Band 3-hemoglobin associations. The band 3 tetramer is the oxyhemoglobin binding site

Hemoglobin Binding to the Red Blood Cell (RBC) Membrane Is Associated with Decreased Cell Deformability

The deformability of red blood cells (RBCs), expressing their ability to change their shape as a function of flow-induced shear stress, allows them to optimize oxygen delivery to the tissues and minimize their resistance to flow, especially in microcirculation. During physiological aging and blood storage, or under external stimulations, RBCs undergo metabolic and structural alterations, one of which is hemoglobin (Hb) redistribution between the cytosol and the membrane. Consequently, part of the Hb may attach to the cell membrane, and although this process is reversible, the increase in membrane-bound Hb (MBHb) can affect the cell's mechanical properties and deformability in particular. In the present study, we examined the correlation between the MBHb levels, determined by mass spectroscopy, and the cell deformability, determined by image analysis. Six hemoglobin subunits were found attached to the RBC membranes. The cell deformability was negatively correlated with the level of four subunits, with a highly significant inter-correlation between them. These data suggest that the decrease in RBC deformability results from Hb redistribution between the cytosol and the cell membrane and the respective Hb interaction with the cell membrane.

Studying Macromolecular Interactions of Cellular Machines by the Combined Use of Analytical Ultracentrifugation, Light Scattering, and Fluorescence Spectroscopy Methods

Cellular machines formed by the interaction and assembly of macromolecules are essential in many processes of the living cell. These assemblies involve homo- and hetero-associations, including protein-protein, protein-DNA, protein-RNA, and protein-polysaccharide associations, most of which are reversible. This chapter describes the use of analytical ultracentrifugation, light scattering, and fluorescence-based methods, well-established biophysical techniques, to characterize interactions leading to the formation of macromolecular complexes and their modulation in response to specific or unspecific factors. We also illustrate, with several examples taken from studies on bacterial processes, the advantages of the combined use of subsets of these techniques as orthogonal analytical methods to analyze protein oligomerization and polymerization, interactions with ligands, hetero-associations involving membrane proteins, and protein-nucleic acid complexes.

Comodulation of NO-Dependent Vasodilation by Erythroid Band 3 and Hemoglobin: A GP.Mur Athlete Study

GP.Mur, a red blood cell (RBC) hybrid protein encoded by glycophorin B-A-B, increases expression of erythroid band 3 (Anion Exchanger-1, SLC4A1). GP.Mur is extremely rare but has a prevalence of 1–10% in regions of Southeast Asia. We unexpectedly found slightly higher blood pressure (BP) among healthy Taiwanese adults with GP.Mur. Since band 3 has been suggested to interact with hemoglobin (Hb) to modulate nitric oxide (NO)-dependent hypoxic vasodilation during the respiratory cycle, we hypothesized that GP.Mur red cells could exert differentiable effects on vascular tone. Here we recruited GP.Mur-positive and GP.Mur-negative elite male college athletes, as well as age-matched, GP.Mur-negative non-athletes, for NO-dependent flow-mediated dilation (FMD) and NO-independent dilation (NID). The subjects were also tested for plasma nitrite and nitrate before and after arterial occlusion in FMD. GP.Mur+ and non-GP.Mur athletes exhibited similar heart rates and blood pressure, but GP.Mur+ athletes showed significantly lower FMD (4.8 ± 2.4%) than non-GP.Mur athletes (6.5 ± 2.1%). NO-independent vasodilation was not affected by GP.Mur. As Hb controls intravascular NO bioavailability, we examined the effect of Hb on limiting FMD and found it to be significantly stronger in GP.Mur+ subjects. Biochemically, plasma nitrite levels were directly proportional to individual band 3 expression on the red cell membrane. The increase of plasma nitrite triggered by arterial occlusion also showed small dependency on band 3 levels in non-GP.Mur subjects. By the GP.Mur comparative study, we unveiled comodulation of NO-dependent vasodilation by band 3 and Hb, and verified the long-pending role of erythroid band 3 in this process.

