The kinetics of intramolecular cross-linking of the band 3 protein in the red blood cell membrane by 4,4'-diisothiocyano dihydrostilbene-2,2'-disulfonic acid (H2DIDS)

Consequences of point mutations in trout anion exchanger 1 (tAE1) transmembrane domains: evidence that tAE1 can behave as a chloride channel

In this study, we have shown that, when expressed in Xenopus oocytes, trout anion exchanger 1 (tAE1) was able to act as a bifunctional protein, either an anion exchanger or a chloride conductance. Point mutations of tAE1 were carried out and their effect on Cl- conductance and Cl- unidirectional flux were studied. We have shown that mutations made in transmembrane domain 7 had dramatic effects on tAE1 function. Indeed, when these residues were mutated, either individually or together (mutants E632K, D633G, and ED/KG), Cl- conductance was reduced to 28-44% that of wild-type tAE1. Moreover, ion substitution experiments showed that anion selectivity was altered. However, the exchanger function was unchanged, as evidenced by the fact that Cl- influx and K(m) were identical for each of these mutants and similar to the wild-type protein parameters. By contrast, mutations made in the C-terminal domains of the protein (R819M, Q829K) affected both transport functions. Cl- conductance was increased by approximately 200% with respect to tAE1 and anion selectivity was impaired. Likewise, Cl- influx was increased by approximately 260% and was no longer saturable. These and other mutations carried out in transmembrane domains 7, 8, 12-14 of tAE1 allow us to demonstrate without doubt that, in addition to its anion exchanger activity, tAE1 can also function as a chloride channel. Above all, this work led us to identify amino acids involved in this double function organization.

Relocation of the Disulfonic Stilbene Sites of AE1 (Band 3) on the Basis of Fluorescence Energy Transfer Measurements

Previous fluorescence resonance energy transfer (FRET) measurements, using BIDS (4-benzamido-4'-isothiocyanostilbene-2,2'-disulfonate) as a label for the disulfonic stilbene site and FM (fluorescein-5-maleimide) as a label for the cytoplasmic SH groups on band 3 (AE1), combined with data showing that the cytoplasmic SH groups lie about 40 A from the cytoplasmic surface of the lipid bilayer, would place the BIDS sites very near the membrane's inner surface, a location that seems to be inconsistent with current models of AE1 structure and mechanism. We reinvestigated the BIDS-FM distance, using laser single photon counting techniques as well as steady-state fluorescence of AE1, in its native membrane environment. Both techniques agree that there is very little energy transfer from BIDS to FM. The mean energy transfer (E), based on three-exponential fits to the fluorescence decay data, is 2.5 +/- 0.7% (SEM, N = 12). Steady-state fluorescence measurements also indicate <3% energy transfer from BIDS to FM. These data indicate that the BIDS sites are probably over 63 A from the cytoplasmic SH groups, placing them near the middle or the external half of the lipid bilayer. This relocation of the BIDS sites fits with other evidence that the disulfonic stilbene sites are located farther toward the external membrane surface than Glu-681, a residue near the inner membrane surface whose modification affects the pH dependence and anion selectivity of band 3. The involvement of two relatively distant parts of the AE1 protein in transport function suggests that the transport mechanism requires coordinated large-scale conformational changes in the band 3 protein.

Binding properties of the stilbene disulfonate sites on human erythrocyte AE1: kinetic, thermodynamic, and solid state deuterium NMR analyses

A novel stilbene disulfonate, 4-trimethylammonium-4'-isothiocyanostilbene-2,2'-disulfonic acid (TIDS), has been chemically synthesized, and the interaction of this probe with human erythrocyte anion exchanger (AE1) was characterized. Covalent labeling of intact erythrocytes by [N(+)((14)CH(3))(3)]TIDS revealed that specific modification of AE1 was achieved only after removal of other ligand binding sites by external trypsinization. Following proteolysis, (1.2 +/- 0.4) x 10(6) TIDS binding sites per erythrocyte could be blocked by prior treatment with 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), a highly specific inhibitor of AE1. Inhibition of sulfate equilibrium exchange by TIDS in whole cells was described by a Hill coefficient of 1.10 +/- 0.06, which reduced to 0.51 +/- 0.01 following external trypsinization. The negative cooperativity of TIDS binding following external trypsinization suggests that trypsin-sensitive proteins modulate allosteric coupling between AE1 monomers. Thermodynamic analysis revealed that TIDS binding induces smaller conformational changes in AE1 than is observed following DIDS binding. The similar inhibitory potencies of both TIDS (IC(50) = 0.71 +/- 0.48 microM) and DIDS (IC(50) = 0.2 microM) imply that there is no correlation between the ability of stilbene disulfonates to arrest anion exchange function and the magnitude of ligand-induced conformational changes in AE1. Solid state (2)H NMR analysis of a [N(+)(CD(3))(3)]TIDS-AE1 complex in both unoriented and macroscopically oriented membranes revealed that large amplitude "wobbling" motions describe ligand dynamics. The data are consistent with a model where TIDS bound to AE1 is located exofacially in contact with the bulk aqueous phase.

