Calorimetric evidence for allosteric subunit interactions associated with inhibitor binding to band 3 transporter

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.

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

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

Mechanistic basis for site-site interactions in inhibitor and substrate binding to band 3 (AE1): evidence distinguishing allosteric from electrostatic effects

Kinetic studies suggest that stilbenedisulfonates inhibit erythrocyte anion exchange by competing with substrate anions for binding to band 3 (AE1). Such competition seems to involve site-site interactions between distinct inhibitor and substrate binding sites. The molecular basis for site-site interactions could be allosteric or electrostatic. In this paper, inhibitor binding kinetic studies are reviewed, and 35Cl(-) NMR line-broadening experiments are presented, both of which seem to rule out an electrostatic hypothesis. The results are consistent with an allosteric site-site interaction mechanism in the binding of stilbenedisulfonate and substrate anions to band 3.

Stilbenedisulfonate Binding Kinetics to Band 3 (AE 1): Relationship between Transport and Stilbenedisulfonate Binding Sites and Role of Subunit Interactions in Transport

Stilbenedisulfonates are competitive inhibitors of band 3 (AE1) anion exchange. It has been assumed that competitive binding implies that the stilbenedisulfonates bind to the transport site. In this paper, I summarize briefly an extensive series of stopped-flow fluorescence kinetic studies which indicate that stilbenedisulfonates do not compete with substrate anions directly, but rather that they behave as allosteric competitive inhibitors of monovalent anion binding to band 3. Monovalent anions lower stilbenedisulfonate affinity by accelerating the rate of their release from band 3, without changing the value of the initial second-order "on" kinetic constant. In addition, partial covalent labeling of the band 3 population with stilbenedisulfonates revealed subunit interaction effects: (a) in steady-state and pre-steady-state transport kinetic studies, (b) in studies on the kinetics of reversible stilbenedisulfonate binding to the unlabeled portion of the band 3 population, and (c) in microcalorimetric studies of the thermal unfolding of the membrane domain of band 3. Studies on the kinetics of reversible stilbenedisulfonate binding to erythrocyte membranes from an individual with Southeast Asian ovalocytosis also revealed subunit interaction effects. The demonstration of allosteric competition between stilbenedisulfonates and substrate anions and the observation of numerous examples of subunit interaction effects suggest that allosteric effects may play a significant role in band 3 function.

Anion binding characteristics of the band 3 / 4,4'-dibenzamidostilbene-2,2'-disulfonate binary complex: Evidence for both steric and allosteric interactions

A novel kinetic approach was used to measure monovalent anion binding to better define the mechanistic basis for competition between stilbenedisulfonates and transportable anions on band 3. An anion-induced acceleration in the release of 4,4prime-dibenzamidostilbene-2,2prime-disulfonate (DBDS) from its complex with band 3 was measured using monovalent anions of various size and relative affinity for the transport site. The K1/2 values for anion binding were determined and correlated with transport site affinity constants obtained from the literature and the dehydrated radius of each anion. The results show that anions with ionic radii of 120-200 pm fall on a well-defined correlation line where the ranking of the K1/2 values matched the ranking of the transport site affinity constants (thiocyanate < nitrate equivalent to bromide < chloride < fluoride). The K1/2 values for the anions on this line were about 4-fold larger than expected for anion binding to inhibitor-free band 3. Such a lowered affinity can be explained in terms of allosteric site-site interactions, since the K1/2 values decreased with increasing anionic size. In contrast, iodide, with an ionic radius of about 212 pm, had a 10-fold lower affinity than predicted by the correlation line established by the smaller monovalent anions. These results indicate that smaller monovalent anions have unobstructed access to the transport site within the band 3 / DBDS binary complex, while iodide experiences significant steric hindrance when binding. The observation of steric hindrance in iodide binding to the band 3 / DBDS binary complex, but not in the binding of smaller monovalent anions, suggests that the stilbenedisulfonate binding site is located at the outer surface of an access channel leading to the transport site.Key words: band 3, anion transport, membrane protein structure, red cell membrane.