Erythrocyte detergent-resistant membrane proteins: their characterization and selective uptake during malarial infection

Infection of human erythrocytes by the apicomplexan malaria parasite Plasmodium falciparum results in endovacuolar uptake of 4 host proteins that reside in erythrocyte detergent-resistant membranes (DRMs). Whether this vacuolar transport reflects selective uptake of host DRM proteins remains unknown. A further complication is that DRMs of vastly different protein and cholesterol contents have been isolated from erythrocytes. Here we show that isolated DRMs containing the highest cholesterol-to-protein ratio have low protein mass. Liquid chromatography, mass spectrometry, and antibody-based studies reveal that the major DRM proteins are band 3, flotillin-1 and -2, peroxiredoxin-2, and stomatin. Band 3 and stomatin, which reflect the bulk mass of erythrocyte DRM proteins, and all tested non-DRM proteins are excluded from the vacuolar parasite. In contrast, flotillin-1 and -2 and 8 minor DRM proteins are recruited to the vacuole. These data suggest that DRM association is necessary but not sufficient for vacuolar recruitment and there is active, vacuolar uptake of a subset of host DRM proteins. Finally, the 10 internalized DRM proteins show varied lipid and peptidic anchors indicating that, contrary to the prevailing model of apicomplexan vacuole formation, DRM association, rather than lipid anchors, provides the preferred criteria for protein recruitment to the malarial vacuole.

Regulation of nitric oxide consumption by hypoxic red blood cells

The homeostasis of nitric oxide (NO) is attained through a balance between its production and consumption. Shifts in NO bioavailability have been linked to a variety of diseases. Although the regulation of NO production has been well documented, its consumption is largely thought to be unregulated. Here, we have demonstrated that under hypoxic conditions, NO accelerates its own consumption by increasing its entry into RBCs. When RBCs were exposed to NO (1:400 NO/heme ratio) under hypoxic conditions to form HbFe(II)NO, the consumption rate of NO increased significantly. This increase in NO consumption converted the bioactivity of serotonin from a vasodilator to a vasoconstrictor in isolated coronary arterioles. We identified HbFe(II)NO as a potential mediator of accelerated NO consumption. Accelerated NO consumption by HbFe(II)NO-bearing RBCs may contribute to hypoxic pulmonary vasoconstriction and the rebound effect seen on termination of NO inhalation therapy. Furthermore, accelerated NO consumption may exacerbate ischemia-mediated vasospasm and nitrate tolerance. Finally, this phenomenon may be an evolved mechanism to stabilize the vasculature in sepsis.

Preparation and analysis of small unilamellar phospholipid vesicles of a uniform size

The interaction of carbonmonoxyhemoglobin and heme with small unilamellar phospholipid vesicles was studied using dynamic light scattering. Addition of carbonmonoxyhemoglobin to dimyristoylphosphatidylcholine:dimyristoylphosphatidylserine small unilamellar vesicles resulted in an increase of average vesicle size from 17.4 to 32.0nm. Addition of heme to vesicles produced a smaller size increase, from 17.4 to 21.0nm. Also reported is a method for preparing small unilamellar lipid vesicles of a uniform size, suitable for use in NMR spectroscopy.

The complex of band 3 protein of the human erythrocyte membrane and glyceraldehyde-3-phosphate dehydrogenase: stoichiometry and competition by aldolase

The cytoplasmic domain of band 3, the main intrinsic protein of the erythrocyte membrane, possesses binding sites for a variety of other proteins of the membrane and the cytoplasm, including the glycolytic enzymes glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and aldolase. We have studied the stoichiometry of the complexes of human band 3 protein and GAPDH and the competition by aldolase for the binding sites. In addition, we have tried to verify the existence of mixed band 3/GAPDH/aldolase complexes, which could represent the nucleus of a putative glycolytic multienzyme complex on the erythrocyte membrane. The technique applied was analytical ultracentrifugation, in particular sedimentation equilibrium analysis, on mixtures of detergent-solubilized band 3 and dye-labelled GAPDH, in part of the experiments supplemented by aldolase. The results obtained were analogous to those reported for the binding of hemoglobin, aldolase and band 4.1 to band 3: (1) the predominant or even sole band 3 oligomer forming the binding site is the tetramer. (2) The band 3 tetramer can bind up to four tetramers of GAPDH. (3) The band 3/GAPDH complexes are unstable. (4) Artificially stabilized band 3 dimers also represent GAPDH binding sites. In addition it was found that aldolase competes with GAPDH for binding to the band 3 tetramer, and that ternary complexes of band 3 tetramers, GAPDH and aldolase do exist.