Characterization of the stilbenedisulphonate binding site on band 3 Memphis variant II (Pro-854-->Leu)

Band 3 Memphis variant II is a mutant anion-exchange protein associated with the Diego a+ blood group antigen. There are two mutations in this transporter: Lys-56-->Glu within the cytoplasmic domain, and Pro-854-->Leu within the membrane-bound domain. The Pro-854 mutation, which is thought to give rise to the antigenicity, is located within the C-terminal subdomain of the membrane-bound domain. Yet, there is an apparent enhancement in the rate of covalent binding of H2DIDS (4,4'-di-isothiocyanatodihydro-2, 2'-stilbenedisulphonate) to 'lysine A' (Lys-539) in the N-terminal subdomain, suggesting widespread conformational changes. In this report, we have used various kinetic assays which differentiate between conformational changes in the two subdomains, to characterize the stilbenedisulphonate site on band 3 Memphis variant II. We have found a significantly higher H2DIDS (a C-terminal-sensitive inhibitor) affinity for band 3 Memphis variant II, due to a lower H2DIDS 'off' rate constant, but no difference was found between mutant and control when DBDS (4,4'-dibenzamido-2,2'-stilbenedisulphonate) (a C-terminal-insensitive inhibitor) 'off' rates were measured. Furthermore, there were no differences in the rates of covalent binding to lysine A, for either DIDS (4,4'-di-isothiocyanato-2,2'-stilbenedisulphonate) or H2DIDS. However, the rate of covalent intrasubunit cross-linking of Lys-539 and Lys-851 by H2DIDS was abnormally low for band 3 Memphis variant II. These results suggest that the Pro-854-->Leu mutation causes a localized conformational change in the C-terminal subdomain of band 3.

Differential sensitivity of stilbenedisulfonates in their reaction with band 3 HT (Pro-868-->Leu)

Band 3 HT (Pro-868-->Leu) is a mutant anion exchange protein which has several phenotypic characteristics, including a 2- to 3-fold larger Vmax, and reduced covalent binding of the anion transport inhibitor 4,4'-diisothiocyanodihydrostilbene-2,2'-disulfonate (H2DIDS). We have used fluorescence kinetic methods to study inhibitor binding to band 3 to determine if the point mutation in band 3 HT produces localized or wide-spread conformational changes within the membrane-bound domain of this transporter. Our results show that covalent binding of H2DIDS by band 3 HT is slower by a factor of 10 to 20 compared with the wild-type protein. In contrast, no such difference in the kinetics was observed for covalent binding of 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS). In addition, the kinetics of H2DIDS release from band 3 HT was abnormal, while the kinetics of 4,4'-dibenzamidostilbene-2,2'-disulfonate (DBDS) release showed no difference when compared with the wild-type protein. We conclude that substitution of leucine for proline at position 868 does not perturb the structure of "lysine A" in the membrane-bound domain of band 3 but rather produces an apparently localized conformational change in the C-terminal subdomain of the protein which alters H2DIDS affinity. When combined with the observation of an increased Vmax, these results suggest that protein structural changes at position 868 influence a turnover step in the transport cycle.

Characterization of the stilbenedisulfonate binding site on band 3

Stilbenedisulfonates are potent inhibitors of Band 3 mediated anion exchange. They bind tightly to the protein and form a 1-to-1 reversible complex. Those stilbenedisulfonates which contain isothocyanato groups such as DIDS (4,4'-diisothiocyanato-2,2'-stilbenedisulfonate) and H2DIDS (4,4'-diisothiocyanatodihydrostilbene-2,2'-disulfonate) can also react rapidly with lysine residues within the binding pocket to yield an irreversible covalent adduct. The reactive lysine residue is known as lysine-A, and is thought to have an unusually low pKa. In this report, we characterize the kinetics of DIDS adduct formation with respect to the effect of substrate anions, competitive inhibitory anions, and pH on the rate of covalent adduct formation. We investigate the following: (a) whether stilbenedisulfonates bind to or block access of substrate anions to the transport site; (b) whether the rapidity of the covalent reaction of DIDS at neutral pH is due to a low pKa for lysine-A within the binding pocket; and (c) whether once bound, DIDS and H2DIDS isothiocyanato groups are accessible to reagents. For this latter experiment, we have utilized a newly discovered reaction of the DIDS isothiocyanato groups with azide to test for accessibility. Our results show that substrate anions, DIDS, and Band 3 form a ternary complex. Significantly, the binding of large substrate anions, such as iodide, is not weakened by DIDS to any greater extent than is the binding of smaller substrates such as chloride or fluoride. These results are not consistent with a "partial blockade" hypothesis for the relationship between the stilbenedisulfonate and transport sites. Rather, they support an allosteric site-site interaction hypothesis. Our pH dependence results show that the apparent pKa for the DIDS/lysine-A reaction is greater than 9.26. This is consistent with typical lysine pKa values, and indicates that lysine-A does not have an unusually low pKa. Finally, we show that azide can react with the isothiocyanato groups of DIDS and H2DIDS within their Band 3 complexes, indicating that the stilbenedisulfonate binding site is accessible to solute. These results support a view which suggests that the stilbenedisulfonate site is a superficial inhibitory site on Band 3 which inhibits transport by allosteric interactions within the protein, rather than by either direct or partial blockade of the transport site.