Glucose-induced thermal stabilization of the native conformation of GLUT 1

The glucose transporter, GLUT 1, was purified from erythrocyte membranes and incorporated into vesicles of erythrocyte lipids. These protein-containing vesicles were studied with differential scanning calorimetry. It was found that the protein underwent an irreversible denaturation at 68.5 +/- 0.2 degreesC (at a scan rate of 0.25 degreesC/min) which was shifted to 72.6 +/- 0.2 degreesC in the presence of 500 mM D-glucose, while 500 mM L-glucose or 10 microM cytochalasin B did not produce a significant shift. The calorimetric enthalpy was found to be 150 kcal/mol, independent of the presence of D-glucose. On a weight basis this value is lower than that for soluble proteins, but it is comparable to values obtained with other integral membrane proteins. The van't Hoff enthalpy is similar to the calorimetric enthalpy, within the experimental error, indicating that the transition is not likely to be cooperative. The activation energy is estimated from both the scan rate dependence of the transition temperature and from the shape of the DSC curve. The presence of 500 mM D-glucose slightly decreases the activation energy. It is concluded that the shift to a higher denaturation transition temperature in the presence of D-glucose is not a result of increased kinetic stability of GLUT 1.

Gel filtration chromatographic studies of the isolated membrane domain of band 3

We have investigated the oligomeric state of the membrane domain of band 3 (MDB3) in non-ionic detergent solution using Sepharose CL-4B gel filtration chromatography to study the hydrodynamic properties of the protein as a function of its concentration. The studies were performed in a C12E9 (polyoxyethylene-9-lauryl ether) buffer containing phosphatidylcholine and sodium chloride, which significantly slow a dilution-induced band 3 conformational change, and an associated aggregation process. Under these conditions native MDB3 eluted predominantly as a single Gaussian peak with a Stokes radius of 76 +/- 14 A, at all protein concentrations studies between 0.2 and 12 microM. This value agrees with the calculated Stokes radius (74 A) determined from the crystal structure of the MDB3 dimer. The Stokes radius of the MDB3 monomer was obtained experimentally by treating native MDB3 with 0.5% SDS, and exchanging the SDS for C12E9 on the Sepharose column. SDS-treated MDB3 showed two peaks whose ratio was strongly dependent on applied protein concentration. The peak representing the largest material had a Stokes radius of 69.7 +/- 14 A, which is essentially the same as the native MDB3 dimer. The peak representing the smaller material had a Stokes radius of 36 +/- 9 A, and was assigned as the MDB3 monomer in C12E9. Evidence is discussed which indicates that the C12E9 monomer specifically self-associates to form a functional MDB3 dimer. We conclude that native MDB3 exists as a stable dimer in mixed micellar solutions composed of C12E9 and phosphatidylcholine, and that the dimer can be dissociated to monomers only by denaturation.

Interactions between mutant and wild-type band 3 subunits in hereditary Southeast Asian ovalocytic red blood cell membranes

Red cell membranes from individuals with Southeast Asian ovalocytosis (SAO) contain approximately equal proportions of wild-type band 3 and a mutant SAO band 3 which lacks residues 400-408. It is known that the Vmax for anion exchange in SAO cells is reduced by about 50%, that SAO band 3 does not transport anions when expressed alone in a cellular expression system, that SAO band 3 does not bind stilbenedisulfonates, and that about 50% of the band 3 exists as wild-type/SAO heterodimers. In this report, we show that the kinetics of H2DIDS (4,4'-diisothiocyanatodihydro-2,2'-stilbenedisulfonate) release from the wild-type band 3 in SAO membranes is biphasic. The two phases were present in about equal proportions, with rate constants differing by about 5-fold. In contrast; control cells showed monophasic, exponential kinetics with a rate constant comparable to that of the fast phase of SAO membranes. We assign the fast phase in SAO membranes to H2DIDS release from wild-type subunits within homodimers and the slow phase to H2DIDS release from the wild-type subunit within the heterodimer. No differences were observed in kinetic studies of H2DIDS binding. These results suggest that the mutant band 3 subunit alters the conformation of its neighboring wild-type subunit within the heterodimer, resulting in about a 4-fold higher H2DIDS affinity. Additional evidence suggesting that the interactions in the heterodimer may be confined to a region of the wild-type subunit containing the C-terminal subdomain is presented. The relationship of these subunit interactions to the observation of a reduced cellular anion transport function is discussed.