Hydroxytamoxifen interaction with human erythrocyte membrane and induction of permeabilization and subsequent hemolysis

4-Hydroxytamoxifen (OHTAM) is the most active metabolite of the widely prescribed anticancer drug tamoxifen (TAM) used in breast cancer therapy. This work describes the effects of OHTAM on isolated human erythrocytes, using standardized test conditions, to check for a putative contribution to the TAM-induced hemolysis and to study basic mechanisms involved in the interaction of OHTAM with cell membranes. Incubation of isolated human erythrocytes with relatively high concentrations of OHTAM results in a concentration-dependent hemolysis, its hemolytic effect being about one-third of that induced by TAM. OHTAM-induced hemolysis is prevented by either alpha-tocopherol (alpha-T) or alpha-tocopherol acetate (alpha-TAc) and it occurs in the absence of oxygen consumption and hemoglobin oxidation, ruling out the oxidative damage of erythrocytes. However, OHTAM remarkably increases the osmotic fragility of erythrocytes, increasing the susceptibility of erythrocytes to hypotonic lysis. Additionally, the hemoglobin release induced by OHTAM is preceded by a rapid efflux of intracellular K(+). Therefore, our data suggest that OHTAM-induced hemolysis does not contribute to TAM-induced hemolytic anemia and it is a much weaker toxic drug as compared with TAM. Moreover, at variance with the membrane disrupting effects of TAM, OHTAM promotes perturbation of the membrane's backbone region due to its strong binding to proteins with consequent formation of membrane paths of permeability to small solutes and retention of large solutes like hemoglobin, followed by osmotic swelling and cell lysis. The prevention of OHTAM-induced hemolysis by alpha-T and alpha-TAc is probably committed to the permeability sealing resulting from structural stabilization of membrane.

The reduction of nitroblue tetrazolium by red blood cells: a measure of Red Cell membrane antioxidant capacity and hemoglobin-membrane binding sites

The reduction of nitroblue tetrazolium (NBT) with intact Red Blood Cells (RBCs) is biphasic with an initial rapid reduction followed by a slower second phase. This biphasic kinetics has been explained with the initial rapid phase attributed to antioxidants in the red cell which reduce membrane bound NBT and the slower phase associated with the reaction of NBT with membrane bound hemoglobin. This model has been confirmed by a utilization of a number of red cell modifications which either increase the red cell antioxidants (vitamin C and vitamin E) or damage the red cell membrane (cumene hydroperoxide and N-ethylmaleimide). The utilization of this assay for human blood samples was investigated by studying a series of 20 human subjects ranging between 34 and 87 years of age. It was possible to fit all of these samples with two adjustable parameters which reflect the red cell membrane antioxidant capacity (x) and the hemoglobin membrane interactions (m). The antioxidant capacity shows a significant (p < .002; R = -.67) decrease with age. This finding is consistent with a decrease in the level of antioxidants in aged subjects. In addition, the number of hemoglobin membrane sites are negatively correlated with the antioxidant capacity (p < .02; R = -.52) suggesting that the oxidative stress associated with reduced antioxidants results in increased hemoglobin-membrane interactions.

Influence of oxidation and crosslinking on oxygen binding properties of mouse erythrocytes

Different chemical treatments for mouse erythrocyte modification has been used. Oxidation treatments with Ascorbate/Fe(3+), a system able to react with intracellular proteins, produced a displacement of the O(2) binding equilibrium curve to a higher affinity behaviour with loss of the haemoglobin cooperativity for oxygen binding. Incubation of mouse erythrocytes with diamide showed that at low reagent concentration (0.8 mM) no modification on oxygen binding equilibrium curves was observed. At higher reagent concentration (2.0 mM), an increased affinity and a disappearance of the cooperative behaviour can be observed. Additionally, crosslinking reactions on mouse erythrocytes with band 3 crosslinkers seemed to affect oxygen binding properties when used at a crosslinker concentration of 5 mM. Oxyhaemoglobin levels in crosslinked and diamide-treated erythrocytes are similar to those found in control cells. In contrast, ascorbate/Fe(3+) treatments produced an increment in the proportion of methaemoglobin, decreasing the oxyhaemoglobin levels in these oxidized erythrocytes.