The influence of two anion-transport inhibitors, 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulfonate and 4,4'-dibenzoylstilbene-2,2'-disulfonate, on the self-association of erythrocyte band 3 protein

4,4'-Diisothiocyanatodihydrostilbene-2,2'-disulfonate and 4,4'-dibenzoylstilbene-2,2'-disulfonate potently inhibit the erythrocyte anion transporter. These inhibitors act by binding, with a 1:1 stoichiometry, to the band 3 transport protein. We have studied, by sedimentation equilibrium analysis in an analytical ultracentrifuge, the effect of the two closely related stilbenedisulfonates on the state of association of band 3 in the nonionic detergent nonaethyleneglycol lauryl ether. It was found that covalent binding of 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulfonate to band 3 did not significantly disturb the monomer/dimer/tetramer association equilibrium shown by the unliganded protein. An entirely different result was obtained after addition of 4,4'-dibenzoylstilbene-2,2'-disulfonate to the protein, at both low and high chloride concentrations. The amount of band 3 dimer in the samples increased with increasing inhibitor concentration c1, and for c1 > or = 15 microM virtually all of the protein was present as dimer. After removal of the inhibitor (by gel filtration or dialysis), the original monomer/dimer/tetramer distribution of the band 3 protein was restored. Our data show that the (noncovalent) binding of 4,4'-dibenzoylstilbene-2,2'-disulfonate drastically changes the coupling between band 3 protomers. In addition, a reversible change in the state of association of band 3 induced by ligand binding is demonstrated.

Local structural difference between human and bovine band 3 in the anion transport inhibitor-binding region

We have examined molecular properties of inhibitor-complexed human and bovine band 3, an anion transport protein of erythrocyte membrane, in order to demonstrate the structural characteristics of the inhibitor binding region. Band 3 modified with DIDS (4,4'-diisothiocyano-2,2'-stilbenedisulfonate), a potent anion transport inhibitor, generated a positive circular dichroic band at a wavelength of 345 nm, corresponding to a DIDS chromophore. The dichroic spectra of human band 3-DIDS complex and its bovine counterpart differed markedly in their ellipticity. Under the conditions that H2DIDS (the dihydro-derivative of DIDS) cross-linked two chymotryptic fragments of human band 3, the reagent failed to cross-link the equivalent bovine fragments. The inhibitory effect of PLP (pyridoxal 5'-phosphate), a substrate and affinity label, on phosphate influx into red blood cells was more pronounced for human band 3 than for bovine band 3. The residue Lys-562 of human band 3 was found to be modified with PLP, while the corresponding residue of bovine band 3 was devoid of reactivity with PLP.

DIDS inhibition of deformation-induced cation flux in human erythrocytes

The permeability of human erythrocytes to sodium, potassium and calcium increases when the cells are deformed by shear. We now report that the anion-exchange inhibitor DIDS (4,4'-diisothiocyanostilbene-2,2'-disulfonic acid) inhibited 55-60% of the deformation-induced flux with an apparent K1/2 of 1 microM. Covalently bound DIDS was also effective. In cells partially derivatized at 0 degrees C (pH 7.4), anion exchange and the deformation flux were inhibited in parallel, implying that lysine a is the site of inhibition for both fluxes. Ektacytometry showed that DIDS does not inhibit by lowering the cell's ability to deform. Crosslinking of lysines in Band 3 was not required for inhibition of the stress flux, as demonstrated by electrophoretic analysis of chymotrypsin-cleaved Band 3 after DIDS treatment. Chymotrypsin cleavage itself did not affect the cation flux rates. DNDS, an anion exchange inhibitor that binds to the chloride site on Band 3 but is unable to derivatize lysine a, is an ineffective inhibitor of the deformation flux. Other high-affinity inhibitors of anion exchange were also relatively ineffective against the deformation flux, and anion exchange itself was unchanged by shear. These results suggest that 55-60% of the deformation-induced cation movement traverses a route that includes Band 3, but is distinct from the pathway utilized by anion exchange. Chloride-dependent cation pathways do not participate in the stress induced cation flux, since complete exchange of intracellular chloride for sulfate had no effect on the rates. Deformation of erythrocytes by laminar shear appears to increase the non-specific cation permeability.

