Characterization of the reversible conformational equilibrium in the cytoplasmic domain of human erythrocyte membrane band 3

Organization and Dynamics of the Red Blood Cell Band 3 Anion Exchanger SLC4A1: Insights From Molecular Dynamics Simulations

Molecular dynamics (MD) simulations have provided new insights into the organization and dynamics of the red blood cell Band 3 anion exchanger (AE1, SLC4A1). Band 3, like many solute carriers, works by an alternating access mode of transport where the protein rapidly (104/s) changes its conformation between outward and inward-facing states via a transient occluded anion-bound intermediate. While structural studies of membrane proteins usually reveal valuable structural information, these studies provide a static view often in the presence of detergents. Membrane transporters are embedded in a lipid bilayer and associated lipids play a role in their folding and function. In this review, we highlight MD simulations of Band 3 in realistic lipid bilayers that revealed specific lipid and protein interactions and were used to re-create a model of the Wright (Wr) blood group antigen complex of Band 3 and Glycophorin A. Current MD studies of Band 3 and related transporters are focused on describing the trajectory of substrate binding and translocation in real time. A structure of the intact Band 3 protein has yet to be achieved experimentally, but cryo-electron microscopy in combination with MD simulations holds promise to capture the conformational changes associated with anion transport in exquisite molecular detail.

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

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

Comparative Study on the Protective Effect of Chlorogenic Acid and 3-(3-Hydroxyphenyl) Propionic Acid against Cadmium-Induced Erythrocyte Cytotoxicity: In Vitro and In Vivo Evaluation

The metabolism of chlorogenic acid (CGA) through the intestinal tract was studied. As cadmium is a well-known toxic heavy metal, this study was carried out to investigate the comparative protective effect of CGA and its representative intestinal metabolite (3-(3-hydroxyphenyl) propionic acid, HPPA) against Cd-induced erythrocyte cytotoxicity in vitro and in vivo. We found that CGA and its intestinal metabolite appreciably prevented erythrocyte hemolysis, osmotic fragility, and oxidative stress induced by Cd. Also, we found that HPPA had a stronger protective ability than CGA against Cd-induced erythrocyte injury in vivo, such as increasing the ratio of protein kinase C from 7.7% (CGA) to 12.0% (HPPA). Therefore, we hypothesized that CGA and its microbial metabolite had protective effects against Cd-induced erythrocyte damage via multiple actions including antioxidation and chelation. For humans, CGA supplementation may be favorable for avoiding Cd-induced biotoxicity.

A substrate access tunnel in the cytosolic domain is not an essential feature of the solute carrier 4 (SLC4) family of bicarbonate transporters

Anion exchanger 1 (AE1; Band 3; SLC4A1) is the founding member of the solute carrier 4 (SLC4) family of bicarbonate transporters that includes chloride/bicarbonate AEs and Na(+)-bicarbonate co-transporters (NBCs). These membrane proteins consist of an amino-terminal cytosolic domain involved in protein interactions and a carboxyl-terminal membrane domain that carries out the transport function. Mutation of a conserved arginine residue (R298S) in the cytosolic domain of NBCe1 (SLC4A4) is linked to proximal renal tubular acidosis and results in impaired transport function, suggesting that the cytosolic domain plays a role in substrate permeation. Introduction of single and double mutations at the equivalent arginine (Arg(283)) and at an interacting glutamate (Glu(85)) in the cytosolic domain of human AE1 (cdAE1) had no effect on the cell surface expression or the transport activity of AE1 expressed in HEK-293 cells. In addition, the membrane domain of AE1 (mdAE1) efficiently mediated anion transport. A 2.1-Å resolution crystal structure of cdΔ54AE1 (residues 55-356 of cdAE1) lacking the amino-terminal and carboxyl-terminal disordered regions, produced at physiological pH, revealed an extensive hydrogen-bonded network involving Arg(283) and Glu(85). Mutations at these residues affected the pH-dependent conformational changes and stability of cdΔ54AE1. As these structural alterations did not impair functional expression of AE1, the cytosolic and membrane domains operate independently. A substrate access tunnel within the cytosolic domain is not present in AE1 and therefore is not an essential feature of the SLC4 family of bicarbonate transporters.

pH-sensitive self-associations of the N-terminal domain of NBCe1-A suggest a compact conformation under acidic intracellular conditions