Hemolysis of human erythrocytes induced by tamoxifen is related to disruption of membrane structure

Tamoxifen (TAM), the antiestrogenic drug most widely prescribed in the chemotherapy of breast cancer, induces changes in normal discoid shape of erythrocytes and hemolytic anemia. This work evaluates the effects of TAM on isolated human erythrocytes, attempting to identify the underlying mechanisms on TAM-induced hemolytic anemia and the involvement of biomembranes in its cytostatic action mechanisms. TAM induces hemolysis of erythrocytes as a function of concentration. The extension of hemolysis is variable with erythrocyte samples, but 12.5 microM TAM induces total hemolysis of all tested suspensions. Despite inducing extensive erythrocyte lysis, TAM does not shift the osmotic fragility curves of erythrocytes. The hemolytic effect of TAM is prevented by low concentrations of alpha-tocopherol (alpha-T) and alpha-tocopherol acetate (alpha-TAc) (inactivated functional hydroxyl) indicating that TAM-induced hemolysis is not related to oxidative membrane damage. This was further evidenced by absence of oxygen consumption and hemoglobin oxidation both determined in parallel with TAM-induced hemolysis. Furthermore, it was observed that TAM inhibits the peroxidation of human erythrocytes induced by AAPH, thus ruling out TAM-induced cell oxidative stress. Hemolysis caused by TAM was not preceded by the leakage of K(+) from the cells, also excluding a colloid-osmotic type mechanism of hemolysis, according to the effects on osmotic fragility curves. However, TAM induces release of peripheral proteins of membrane-cytoskeleton and cytosol proteins essentially bound to band 3. Either alpha-T or alpha-TAc increases membrane packing and prevents TAM partition into model membranes. These effects suggest that the protection from hemolysis by tocopherols is related to a decreased TAM incorporation in condensed membranes and the structural damage of the erythrocyte membrane is consequently avoided. Therefore, TAM-induced hemolysis results from a structural perturbation of red cell membrane, leading to changes in the framework of the erythrocyte membrane and its cytoskeleton caused by its high partition in the membrane. These defects explain the abnormal erythrocyte shape and decreased mechanical stability promoted by TAM, resulting in hemolytic anemia. Additionally, since membrane leakage is a final stage of cytotoxicity, the disruption of the structural characteristics of biomembranes by TAM may contribute to the multiple mechanisms of its anticancer action.

Hemolysis of human erythrocytes with saponin affects the membrane structure

Incubation of cells and tissues with saponin makes the lipid bilayer permeable to macromolecules. Ghosts (membrane preparations) of saponin-lysed erythrocytes do not reseal, thus indicating an irreversible damage of the lipid bilayer. We investigated the influence of disturbance of the lipid bilayer on membrane proteins by comparing ghosts of saponin-lysed erythrocytes with ghosts of cells lysed in hypotonic buffer. Transmission electron microscopy revealed destruction of the lipid bilayer and emergence of multilamellar buds in saponin-lysed ghosts. Freeze-fracture electron microscopy showed regions with crystalline lipids and an increase in particle-free areas on fracture faces. The number of protein sulfhydryl groups and the binding of hemoglobin were diminished in saponin-lysed ghosts. A Scatchard plot of hemoglobin binding revealed the decrease of high affinity binding sites. All these results indicate an aggregation of band 3 protein also demonstrated by laser scanning microscopy after incubation of cells labelled with eosin-5-maleimide with sublytic concentration of saponin. Hemolysis with saponin also affected the interaction between transmembrane proteins and the cytoskeleton. Dissociation of peripheral membrane proteins by incubation of ghosts in low salt buffer or by blocking sulfhydryl groups was increased and the association of spectrin with spectrin-depleted vesicles was decreased. The increased incorporation of the fluorescent probe Merocyanine 540 into saponin-lysed ghosts and the increased relative fluorescence quantum yield confirmed the perturbation of the lipid bilayer and the changed interaction between membrane lipids and intrinsic membrane proteins. Our results suggest that permeabilization of the lipid bilayer with saponin to admit the access of antibodies to the cytoplasmic surface of cells can aggregate transmembrane proteins and affect the immunocytochemical localization of associated proteins of the cytoskeleton.