Sulfate concentrations and transport in human bronchial epithelial cells

Inorganic sulfate concentrations in the cytoplasm of human bronchial epithelial cells exceeded levels in the bathing medium under all circumstances tested. Cell sulfate concentrations were directly related to medium sulfate concentrations and inversely related to medium chloride concentrations. In physiological media there was a sulfate compartment of approximately 0.3 mM that exchanged very slowly with extracellular sulfate. In media lacking chloride, sulfate was accumulated by the cells to a level as high as 2 mM. Sulfate uptake was markedly inhibited by external chloride and by stilbene sulfonic acid derivatives but was not affected by sodium in the medium. Efflux of 35SO4(2-) was stimulated by both chloride and sulfate in the bathing medium but inhibited by stilbenes. The following compounds had no effect on sulfate movements: phorbol esters, adenosine 3',5'-cyclic monophosphate derivatives, and okadaic acid. Changes in medium tonicity were likewise without effect. Our results suggest that human bronchial epithelial cells maintain a steady-state disequilibrium for inorganic sulfate. Furthermore, sulfate appears to exist in at least two compartments in the cells: one that is slowly exchangeable with sulfate in the medium and another exchangeable compartment that is of negligible size in physiological media but that becomes very large in media lacking chloride. Sulfate is transported by an anion exchanger of broad specificity that is not influenced by substances known to modulate chloride channels.

Role of Lys 558 and Lys 869 in substrate and inhibitor binding to the murine band 3 protein: a study of the effects of site-directed mutagenesis of the band 3 protein expressed in the oocytes of Xenopus laevis

The effect of mutation of either Lys 558 or Lys 869 or both on mouse erythroid band 3 protein (AE1)-mediated 36Cl- efflux and its inhibition by pyridoxal 5-phosphate (P5-P), DNDS and H2DIDS were studied. Regardless of the mutation, band 3 was always capable of executing Cl- self-exchange. P5-P (5 mM, pH 7.6) produced irreversible inhibition in the wild type (KK) and in the mutant in which Lys 558 (NK) or Lys 869 (KM) had been replaced by asparagine (N) or methionine (M), respectively. However, when both residues were replaced, mutant (NM), irreversible inhibition could no longer be achieved. This shows that P5-P is capable of producing inhibition with either one of the lysine residues, 558 or 869. Inhibition by DNDS changed dramatically upon mutation. The Ki app increased from 6.0 microM in the wild type (KK) to 23 microM in the mutant NK, to 73 microM in the mutant KM and to 474 microM in the double mutant NM. The Km value for activation of the transport system by varying the substrate concentration by isosmotic substitution of Cl- with SO4(2-) decreased from 42 mM in the wild type (KK) to 11.3 mM in the mutant NM. The results show that both Lys 558 and Lys 869 are involved in the maintenance of the structure of the overlapping binding sites for stilbene disulfonates and the substrate Cl-. In the double mutant NM, H2DIDS is no longer able to produce irreversible inhibition at pH 7.6. This is evidently related to the replacement of Lys 558 (pK 8.2) by Asn 558 in this mutant (see Bartel, D., Lepke, S., Layh-Schmitt, G., Legrum, B., Passow, H., 1989. EMBO J. 8:3601-3609). However, at pH 9.5, some irreversible inhibition could still be observed. This suggests that the other lysine residue (pK 10.8) that is known to be involved in covalent binding with the second isothiocyanate group of H2DIDS is still present, and hence, not identical to Lys 869, which had been substituted by a methionine residue. However, this result remains inconclusive since after mutagenesis, the H2DIDS may produce inhibition at a site that is not normally involved in H2DIDS binding.

