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

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.

Effects of Hypoxia on Erythrocyte Membrane Properties-Implications for Intravascular Hemolysis and Purinergic Control of Blood Flow

Intravascular hemolysis occurs in hereditary, acquired, and iatrogenic hemolytic conditions but it could be also a normal physiological process contributing to intercellular signaling. New evidence suggests that intravascular hemolysis and the associated release of adenosine triphosphate (ATP) may be an important mechanism for in vivo local purinergic signaling and blood flow regulation during exercise and hypoxia. However, the mechanisms that modulate hypoxia-induced RBC membrane fragility remain unclear. Here, we provide an overview of the role of RBC ATP release in the regulation of vascular tone and prevailing assumptions on the putative release mechanisms. We show importance of intravascular hemolysis as a source of ATP for local purinergic regulation of blood flow and discuss processes that regulate membrane propensity to rupture under stress and hypoxia.

Identification of adducin-binding residues on the cytoplasmic domain of erythrocyte membrane protein, band 3

Two major complexes form structural bridges that connect the erythrocyte membrane to its underlying spectrin-based cytoskeleton. Although the band 3-ankyrin bridge may account for most of the membrane-to-cytoskeleton interactions, the linkage between the cytoplasmic domain of band 3 (cdb3) and adducin has also been shown to be critical to membrane integrity. In the present paper, we demonstrate that adducin, a major component of the spectrin-actin junctional complex, binds primarily to residues 246-264 of cdb3, and mutation of two exposed glutamic acid residues within this sequence completely abrogates both α- and β-adducin binding. Because these residues are located next to the ankyrin-binding site on cdb3, it seems unlikely that band 3 can bind ankyrin and adducin concurrently, reducing the chances of an association between the ankyrin and junctional complexes that would significantly compromise erythrocyte membrane integrity. We also demonstrate that adducin binds the kidney isoform of cdb3, a spliceoform that lacks the first 65 amino acids of erythrocyte cdb3, including the central strand of a large β-pleated sheet. Because kidney cdb3 is not known to bind any of the common peripheral protein partners of erythrocyte cdb3, including ankyrin, protein 4.1, glyceraldehyde-3-phosphate dehydrogenase, aldolase, and phosphofructokinase, retention of this affinity for adducin was unexpected.

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.

Stored red blood cells: a changing universe waiting for its map(s)

The availability of stored red blood cells (RBCs) for transfusion remains an important aspect of the treatment of polytrauma, acute anemia or major bleedings. RBCs are prepared by blood banks from whole blood donations and stored in the cold in additive solutions for typically six weeks. These far from physiological storage conditions result in the so-called red cell storage lesion that is of importance both to blood bankers and to clinical practitioners. Here we review the current state of knowledge about the red cell storage lesion from a proteomic perspective. In particular, we describe the current models accounting for RBC aging and response to lethal stresses, review the published proteomic studies carried out to uncover the molecular basis of the RBC storage lesion, and conclude by suggesting a few possible proteomic studies that would provide further knowledge of the molecular alterations carried by RBCs stored in the cold for six weeks.

Mapping of glycolytic enzyme-binding sites on human erythrocyte band 3

Previous work has shown that GAPDH (glyceraldehyde-3-phosphate dehydrogenase), aldolase, PFK (phosphofructokinase), PK (pyruvate kinase) and LDH (lactate dehydrogenase) assemble into a GE (glycolytic enzyme) complex on the inner surface of the human erythrocyte membrane. In an effort to define the molecular architecture of this complex, we have undertaken to localize the binding sites of these enzymes more accurately. We report that: (i) a major aldolase-binding site on the erythrocyte membrane is located within N-terminal residues 1-23 of band 3 and that both consensus sequences D6DYED10 and E19EYED23 are necessary to form a single enzyme-binding site; (ii) GAPDH has two tandem binding sites on band 3, located in residues 1-11 and residues 12-23 respectively; (iii) a PFK-binding site resides between residues 12 and 23 of band 3; (iv) no GEs bind to the third consensus sequence (residues D902EYDE906) at the C-terminus of band 3; and (v) the LDH- and PK-binding sites on the erythrocyte membrane do not reside on band 3. Taken together, these results argue that band 3 provides a nucleation site for the GE complex on the human erythrocyte membrane and that other components near band 3 must also participate in organizing the enzyme complex.