NBCe1-A is an integral membrane protein that cotransports Na⁺ and HCO₃^- ions across the basolateral membrane of the proximal tubule. It is essential for maintaining a homeostatic balance of cellular and blood pH. In X-ray diffraction studies, we reported that the cytoplasmic, N-terminal domain of NBCe1-A (NtNBCe1-A) is a dimer. Here, biophysical measurements show that the dimer is in a concentration-dependent dynamic equilibrium among three additional states in solution that are characterized by its hydrodynamic properties, molar masses, emission spectra, binding properties, and stabilities as a function of pH. Under physiological conditions, dimers are in equilibrium with monomers that are pronounced at low concentration and clusters of molecular masses up to 3-5 times that of a dimer that are pronounced at high concentration. The equilibrium can be influenced so that individual dimers predominate in a taut conformation by lowering the pH. Conversely, dimers begin to relax and disassociate into an increasing population of monomers by elevating the pH. A mechanistic diagram for the inter-conversion of these states is given. The self-associations are further supported by surface plasmon resonance (SPR-Biacore) techniques that illustrate NtNBCe1-A molecules transiently bind with one another. Bicarbonate and bicarbonate-analog bisulfite appear to enhance dimerization and induce a small amount of tetramers. A model is proposed, where the Nt responds to pH or bicarbonate fluctuations inside the cell and plays a role in self-association of entire NBCe1-A molecules in the membrane.

Ribonucleotide and ribonucleoside determination by ambient pressure ion mobility spectrometry

Detection limits and reduced mobilities for 12 ribonucleotides and 4 ribonucleosides were measured by ambient pressure electrospray ionization-ion mobility spectrometry (ESI-IMS). With the instrument used in this study it was possible to separate some of these compounds within mixtures. Detection limits reported for ribonucleotides and ribonucleosides ranged from 15 to 300 pmol and the reduced mobilities ranged from 41 to 56 suggesting that ambient pressure ESI-IMS may be used for their rapid and sensitive separation and detection. This report demonstrates that it was possible to use ion mobility spectrometry (IMS) to obtain a spectrum for the separation of nucleotides and nucleosides in less than 1 min. The application holds great promise for nucleotide analysis in the area of separating DNA fragments in genome sequencing and also for forensics DNA typing examinations used for the identification of blood stains in crime scenes and paternity testing.

Shapes of Red Blood Cells: Comparison of 3D Confocal Images with the Bilayer-Couple Model

Cells and organelles are shaped by the chemical and physical forces that bend cell membranes. The human red blood cell (RBC) is a model system for studying how such forces determine cell morphology. It is thought that RBCs, which are typically biconcave discoids, take the shape that minimizes their membrane-bending energies, subject to the constraints of fixed area and volume. However, recently it has been hypothesized that shear elasticity arising from the membrane-associated cytoskeleton (MS) is necessary to account for shapes of real RBCs, especially ones with highly curved features such as echinocytes. In this work we tested this hypothesis by following RBC shape changes using spherical harmonic series expansions of theoretical cell surfaces and those estimated from 3D confocal microscopy images of live cells. We found (i) quantitative agreement between shapes obtained from the theoretical model including the MS and real cells, (ii) that weakening the MS, by using urea (which denatures spectrin), leads to the theoretically predicted gradual decrease in spicule number of echinocytes, (iii) that the theory predicts that the MS is essential for stabilizing the discocyte morphology against changes in lipid composition, and that without it, the shape would default to the elliptocyte (a biconcave ellipsoid), (iv) that we were able to induce RBCs to adopt the predicted elliptocyte morphology by treating healthy discocytes with urea. The latter observation is consistent with the known connection between the blood disease hereditary elliptocytosis and spectrin mutations that weaken the cell cortex. We conclude that while the discocyte, in absence of shear, is indeed a minimum energy shape, its stabilization in healthy RBCs requires the MS, and that elliptocytosis can be explained based on purely mechanical considerations.