Probing the active site of the reconstituted aspartate/glutamate carrier from bovine heart mitochondria: carbodiimide-catalyzed acylation of a functional lysine residue

Upon modification of the reconstituted aspartate/glutamate carrier by various amino acid-reactive chemicals a functional lysine residue at the exofacial binding site was identified. The inactivation of transport function by the lysine-specific reagents pyridoxal phosphate (PLP, IC50 400 microM) and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonate (SITS, IC50 300 microM) could specifically be suppressed by the substrates aspartate and glutamate; a 50% substrate protection was observed at half-saturation of the external binding site. The same held true for 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC, IC50 500 microM) and diethyl pyrocarbonate (DEPC, IC50 20 microM), two reagents known to modify carboxylic or histidinyl side-chains, respectively. EDC, however, turned out to catalyze an acylation of the active site lysine by activating carboxyls that had to be present in the incubation medium. This special mechanism, which was proven by protein labelling using EDC/[14C]succinate, necessitates a lysine side-chain of high reactivity and low pK, since the modification had to occur at pH less than or equal to 6.5, i.e. not too far from the pK of the carboxyl to be activated. All reagents applied, additionally including 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS, IC50 10 microM), were effective at this pH. Competition experiments revealed interaction of EDC, PLP, SITS and probably DIDS at the same active site lysine. For DEPC a lysine modification could not be ruled out. Yet, a model comprising a histidine juxtaposed to the lysine seems to be appropriate.

pH-dependence of inhibition by H2DIDS of mouse erythroid band 3-mediated Cl- transport in Xenopus oocytes. The effect of oligonucleotide-directed replacement of Lys-558 by an Asn residue

The rapid reversible inhibition of band 3-mediated inorganic anion transport by 4,4'-diisothiocyanodihydrostilbene-2,2'-disfulfonate (H2DIDS) turns slowly into irreversible inhibition. This is due to covalent bond formation of the two isothiocyanate groups of the inhibitor with two lysine residues on band 3, called Lys a and Lys b. In the red cell membrane, the pK value of Lys a is about 2.5 pK units lower than the pK value of Lys b. Hence the susceptibility of Lys a to irreversible modification by H2DIDS far exceeds the susceptibility to Lys b. In the present paper, we have expressed in Xenopus oocytes cRNA's derived from cDNA clones encoding wild-type mouse band 3 and mouse band 3 in which Lys a (Lys-558) had been replaced by an Asn residue by oligonucleotide-directed mutagenesis. In accord with previous findings, in the oocytes both wild-type and mutated band 3 mediate Cl- exchange. After determining the uninhibited exchange rate the oocytes were exposed for a fixed length of time to H2DIDS at a concentration (20 microM) which saturates all H2DIDS binding sites with reversibly bound H2DIDS (KI = 0.3 microM and 1.1 microM, respectively, for wild-type and mutant). Exposure was terminated by washing with a medium in which H2DIDS was replaced by bovine serum albumin to remove free and reversibly bound H2DIDS from the extracellular phase. Subsequent measurements of Cl- efflux yielded a measure for the irreversible inhibition that persisted. Since the transition from reversible to irreversible H2DIDS binding was found to follow first-order kinetics it was possible to calculate rate constants. From the pH dependence of the rate constants, pK values were calculated. These calculations could be made since in the wild-type, in which Lys a and Lys b are present, the exposure to H2DIDS could be confined to a pH range in which little if any covalent binding to Lys b takes place. The data could be represented by a single pK value of 8.3. In the mutant, Lys a is missing. Hence, covalent reaction can only take place with Lys b. Measurements over the appropriate pH range could be described by a single pK of 10.8. These values are 0.8-0.9 pK units higher than those previously obtained in experiments with band 3 in the red cell membrane (Kampmann et al. (1982) J. Membr. Biol. 70, 199-216).(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for the development of an intermonomeric asymmetry in the covalent binding of 4,4'-diisothiocyanatostilbene-2,2'-disulfonate to human erythrocyte band 3

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) studies have identified two oligomeric forms of band 3 whose proportions on gel profiles were modulated by the particular ligand occupying the intramonomeric stilbenedisulfonate site during intermonomeric cross-linking by BS3 [bis-(sulfosuccinimidyl) suberate] [Salhany et al. (1990) J. Biol. Chem. 265, 17688-17693]. When DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonate) was irreversibly attached to all monomers, BS3 covalent dimers predominated, while with DNDS (4,4'-dinitrostilbene-2,2'-disulfonate) present to protect the intramonomeric stilbenedisulfonate site from attack by BS3, a partially cross-linked band 3 tetramer was observed. In the present study, we investigate the structure of the protected stilbenedisulfonate site within the tetrameric complex by measuring the ability of patent monomers to react irreversibly with DIDS. Our results show two main populations of band 3 monomers present after reaction with DNDS/BS3: (a) inactive monomers resulting from the displacement of reversibly bound DNDS molecules and subsequent irreversible attachment of BS3 to the intramonomeric stilbenedisulfonate site and (b) residual, active monomers. All of the residual activity was fully inhibitable by DIDS under conditions of reversible binding, confirming expectations that all of the monomers responsible for the residual activity have patent stilbenedisulfonate sites. However, within this active population, two subpopulations could be identified: (1) monomers which were irreversibly reactive toward DIDS and (2) monomers which were refractory toward irreversible binding of DIDS at pH 6.9, despite being capable of binding DIDS reversibly. Increasing the pH to 9.5 during treatment of DNDS/BS3-modified cells with 300 microM DIDS did not cause increased irreversible transport inhibition relative to that seen for cells treated at pH 6.9.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics of DIDS inhibition of HL-60 cell anion exchange rules out ping-pong model with slippage