Oxygen sensing in the circulation: "cross talk" between red blood cells and the vasculature

Oxygen (O(2)) sensing in blood and regulation of microvascular tone appear to involve hemoglobin (Hb) conformational changes resulting from O(2) desaturation. This observation has prompted the thought that Hb functions as both an O(2) sensor and regulator of microvasular blood flow to meet local tissue oxygen demand. The mechanism(s) by which this is accomplished has recently been the subject of increasing debate. Three primary hypotheses are described within the literature and include release of adenosine 5'-triphosphate by red blood cells (RBCs), release of S-nitrosylated molecules from RBCs originally bound to beta93 cysteine residues of oxyHb, and nitrite conversion and storage of nitric oxide by Hb at the site of ferric (Fe(3+)) and ferrous (Fe(2+)) Hb. Within extravascular cells, the global regulator of oxygen homeostasis is hypoxia-inducible factor-1 (HIF- 1). This transcriptional factor is tightly regulated by O(2) and cellular redox-sensitive mechanisms. HIF-1 activation is responsible for the up-regulation of proteins, which increase O(2) supply. We believe that there are important and yet unexplored mechanisms by which RBCs can directly or indirectly communicate via redox intermediates with extravascular sites as part of the global O(2) sensing mechanism.

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.

Identification of a critical ankyrin-binding loop on the cytoplasmic domain of erythrocyte membrane band 3 by crystal structure analysis and site-directed mutagenesis

The cytoplasmic domain of erythrocyte membrane band 3 (cdb3) serves as a center of membrane organization, interacting with such proteins as ankyrin, protein 4.1, protein 4.2, hemoglobin, several glycolytic enzymes, a tyrosine phosphatase, and a tyrosine kinase, p72(syk). The crystallographic structure of the cdb3 dimer has revealed that residues 175-185 assume a beta-hairpin loop similar to a putative ankyrin-binding motif at the cytoplasmic surface of the Na(+)/K(+)-ATPase. To test whether this hairpin loop constitutes an ankyrin-binding site on cdb3, we have deleted amino acids 175-185 and substituted the 11-residue loop with a Gly-Gly dipeptide that bridges the deletion without introducing strain into the structure. Although the deletion mutant undergoes the same native conformational changes exhibited by wild type cdb3 and binds other peripheral proteins normally, the mutant exhibits no affinity for ankyrin. This suggests that the exposed beta-hairpin turn indeed constitutes a major ankyrin-binding site on cdb3. Other biochemical studies suggest that ankyrin also docks at the NH(2) terminus of band 3. Thus, antibodies to the NH(2) terminus of cdb3 block ankyrin binding to the cdb3, and ankyrin binding to cdb3 prevents p72(syk) phosphorylation of cdb3 at its NH(2) terminus (predominantly at Tyr-8). However, a truncation mutant of cdb3 lacking the NH(2)-terminal 50 residues displays the same binding affinity as wild type cdb3. These data thus suggest that the NH(2) terminus of cdb3 is proximal to but not required for the cdb3-ankyrin interaction.

Flexibility of the cytoplasmic domain of the anion exchange protein, band 3, in human erythrocytes

The rotational flexibility of the cytoplasmic domain of band 3, in the region that is proximal to the inner membrane surface, has been investigated using a combination of time-resolved optical anisotropy (TOA) and saturation-transfer electron paramagnetic resonance (ST-EPR) spectroscopies. TOA studies of rotational diffusion of the transmembrane domain of band 3 show a dramatic decrease in residual anisotropy following cleavage of the link with the cytoplasmic domain by trypsin (E. A. Nigg and R. J. Cherry, 1980, Proc. Natl. Acad. Sci. U.S.A. 77:4702-4706). This result is compatible with two independent hypotheses: 1) trypsin cleavage leads to dissociation of large clusters of band 3 that are immobile on the millisecond time scale, or 2) trypsin cleavage leads to release of a constraint to uniaxial rotational diffusion of the transmembrane domain. ST-EPR studies at X- and Q-band microwave frequencies detect rotational diffusion of the transmembrane domain of band 3 about the membrane normal axis of reasonably large amplitude that does not change upon cleavage with trypsin. These ST-EPR results are not consistent with dissociation of clusters of band 3 as a result of cleavage with trypsin. Global analyses of the ST-EPR data using a newly developed algorithm indicate that any constraint to rotational diffusion of the transmembrane domain of band 3 via interactions of the cytoplasmic domain with the membrane skeleton must be sufficiently weak to allow rotational excursions in excess of 32 degrees full-width for a square-well potential. In support of this result, analyses of the TOA data in terms of restricted amplitude uniaxial rotational diffusion models suggest that the membrane-spanning domain of that population of band 3 that is linked to the membrane skeleton is constrained to diffuse in a square-well of approximately 73 degrees full-width. This degree of flexibility may be necessary for providing the unique mechanical properties of the erythrocyte membrane.