Steric hindrance effects on surface reactions: applications to BIAcore

Because surface-volume reactions occur in many biological and industrial processes, understanding the rate of such reactions is important. The BIAcore surface plasmon resonance (SPR) biosensor for measuring rate constants has such a geometry. Though several models of the BIAcore have been presented, few take into account that large ligand molecules can block multiple receptor sites, thus skewing the sensogram data. In this paper some general mathematical principles are stated for handling this phenomenon, and a surface-reaction model is presented explicitly. An integro-partial differential equation results, which can be simplified greatly using perturbation techniques, yielding linear and nonlinear integrodifferential equations. Explicit and asymptotic solutions are constructed for cases motivated by experimental design. The general analysis can provide insight into surface-volume reactions occurring in various contexts. In particular, the steric hindrance effect can be quantified with a single dimensionless parameter.

Verapamil prevents impairment in filterability of human erythrocytes exposed to oxidative stress

Effects of oxidative stress on intact human erythrocytes were investigated using tert-butyl hydroperoxide (tBHP). Exposure of erythrocytes to tBHP caused a marked decrease in filterability in a time-dependent manner. Erythrocytes exposed to tBHP also show an increase in mean corpuscular volume and a remarkable formation of methemoglobin (met-Hb) without any appearance of hemichromes that form Heinz bodies. High performance liquid chromatography demonstrated that the tBHP-treated erythrocytes exhibited an apparent decrease in the membrane phospholipid, phosphatidylethanolamine (PE). The decrease in PE was inhibited by pretreatment with ascorbate, but not with verapamil. SDS-polyacrylamide gel electrophoresis of the tBHP-treated erythrocyte membrane showed a degradation of spectrin, band 3, band 4.2, and band 4.5, accompanied by the appearance of low-molecular-weight products. The degradation of the membrane proteins was not prevented by pretreatment with verapamil or ascorbate. However, the pretreatment with verapamil but not with ascorbate revealed significant inhibition of the tBHP-induced impairment in filterability in the presence of extracellular Ca2+. Thus, the present study shows that verapamil, a potent drug in reperfusion therapy, plays an important role in protection against oxidative injury, based on a close linkage among decreased filterability, met-Hb formation, and impaired membrane integrity.

Expression and purification of recombinant cytoplasmic domain of human erythrocyte band 3 with hexahistidine tag or chitin-binding tag in Escherichia coli

The cytoplasmic domain of erythrocyte band 3 (cdb3) serves as a center of membrane organization in the erythrocytes by its interaction with multiple proteins including ankyrin, protein 4.1, protein 4.2, hemoglobin, and several glycolytic enzymes. In this paper, human cdb3 was cloned into three different expression vectors controlled by T7 polymerase promoter and induced with isopropyl beta-D-thiogalactopyranoside in Escherichia coli. Two of the fusion proteins containing hexahistidine tag in the N-terminal or C-terminal were purified by immobilized metal affinity column chromatography. The third recombinant cdb3 containing the affinity chitin-binding tag was purified using chitin beads followed by specific self-cleavage, which released cdb3 according to the mechanism of protein splicing. The molecular weights of purified recombinant proteins were verified by mass spectrometry. The pH-dependent properties of the intrinsic tryptophan fluorescence of the three kinds of recombinant cdb3 were compared with that of the cdb3 extracted from the erythrocytes, showing that there were no significant differences between them. Using pull-down assay combined with Western blot analysis, the interaction between recombinant cdb3 and protein 4.2 C3 fragment was verified. These demonstrated that the recombinant proteins were both structurally and functionally active. The typical yields of cdb3 purified with hexahistidine tag in the N-terminal, C-terminal, and cleaved from chitin bead were 10.6, 9.6, and 1.5 mg from 1L culture medium, respectively.

Use of luminescence resonance energy transfer to measure distances in the AE1 anion exchange protein dimer

To understand how red blood cell and other proteins carry out their functions, it is necessary not only to have high-resolution crystal structures, but also to have methods that can measure changes in position of parts of the protein on the scale of Angstroms. The method of luminescence resonance energy transfer (LRET) has considerable advantages for this purpose, particularly for proteins, such as the AE1 anion exchange protein in the red cell, that are homodimers. We have applied this method, using a terbium maleimide chelate (TbM) as donor and fluorescein maleimide (FM) as acceptor, to measure the distance between the C201 residues in adjacent dimerized cytoplasmic domains of AE1 (cdAE1). The distance measured by LRET (40.8 A) corresponds closely with that calculated from the crystal structure of the cdAE1, indicating that the method can provide useful information for testing hypotheses concerning motions in this and other blood cell proteins.