According to the ping-pong model of band 3-mediated anion exchange, the transport protein has a single transport site, which can exist in either an inward-facing or an outward-facing conformation. Anions bind to these unloaded forms of the carrier, and translocation takes place only when a suitable anion is bound to the transport site. In a previous paper [Am. J. Physiol. 257 (Cell Physiol. 26): C520-C527, 1989], we had shown that the substrate kinetics of Cl-Cl exchange in the promyelocytic HL-60 cell cannot be explained by this simple ping-pong model of anion exchange but is consistent with a simultaneous model according to which both extracellular and intracellular anions must bind before simultaneous translocation can take place. In the present paper we show that external 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) inhibits anion exchange in HL-60 cells by competing with Cl- for binding to the outward-facing transport site. Furthermore, there is a linear dependence of the slope of the Dixon plot for inhibition by DIDS on the reciprocal of the intracellular Cl- concentration. This result clearly rules out a simple ping-pong scheme. In addition, the data also rule out a ping-pong model in which some translocation of the unloaded carrier is allowed (ping-pong model with slippage). The observed inhibition kinetics can be modeled by a simultaneous model of Cl-Cl exchange with competitive inhibition by DIDS.

Identification by site-directed mutagenesis of Lys-558 as the covalent attachment site of H2DIDS in the mouse erythroid band 3 protein

After functional expression of mouse erythroid band 3 by cRNA microinjection into Xenopus oocytes, 36Cl- efflux is irreversibly inhibited by H2DIDS. When a cRNA is injected that is derived from a cDNA in which the nucleotides encoding for lysine-558 were replaced by nucleotides encoding for asparagine, transport and inhibition of transport by H2DIDS still occur. However, when measured under conditions where no intramolecular crosslinking takes place the inhibition by H2DIDS is no longer irreversible. This indicates that thiourea bond formation between H2DIDS and band 3 takes place at Lys-558.

The mechanisms of inhibition of anion exchange in human erythrocytes by 1-ethyl-3-[3-(trimethylammonio)propyl]carbodiimide

Treatment of human erythrocytes with the membrane-impermeant carbodiimide 1-ethyl-3-[3-(trimethylammonio)propyl]carbodiimide (ETC) in citrate-buffered sucrose leads to irreversible inhibition of phosphate-chloride exchange. The level of transport inhibition produced was dependent on the concentration of citrate present during treatment, with a maximum of approx. 60% inhibition. [14C]Citric acid was incorporated into Band 3 (Mr = 95,000) in proportion to the level of transport inhibition, reaching a maximum stoichiometry of 0.7 mol citrate per mol Band 3. The citrate label was localized to a 17 kDa transmembrane fragment of the Band 3 polypeptide. Citrate incorporation was prevented by the transport inhibitors 4,4'-diisothiocyano- and 4,4'-dinitrostilbene-2,2'-disulfonate. ETC plus citrate treatment also dramatically reduced the covalent labeling of Band 3 by [3H]4,4'-diisothiocyano-2,2'-dihydrostilbene disulfonate (3H2DIDS). Noncovalent binding of stilbene disulfonates to modified Band 3 was retained, but with reduced affinity. We propose that the inhibition of anion exchange in this case is due to carbodiimide-activated citrate modification of a lysine residue in the stilbenedisulfonate binding site, forming a citrate-lysine adduct that has altered transport function. The evidence is consistent with the hypothesis that the modified residue may be Lys a, the lysine residue involved in the covalent reaction with H2DIDS. Treatment of erythrocytes with ETC in the absence of citrate resulted in inhibition of anion exchange that reversed upon prolonged incubation. This reversal was prevented by treatment in the presence of hydrophobic nucleophiles, including phenylalanine ethyl ester. Thus, inhibition of anion exchange by ETC in the absence of citrate appears to involve modification of a protein carboxyl residue(s) such that both the carbodiimide- and the nucleophile-adduct result in inhibition.

