes when 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS) or diethypyrocarbonate (DEPC) was present. DNDS and DEPC fix band 3 in the outward and inward conformations, respectively. HPG-modification was unaffected in the presence of DEPC but decreased in the presence of DNDS. In addition to that, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), which specifically reacts with the outward conformation of band 3, did not react with HPG-modified membranes. Furthermore, we expressed a band 3 mutant in which Arg 901 was replaced by alanine (R901A) on yeast membranes. The kinetic parameters indicated that the R901A mutation affected the rate of conformational change of the band 3 protein. From these results, we conclude that the most C-terminal arginine, Arg 901, has a functional role in the conformational change that is necessary for anion transport.

Electrogenic sulfate/chloride exchange in Xenopus oocytes mediated by murine AE1 E699Q

Functional evaluation of chemically modified human erythrocytes has led to the proposal that amino acid residue E681 of the band 3 anion exchanger AE1 lies on the anion translocation pathway and is a proton carrier required for H+/SO4(2-) cotransport. We have tested in Xenopus oocytes the functional consequences of mutations in the corresponding residue E699 of mouse AE1. Most mutations tested abolished AE1-mediated Cl- influx and efflux. Only the E699Q mutation increased stilbene disulfonate-sensitive efflux and influx of SO4(2-). E699Q-mediated Cl- influx was activated by elevation of intracellular SO4(2-), but E699Q-mediated Cl- efflux was undetectable. The DNDS (4,4'-dinitrostilbene-2,2'-disulfonic acid) sensitivity of E699Q-mediated SO4(2-) efflux was indistinguishable from that of wt AE1-mediated Cl- efflux. The extracellular anion selectivity of E699Q-mediated SO4(2-) efflux was similar to that of wt AE1-mediated Cl- efflux. The stoichiometry of E699Q-mediated exchange of extracellular Cl- with intracellular SO4(2-) was 1:1. Whereas SO4(2-) injection into oocytes expressing wt AE1 produced little change in membrane potential or resistance, injection of SO4(2-), but not of Cl- or gluconate, into oocytes expression E699Q depolarized the membrane by 17 mV and decreased membrane resistance by 66%. Replacement of bath Cl- with isethionate caused a 28-mV hyperpolarization in SO4(2-)-loaded oocytes expressing E699Q, but had no effect on oocytes expressing wt AE1. Extracellular Cl(-)-dependent depolarization of SO4(2-)-preloaded oocytes was blocked by DNDS. AE1 E699Q-mediated inward current measured in the presence of extracellular Cl- was of magnitude sufficient to account for measured 35SO4(2-) efflux. Thus, AE1 E699Q-mediated SO4(2-)/Cl- exchange operated largely, if not exclusively, as an electrogenic, asymmetric, 1:1 anion exchange. The data confirm the proposal that E699 resides on or contributes to the integrity of the anion translocation pathway of AE1. A single amino acid change in the sequence of AE1 converted electroneutral to electrogenic anion exchange without alteration of SO4(2-)/Cl- exchange stoichiometry.

Towards the localization of the essential arginine residues in the band 3 protein of human red blood cell membranes

The effects of 4-hydroxy-3-nitrophenylglyoxal (HNPG), on the binding of eosin-5-maleimide (EMA), and diethyl pyrocarbonate (DEPC) to the anion transport system in the human red blood cell membrane, have been investigated. HNPG is a reversibly binding, arginine-specific, anion transport competitive inhibitor, known to act on the anion binding site. The EMA reaction site is an external facing lysine residue (Lys-430) in the 17 kDa transmembrane segment. The DEPC reaction site is an intracellular histidine (His-819) in the 35 kDa fragment. The results show that inhibition of the transport system with EMA increases in presence of HNPG to about 2.3 times. This finding suggests a positive cooperativity between the HNPG and EMA binding site and give evidence that the essential arginine is either nearby or allosterically linked to Lys-430. The inhibition of the cells with DEPC was nearly unchanged or slightly decreased in the presence of 10 mM HNPG. These results suggest that the intracellular His-residue which reacts with DEPC is not a part of the transport pathway. Our experiments with 4,4'-dinitrostilbene-2,2'-disulfonate (DNDS) have shown that its affinity to the transport system does not change after pre-treatment with phenylglyoxal (PG). We also found that the binding of [14C]phenylglyoxal (PG) to band 3 reduces significantly in presence of chloride. This is another evidence for the direct involvement of arginine residues in substrate binding.

Inhibition of mouse erythroid band 3-mediated chloride transport by site-directed mutagenesis of histidine residues and its reversal by second site mutation of Lys 558, the locus of covalent H2DIDS binding

Substitution by site-directed mutagenesis of any one of the histidine residues H721, H837, and H852 by glutamine, or of H752 by serine, inhibits Cl- flux mediated by band 3 expressed in Xenopus oocytes. Mutation of Lys 558 (K558N), the site of covalent binding of H2DIDS (4,4'-diisothiocyanostilbene-2,2'-disulfonate) in the outer membrane surface, in combination with any one of the His/Gln mutations leads to partial (H721Q; H837Q) or complete (H852Q) restoration of Cl- flux. In contrast, inhibition of Cl- flux by mutation of proline or lysine residues in the vicinity of His 837 at the inner membrane surface cannot be reversed by the second-site mutation K558N, indicating specificity of interaction between Lys 558 and His 837. The histidine-specific reagent diethyl pyrocarbonate (DEPC) is known to inhibit band 3-mediated anion exchange in red blood cells [Izuhara, K., Okubo, K., & Hamasaki, N. (1989) Biochemistry 28, 4725-4728]. It was also found to inhibit transport after expression in the oocyte of wild-type band 3, of the double mutants of the histidines listed above, and of the single mutant H752S. The effects on the wild type and the double mutants were indistinguishable, while the mutant H752S exhibited a considerably reduced sensitivity to inhibition, suggesting that His 752 is the most prominent site of action of DEPC. According to a hydrophobicity plot of band 3 and further independent evidence, Lys 558, the mutated histidines, and Glu 699, the mutation of which was also found to inhibit Cl- flux [Müller-Berger, S., Karbach, D., Kang, D., Aranibar, N., Wood, P. G., Rüterjans, H., & Passow, H. (1995) Biochemistry 34, 9325-9332], are most likely located in five different transmembrane helices. The interactions between Lys 558 and the various histidines suggest that these helices reside in close proximity. Together with the helix carrying Glu 699, they could form an access channel lined with an array of alternating histidine and glutamate residues. Together with a chloride ion bridging the gap between His 852 and His 837, they could have the potential to form, at low pH, a transmembrane chain of hydrogen bonds. The possible functional significance of such channel is discussed.

Inhibition of chloride binding to the anion transport site by diethylpyrocarbonate modification of Band 3

The line widths of 35Cl- nuclear magnetic resonances were used to measure chloride binding by Band 3. Since this procedure relates directly to binding, the data obtained may be interpreted more unequivocally than affinities derived from kinetic data which could be related to either translocation or binding. Chloride binding to the active sites in Band 3 was assessed from that portion of the total line width which was sensitive to 4,4'-dinitrostilbene-2,2'-disulfonic acid. These sites appeared to be completely inhibited by treatment of erythrocyte membranes with diethylpyrocarbonate. This result is consistent with our previous observation that this reagent inhibits anion transport in resealed erythrocyte ghosts (Izuhara, Okubo & Hamasaki, 1989, Biochemistry 28:4725-4728). Hydroxylamine could not reverse the diethylpyrocarbonate inhibition of chloride binding to Band 3. The pH-dependence of diethylpyrocarbonate reactivity suggests that the modified residues may be those of histidine.