Effect of pH on the self-association of erythrocyte band 3 in situ

The human erythrocyte anion exchanger (band 3) contains a cytoplasmic domain (cdb3) that exists in a reversible, pH-dependent structural equilibrium among three native conformations. To understand how this conformational equilibrium might influence the association state of band 3, we have incubated stripped erythrocyte membranes in solutions ranging from pH 6.0 to pH 10.5 and have examined the oligomeric state of the protein by size exclusion high performance liquid chromatography. We demonstrate that incubation of membranes in slightly acidic conditions favors dimer formation, whereas extended incubation at higher pHs (pH>9) leads to irreversible formation of an oligomeric species larger than the tetramer. Since the pH dependence of the conformational state of the cytoplasmic domain exhibits a similar pH profile, we suggest that the conformation of the cytoplasmic domain can modulate the self-association of band 3. Importantly, this modulation would appear to require the structural interactions present within the intact protein, since the isolated membrane-spanning domain does not display any pH dependence of association. The irreversible nature of the alkali-induced aggregation further suggests that a secondary reaction subsequent to band 3 association is required to stabilize the high molecular weight aggregate. Although we were able to eliminate covalent bond formation in this irreversible aggregation process, the exact nature of the secondary reaction remains to be elucidated.

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

The cytoplasmic domain of erythrocyte membrane band 3 (cdb3) serves as a center of membrane organization, interacting with such proteins as ankyrin, protein 4.1, protein 4.2, hemoglobin, several glycolytic enzymes, and a tyrosine kinase, p72syk. cdb3 exists in a reversible, pH-dependent conformational equilibrium characterized by large changes in Stokes radius (11 A) and intrinsic fluorescence (2-fold). Based on the crystallographic structure of the cdb3 dimer, we hypothesized that the above conformational equilibrium might involve the movement of flanking peripheral protein binding domains away from a shared dimerization domain. To test this hypothesis, we have mutated both donor (W105L) and acceptor (D316A) residues of a prominent H bond that bridges the above two domains and have examined the effect on the resulting conformational equilibrium. Analysis of the intrinsic fluorescence, Stokes radius, thermal stability, urea stability, and segmental mobility of these mutants reveals that the above H bond is indeed present in the low pH conformation of cdb3 and broken in a higher pH conformation. The data further reveal that cdb3 exists in three native pH-dependent conformations and that rupture of the aforementioned H bond occurs only during conversion of the low pH conformation to the mid-pH conformation. Conversion of the mid-pH conformation to the high pH conformation would now appear to involve structural changes primarily in the peripheral protein binding domain. Because ankyrin associates avidly with the low pH conformation of cdb3, ankyrin occupancy should strongly influence this structural equilibrium and thereby affect band 3 and perhaps global membrane properties.

Role of erythrocyte in regulating local O2 delivery mediated by hemoglobin oxygenation

The release of ATP from red blood cells (RBC) in response to low O2 levels is linked to ATP production and the oxygenation state of hemoglobin. Because O2 is unloaded from the RBC, the concentration of deoxygenated hemoglobin increases, displacing phosphofructokinase from the cytoplasmic domain of band 3. We hypothesize that the ATP molecules produced through this glycolytic stimulation at the membrane surface result in the release of ATP from the RBC. Rat whole blood exposed to 5 min of low PO2 in vitro increased plasma [ATP] by 1.0 miccroM (+45%). This increase was reduced to 0.1 microM (+12%, P < 0.05) after citrate incubation and reversed after fluoride treatment (both glycolytic inhibitors) by -0.2 microM (-23%, P < 0.05). Plasma [ATP] of control RBC decreased -0.3 microM (-12%) when 8% CO (P < 0.05) was added to the chamber. Because CO and O2 bind competitively to heme, these results support our hypothesis that the release of ATP from RBC is linked to ATP production through the oxygenation state of the hemoglobin molecule.

Spectrin oligomerization is cooperatively coupled to membrane assembly: a linkage targeted by many hereditary hemolytic anemias?