Hydrodynamic properties of human erythrocyte band 3 solubilized in reduced Triton X-100

The oligomeric state and function of band 3, purified by sulfhydryl affinity chromatography in reduced Triton X-100, was investigated. Size exclusion high-performance liquid chromatography showed that a homogeneous population of band 3 dimers could be purified from whole erythrocyte membranes. The elution profile of band 3 purified from membranes that had been stripped of its cytoskeleton before solubilization was a broad single peak describing a heterogeneous population of oligomers with a mean Stokes radius of 100 A. Sedimentation velocity ultracentrifugation analysis confirmed particle heterogeneity and further showed monomer/dimer/tetramer equilibrium self-association. Whether the conversion of dimer to the form described by a Stokes radius of 100 A was initiated by removal of cytoskeletal components, alkali-induced changes in band 3 conformation, or alkali-induced loss of copurifying ligands remains unclear. After incubation at 20 degrees C for 24 h, both preparations of band 3 converted to a common form characterized by a mean Stokes radius of 114 A. This form of the protein, examined by equilibrium sedimentation ultracentrifugation, is able to self-associate reversibly, and the self-association can be described by a dimer/tetramer/hexamer model, although the presence of higher oligomers cannot be discounted. The ability of the different forms of the protein to bind stilbene disulfonates revealed that the dimer had the highest inhibitor binding affinity, and the form characterized by a mean Stokes radius of 114 A to have the lowest.

Characterization of the pH dependence of hemoglobin binding to band 3. Evidence for a pH-dependent conformational change within the hemoglobin-band 3 complex

The pH dependence of hemoglobin binding to inside-out red cell membrane vesicles was studied using 90 degrees light scattering (Salhany, J.M. et al., Biochemistry 19 (1980) 1447-1454). Hyperbolic binding curves were observed for high-affinity hemoglobin binding to the cytoplasmic domain of band 3 (CDB3) within the intact transporter. Analysis of these saturation curves yielded the apparent Kd and the maximum light scattering signal change (DeltaLSmax ). The apparent Kd for hemoglobin binding did not change substantially between pH 5.5 and 7.0, while at pH 8, there is no binding. In contrast, DeltaLSmax decreased by about 20-fold between pH 5.5 and 7.0, with an apparent pK of 6.5. These results suggest that hemoglobin binds to CDB3 with high affinity at both neutral and acid pH, a suggestion that was confirmed using a centrifugation method. Thus, the pK for the light scattering signal change is significantly lower than the pK for the actual binding process. We show that the change in DeltaLSmax is not related to a change in band 3 binding capacity, which remains constant as a function of pH. We also show that hemoglobin binding to non-band 3 sites contributes less than 10% to DeltaLSmax under our specific experimental conditions. On the basis of these and previous fluorescence quenching results in the literature, we propose a new model for hemoglobin binding to band 3, where raising the pH from 6 and 7 causes the CDB3-hemoglobin complex to undergo a conformational change leading to the movement of the bound hemoglobin away from the surface of the bilayer. The possible implication of this new mechanistic interpretation is discussed briefly.

Cytoskeleton-membrane connections in the human erythrocyte membrane: band 4.1 binds to tetrameric band 3 protein

Band 4.1 provides, besides ankyrin, the main linkage between the erythrocyte membrane and its cytoskeleton. Its predominant binding sites in the membrane are located on the glycophorins. However, the cytoplasmic domain of band 3 can also bind band 4.1. We have studied which of the different band 3 oligomers observed (monomers, dimers, tetramers) can act as band 4.1 binding sites, by equilibrium sedimentation experiments on mixtures of purified band 3 and dye-labelled band 4.1 in solutions of a nonionic detergent. At low molar ratios of band 4.1 and band 3, the sedimentation equilibrium distributions obtained could all be perfectly fitted assuming that only two dye-labelled particles were present: uncomplexed band 4.1 and a complex formed between one band 4.1 molecule and one band 3 tetramer. The presence of small amounts of complexes containing band 3 monomers or dimers could not be completely ruled out but is unlikely. On the other hand, stabilized band 3 dimers effectively bound band 4.1. At higher molar band 4.1/band 3 ratio, the band 3 tetramer apparently could bind up to at least four band 4.1 molecules. The band 4.1/band 3 tetramer complex was found to be unstable. The results described, together with those reported previously, point at a prominent role of tetrameric band 3 in ligand binding.