Inhibition of chloride self-exchange with stilbene disulphonates in depolarized skeletal muscle of Rana temporaria

1. The inhibition of 36Cl efflux with stilbene disulphonates, SD, has been studied under conditions of chloride equilibrium in depolarized fibre bundles from frog semitendinosi. The chosen probes were the aminoreactive derivative SITS and the derivative DNDS with no aminoreactive group. SD were added to the medium during 36Cl efflux allowing the estimation of fractional inhibition after a single 36Cl loading. 2. Both probes inhibited chloride self-exchange reversibly within the pH range 5.5-9.5 under study. 3. At SD concentrations above the half-inhibition concentration the inhibition reached a steady level with a time lag equal to that required for extracellular fluid change. The time constant for reversibility upon the removal of SD increased with decreasing pH, but rapid reversibility always appeared upon an increase of pH to 7.2. These findings suggest that SD may enter the membrane at low pH, but that the inhibitory action is confined to superficial membrane sites. 4. The inhibitory power of both probes showed a pronounced pH dependence, pK approximately 7. The half-inhibition concentration increased about 6-7 times when pH was lowered one unit from the pK value. 5. The apparent affinity of SITS to the transport system was about 5 times higher than that of DNDS. The apparent dissociation constants at neutral pH were 8.5 x 10(-5) M (SITS) and 4.5 x 10(-4) M (DNDS). Both probes showed a maximal inhibition close to 100% at neutral pH and approximately 85% at pH 5.5. 6. The inhibition depended on the chloride concentration in a way consistent with competitive inhibition in both neutral and acid media. 7. The results are consistent with the classical model of anion transport in frog muscle, suggesting that SD and chloride may compete for binding to a site with increasing anion affinity upon protonation; the results do not, however, exclude that the conductive and the non-conductive chloride transport modes in frog muscle are mediated by separate SD-sensitive transport pathways.

Sulfate transport in apical membrane vesicles isolated from tracheal epithelium

Sulfate uptake in apical membrane vesicles isolated from bovine tracheal epithelium is shown to occur into an osmotically sensitive intravesicular space, via a carrier-mediated system. This conclusion is based on three lines of evidence: 1) saturation kinetics; 2) substrate specificity; and 3) inhibition by the anion transport inhibitors SITS and DIDS. The affinity of the transport system is highest in low ionic strength media (apparent Km = 0.13 mM) and decreases in the presence of gluconate (apparent Km = 0.68 mM). Chloride appears to cis-inhibit sulfate uptake and to trans-stimulate sulfate efflux. Cis-inhibition and trans-stimulation studies with a variety of anions indicate that this exchange system may be shared by HCO3-, S2O3(2-), SeO4(2-), and MoO4(2-) but not by H2PO4- or HAsO4(2-). Studies indicate that protons may play two distinct roles in sulfate transport in this system. 1) Their possible modifier role is suggested by the fact that protons affect SO2-4 transport in an uncompetitive manner. 2) The possibility that the proton gradient may act as an energy source for a secondary active transport is indicated by the fact that the imposition of a proton gradient stimulates a transient movement of sulfate in to the tracheal apical membrane vesicle, against its concentration gradient, causing an "overshoot" phenomenon. Our studies show that the carrier-mediated system can function in the absence of chloride. The overshoot observed in the presence of a proton gradient (OH- gradient) indicates that under those conditions the mechanism of transport may be a SO4(2-)-OH- exchange. The fact that chloride cis-inhibits and trans-stimulates SO4(2-) transport indicates that SO2-4 uptake may also occur via a SO4(2-)-Cl- exchange. Studies carried out so far do not enable us to conclude unequivocally whether the tracheal apical membrane system displays two distinct carrier activities (SO4(2-)-Cl-; SO4(2-)-OH-) or one anion exchanger, which like the erythrocyte anion exchanger, may interact with SO4(2-), Cl-, and H+. The fact that the anion transport inhibitors DIDS and SITS inhibit SO4(2-) transport in the presence or absence of chloride suggests that the latter possibility may be the case.

Characterization of eosin 5-isothiocyanate binding site in band 3 protein of the human erythrocyte

The characteristics of the anion transport system in human erythrocyte, which can be modified by eosin 5-isothiocyanate (EITC), were studied using the pH titration method and by measuring the sulfate efflux. Based on the pH dependence of EITC binding to the erythrocyte ghosts, it was found that the reaction rate was maximal at about pH 6.4, and that the pH profile of EITC binding was similar to that of divalent anion transport. The interaction between EITC and ghosts was interpreted by a two-step reaction, a fast ionic-binding reaction and a slow covalent-binding reaction. The induced CD spectrum of the EITC-ghost system was also dependent on pH. The intensity of the CD band at 530 nm was decreased in acidic pH region, and the inflection point was observed at about pH 6.3, indicating a participation of the histidine residue in the interaction of EITC with band 3. In order to characterize the EITC-binding site, the kinetics of sulfate efflux in intact and EITC-modified cells were examined at various pH values. The inhibitory effect of EITC was dependent on pH. From the experimental results, the followings are suggested. The rate of ionic interaction in the early stage is much slower than that in a general ionic reaction. A conformational change may participate in the reaction. The conformation of the EITC-binding site depends on pH, relating to the dissociation of the histidine residues. The EITC molecules act also as a competitive inhibitor to the sulfate efflux after binding covalently to band 3 protein.