Fluorescence analysis of aldolase dissociation from the N-terminal of the cytoplasmic domain of band 3 induced by lanthanide

The cytoplasmic domain of band 3 (CDB3) offers binding sites for several glycolytic enzymes and regulates the glycolysis of erythrocyte. The interaction between recombinant (His)(6)-tagged CDB3 and aldolase, one of the key enzymes that participated in erythrocyte glycolysis, was investigated in the presence of lanthanide. The results indicate that trace lanthanide blocks the inhibition of CDB3-(His)(6) to aldolase and leads to enhancement of aldolase activity. In agreement with activity studies, fluorescence spectra reveal that 4 microM lanthanum ions induce the complete dissociation of aldolase from the N-terminal of CDB3-(His)(6). Interestingly, the synchronous scanning fluorescence spectra of proteins in the presence of various concentrations of lanthanum ions suggest that the conformational change of CDB3-(His)(6) is significantly attributed to the alteration of tryptophan cluster microenvironment, while the aldolase conformation change is mainly derived from tyrosine microenvironment changes. Based on the observation that lanthanide ions induce the dissociation of aldolase from CDB3-(His)(6), it is suggested that the existence of trace lanthanide may affect the glycolysis of erythrocyte.

Binding of hemoglobin to red cell membranes with eosin-5-maleimide-labeled band 3: analysis of centrifugation and fluorescence data

We have studied the binding of hemoglobin to the red cell membrane by centrifugation and fluorescence methods. The intact red cell was labeled with eosin-5-maleimide (EM), which specifically reacts with lysine 430 of band 3. Even though this residue is not part of the cytoplasmic domain of band 3 (cdb3) associated with hemoglobin binding, fluorescence quenching was observed when hemoglobin bound to inside-out vesicles (IOVs). The use of fluorescence quenching to measure band 3 binding was quantitatively compared with the binding determined by centrifugation, which measures binding to band 3 and non-band 3 sites. For the centrifugation it was necessary to include the non-band 3 association constants determined from chymotrypsin-treated IOVs. The binding of hemoglobin to band 3 was interpreted in terms of the binding of two hemoglobin tetramers to each band 3 dimer. An anticooperative interaction associated with the conformational change produced when hemoglobin binds results in a 2.8-fold decrease in the intrinsic constant of (1.54 +/- 0.25) x 10(7) M(-1) for the binding of the second hemoglobin molecule. From the changes in lifetime produced by binding the first and second hemoglobin molecules, it was possible to show that the conformational change associated with binding the second hemoglobin molecule results in a decrease of the heme-eosin distance from 47.90 to 44.78 A. Reaction of cyanate with the alpha-amino group of hemoglobin (HbOCN) is shown to produce a very dramatic decrease in the binding of hemoglobin to both the band 3 and non-band 3 sites. The intrinsic constant for binding the first hemoglobin molecule to band 3 decreases by a factor of 29 to (5.34 +/- 0.15) x 10(5) M(-1). The anticooperative interaction is greater with the intrinsic constant decreasing by a factor of 3.8 for the binding of the second hemoglobin tetramer to band 3. In addition, the nature of the conformational change produced by binding hemoglobin is very different with the second HbOCN increasing the heme-eosin distance to 55.99 A. The utilization of eosin-5-maleimide-reacted red cell membrane to study hemoglobin binding makes it possible to directly study the binding to band 3. At the same time a sensitive probe of the conformational changes, which occur when hemoglobin binds to band 3, is provided.

Band 3 anion exchanger and its involvement in erythrocyte and kidney disorders

Recent developments in the structure of erythrocyte band 3 and its role in hereditary spherocytosis and distal renal tubular acidosis are described. The crystal structure of the N-terminal cytoplasmic domain provides a basis for understanding the organization of ankyrin and other peripheral membrane proteins around band 3. Band 3 also binds integral membrane proteins, including the Rh protein complex and CD47. Band 4.2 is important in these associations, which link the Rh complex to the skeleton. It is suggested that band 3 forms the scaffold for a protein assembly that could transduce signals from the cell exterior and modulate the transport and mechanical properties of the erythrocyte. The involvement of band 3 in distal renal tubular acidosis is reviewed. The article discusses a likely mechanism for dominant distal renal tubular acidosis in which associations between the normal and mutant protein alter the plasma membrane targeting of the normal protein in the kidney.