Forces and factors that contribute to the structural stability of membrane proteins

While a considerable amount of literature deals with the structural energetics of water-soluble proteins, relatively little is known about the forces that determine the stability of membrane proteins. Similarly, only a few membrane protein structures are known at atomic resolution, although new structures have recently been described. In this article, we review the current knowledge about the structural features of membrane proteins. We then proceed to summarize the existing literature regarding the thermal stability of bacteriorhodopsin, cytochrome-c oxidase, the band 3 protein, Photosystem II and porins. We conclude that a fundamental difference between soluble and membrane proteins is the high thermal stability of intrabilayer secondary structure elements in membrane proteins. This property manifests itself as incomplete unfolding, and is reflected in the observed low enthalpies of denaturation of most membrane proteins. By contrast, the extramembranous parts of membrane proteins may behave much like soluble proteins. A brief general account of thermodynamics factors that contribute to the stability of water soluble and membrane proteins is presented.

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.

Forces and factors that contribute to the structural stability of membrane proteins

While a considerable amount of literature deals with the structural energetics of water-soluble proteins, relatively little is known about the forces that determine the stability of membrane proteins. Similarly, only a few membrane protein structures are known at atomic resolution, although new structures have recently been described. In this article, we review the current knowledge about the structural features of membrane proteins. We then proceed to summarize the existing literature regarding the thermal stability of bacteriorhodopsin, cytochrome-c oxidase, the band 3 protein, Photosystem II and porins. We conclude that a fundamental difference between soluble and membrane proteins is the high thermal stability of intrabilayer secondary structure elements in membrane proteins. This property manifests itself as incomplete unfolding, and is reflected in the observed low enthalpies of denaturation of most membrane proteins. By contrast, the extramembranous parts of membrane proteins may behave much like soluble proteins. A brief general account of thermodynamics factors that contribute to the stability of water soluble and membrane proteins is presented.

Mammalian exchangers and co-transporters

In the past year, novel mammalian exchanger and co-transporter isoforms have been characterized. Specialized subdomains within these oligomeric transporters have been shown to be involved in biosynthesis, targeting, transport and regulation. Progress on the structural front has been limited due to the lack of high-resolution structures, but transport mutants responsible for disease states continue to be identified.

Band 3 protein: structure, flexibility and function

The electroneutral exchange of chloride and bicarbonate across the human erythrocyte membrane is facilitated by Band 3, a 911 amino acid glycoprotein. The 43 kDa amino-terminal cytosolic domain binds the cytoskeleton, haemoglobin and glycolytic enzymes. The 52 kDa carboxyl-terminal membrane domain mediates anion transport. The protein is a functional dimer, in which the two subunits probably interact with one another by an allosteric mechanism. It is proposed that the link between the mobile cytoplasmic and the membrane-spanning domains of the protein is flexible, based on recent biochemical, biophysical and structural data. This explains the long-standing puzzle that attachment to the cytoskeletal spectrin and actin does not appear to restrict the rotational movement of the Band 3 protein in the erythrocyte membrane. In the Band 3 isoform from the Southeast Asian Ovalocytes (SAO) this link is altered, resulting a tighter attachment of the cytoskeleton to the plasma membrane and a more rigid red blood cell.