Interactions of inhibitors on anion transporter of human erythrocyte

Chloride tracer efflux was measured from intact human erythrocytes into media containing different chloride concentrations and different concentrations of the inhibitors 4,4'-dinitrostilbene-2-2'-disulfonate (DNDS), N-(4-azido-2-nitrophenyl)-2-aminoethylsulfonate (NAP-taurine), phloretin, and sulfate. The data were analyzed to test whether these inhibitors were mutually exclusive with each other or whether they could bind at the same time. Under the assumption that mutual exclusiveness is due to steric interference, the data can be used to map out the protein surface near the outward-facing anion binding-transport site. It is concluded that there are separate domains for NAP taurine and phloretin that do not overlap with each other or with the chloride binding site. These two domains do, however, overlap with the binding domain for DNDS that, in addition, excludes the binding of chloride and sulfate.

A new method for the reconstitution of the anion transport system of the human erythrocyte membrane

The anion transport protein of the human erythrocyte membrane, band 3, was solubilized and purified in solutions of the non-ionic detergent Triton X-100. It was incorporated into spherical lipid bilayers by the following procedure: Dry phosphatidylcholine was suspended in the protein solution. Octylglucopyranoside was added until the milky suspension became clear. The sample was dialyzed overnight against detergent-free buffer. Residual Triton X-100 was removed from the opalescent vesicle suspension by sucrose density gradient centrifugation and subsequent dialysis. Sulfate efflux from the vesicles was studied, under exchange conditions, using a filtration method. Three vesicle subpopulations could be distinguished by analyzing the time course of the efflux. One was nearly impermeable to sulfate, and efflux from another was due to leaks. The largest subpopulation, however, showed transport characteristics very similar to those of the anion transport system of the intact erythrocyte membrane: transport numbers (at 30 degrees C) close to 20 sulfate molecules per band 3 and min, an activation energy of approx. 140 kJ/mol, a pH maximum at pH 6.2, saturation of the sulfate flux at sulfate concentrations around 100 mM, inhibition of the flux by H2DIDS and flufenamate (approx. KI-values at 30 degrees C: 0.1 and 0.7 microM, respectively), and "right-side-out" orientation of the transport protein (as judged from the inhibition of sulfate efflux by up to 98% by externally added H2DIDS). Thus, the system represents, for the first time, a reconstitution of all the major properties of the sulfate transport across the erythrocyte membrane.

Transmembrane effects of intracellular chloride on the inhibitory potency of extracellular H2DIDS. Evidence for two conformations of the transport site of the human erythrocyte anion exchange protein

The ping-pong model for the red cell anion exchange system postulates that the transport protein band 3 can exist in two different conformations, one in which the transport site faces the cytoplasm (Ei) and another in which it faces the outside medium (Eo). This model predicts that an increase in intracellular chloride should increase the fraction of sites in the outward-facing, unloaded form (Eo). Since external H2DIDS is a competitive inhibitor of chloride exchange that does not cross the membrane, it must bind only to the Eo form. Thus, an increase in Eo should cause an increase in H2DIDS inhibition. When intracellular chloride was increased at constant extracellular chloride, the inhibitory potency of H2DIDS rose, as predicted by the ping-pong model. This increase was not due to the concomitant changes in intracellular pH or membrane potential. When the chloride gradient was reversed, the inhibitory potency of H2DIDS decreased, again in qualitative agreement with the ping-pong model. These data provide support for the ping-pong model and also demonstrate that chloride gradients can be used to change the orientation of the transport protein.

Inhibition of anion transport across the red cell membrane by dinitrophenylation of a specific lysine residue at the H2DIDS binding site of the band 3 protein

The inhibition of anion transport by dinitrophenylation of the red cell membrane is brought about by the modification of a single lysine residue located on the 17-kDa segment of the band 3 protein. This residue is identical with Lys a, which is also capable of reacting with one of the two isothiocyanate groups of 4,4'-diisothiocyano dihydro-stilbene-2,2'-disulfonate (H2DIDS). The rate of reaction between Lys a and 1-fluoro-2,4-dinitrobenzene is reduced when a second lysine residue on the 35-kDa segment of the band 3 protein becomes dinitrophenylated. This latter residue is not identical with Lys b which is known to be present on the 35-kDa segment and involved in the cross-linking of this segment with the 17-kDa segment by H2DIDS.