Associations between erythrocyte band 3 protein and aldolase in detergent solution. Determining their stoichiometry by analytical ultracentrifugation

The cytoplasmic domain of band 3, the predominant polypeptide of the erythrocyte membrane, represents a binding site for certain glycolytic enzymes. We have studied the association between human band 3 protein and aldolase, in order to clarify the role of the different band 3 oligomers as ligand binding sites. The experiments were performed on mixtures of solubilized band 3 and aldolase in solutions of a nonionic detergent, nonaethyleneglycol lauryl ether. The main technique applied was sedimentation equilibrium analysis in an analytical ultracentrifuge. In addition, nonequilibrium centrifugation techniques were used. To facilitate the evaluations, the aldolase was labelled with a dye. The following results were obtained. (1) With unmodified band 3, aldolase is bound exclusively or at least predominantly to the band 3 tetramer (but not to monomers or dimers). (2) The band 3 tetramer can bind up to four aldolase tetramers. (3) The band 3 tetramer/aldolase complex is unstable on the time scale of the techniques used. (4) Stable band 3 dimers (stabilized either covalently or noncovalently) can also associate with aldolase and can bind up to two aldolase tetramers. The results described, together with those reported previously, point at a prominent role of the band 3 tetramer in ligand binding.

A tethered adhesive particle model of two-dimensional elasticity and its application to the erythrocyte membrane

A new model of two-dimensional elasticity with application to the erythrocyte membrane is proposed. The system consists of a planar array of self-adhesive particles attached to nearest neighbors with flexible tethers. Stretching from the equilibrium dimension is resisted because force is required to dissociate the particle clusters and to decrease the distribution entropy. Release of the external force is accompanied by a contraction as thermal diffusion randomizes the particles and allows interparticle attachments to form again. Analysis of membrane thermodynamics and mechanics under the two-state particle assumption results in a shear softening stress-strain relation. The shear modulus is found proportional to the square root of the surface density of particles, the interparticle adhesive energy, and is inversely proportional to the tether length. Applied to the erythrocyte membrane under the assumption that band 3 tetramer represents the particle and spectrin the tether, the shear modulus predicted corresponds to the measured value when the interparticle adhesive energy is approximately 4.0-5.9 kT, where kT is the Boltzmann constant multiplied by the temperature. This model suggests a mechanism wherein erythrocyte membrane deformability depends on integral protein homomultimeric interactions and can be modulated from the external surface.

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.

Near-UV circular dichroism of band 3. Evidence for intradomain conformational changes and interdomain interactions

Near-UV circular dichroism (CD) was used to identify differences in the tertiary structure of human erythrocyte band 3, the chloride/bicarbonate exchange protein, consequent to covalent binding of anion transport inhibitors to the intramonomeric stilbenedisulfonate (ISD) site. Isolated intact band 3 and its membrane domain (B3MD) were compared. Spectral differences were observed which involved intradomain effects, in that they were seen both with intact band 3 and with B3MD, or interdomain effects, in that they were observed only for B3MD, but were inhibited when the cytoplasmic domain was attached. The intradomain effect involved a significant loss in optical activity in the Phe/Tyr region of the spectrum below 280 nm. It was seen only when the ISD site had stilbenedisulfonates bound covalently at pH 7.4. Raising the pH to 9.6 after adduct formation "normalized" this spectral change irreversibly. The interdomain effect was identified in the Trp spectral region at 292 nm. There was a significant increase in optical activity at 292 nm when bulky covalent ligands such as DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonate) were bound to B3MD, but not when the same ligands were bound to intact band 3. These latter results offer evidence that certain aspects of the conformational response of the integral domain are inhibited by the presence of an attached cytoplasmic domain. The potential significance of interdomain interactions to band 3 function is discussed briefly.