In the erythrocyte, ankyrin is the major adapter protein linking tetramers of band 3 to the spectrin-actin cytoskeleton. This linkage involves a direct interaction between ankyrin and the 14th-15th repeat unit of beta-spectrin. The spectrin cytoskeleton itself is stabilized by the self-association of spectrin heterodimers into tetramers and larger oligomers, a process mediated by the 17th repeat unit of beta-spectrin and a short NH(2) -terminal sequence in alpha-spectrin. The self-association of spectrin and its ankyrin-mediated membrane binding have generally been considered independent events. We now demonstrate that spectrin self-association, the binding of spectrin to ankyrin, and the binding of ankyrin to the 43-kDa cytoplasmic domain of band 3 (cdb3) are coupled in a positively cooperative way. In solution, [(125)I]-labeled ankyrin was found by ND-PAGE3 to enhance the affinity of spectrin self-association by 10-fold. The reciprocal process was also true, in that spectrin tetramers and oligomers bound ankyrin with enhanced affinity relative to dimer spectrin. Saturation of the beta-spectrin self-association site by an NH(2) -terminal 80-kDa alpha-spectrin peptide enhanced the affinity of spectrin dimer for ankyrin, indicating a direct relationship between ankyrin binding and the occupancy of the beta-spectrin self-association site. cdb3 accentuated these cooperative interactions. Several inherited spectrin mutations that cause hemolytic disease but that do not directly destabilize the self-association or ankyrin-binding sites can be explained by these results. Three classes of mutations appear to disrupt cooperative coupling between self-association and ankyrin binding: (i) mutation of the linker sequences that join helices C and A in repeat units that intervene between the two functional sites, mutations that presumably block repeat-to-repeat transfer of conformational information; (ii) mutations in alpha-spectrin repeats 4 to 6 that disrupt the ability of this region to trans-regulate ankyrin binding by the adjacent beta-spectrin repeats 14-15; and (iii) exon-skipping mutations that shorten alpha-spectrin and force repeats 4 to 6 to fall out-of-register with the ankyrin-binding motif in beta-spectrin. Collectively, these results demonstrate a molecular mechanism whereby a membrane receptor can directly promote cytoskeletal assembly.

Crystallographic structure and functional interpretation of the cytoplasmic domain of erythrocyte membrane band 3

The red blood cell membrane (RBCM) is a primary model for animal cell plasma membranes. One of its major organizing centers is the cytoplasmic domain of band 3 (cdb3), which links multiple proteins to the membrane. Included among its peripheral protein ligands are ankyrin (the major bridge to the spectrin-actin skeleton), protein 4.1, protein 4.2, aldolase, glyceraldehyde-3-phosphate dehydrogenase, phosphofructokinase, deoxyhemoglobin, p72syk protein tyrosine kinase, and hemichromes. The crystal structure of cdb3 is reported at 0.26 nm (2.6 Å) resolution. A tight symmetric dimer is formed by cdb3; it is stabilized by interlocked dimerization arms contributed by both monomers. Each subunit also includes a larger peripheral protein binding domain with an α+  β-fold. The binding sites of several peripheral proteins are localized in the structure, and the nature of the major conformational change that regulates membrane-skeletal interactions is evaluated. An improved structural definition of the protein network at the inner surface of the RBCM is now possible.

Crystallographic structure and functional interpretation of the cytoplasmic domain of erythrocyte membrane band 3

The red blood cell membrane (RBCM) is a primary model for animal cell plasma membranes. One of its major organizing centers is the cytoplasmic domain of band 3 (cdb3), which links multiple proteins to the membrane. Included among its peripheral protein ligands are ankyrin (the major bridge to the spectrin-actin skeleton), protein 4.1, protein 4.2, aldolase, glyceraldehyde-3-phosphate dehydrogenase, phosphofructokinase, deoxyhemoglobin, p72syk protein tyrosine kinase, and hemichromes. The crystal structure of cdb3 is reported at 0.26 nm (2.6 Å) resolution. A tight symmetric dimer is formed by cdb3; it is stabilized by interlocked dimerization arms contributed by both monomers. Each subunit also includes a larger peripheral protein binding domain with an α+  β-fold. The binding sites of several peripheral proteins are localized in the structure, and the nature of the major conformational change that regulates membrane-skeletal interactions is evaluated. An improved structural definition of the protein network at the inner surface of the RBCM is now possible.

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.

Comparative study of the cytoplasmic domain of band 3 from human and rabbit erythrocyte membranes

The cytoplasmic domain of band 3 (cdb3) is thought to play an important role in human erythrocyte aging. In order to investigate the role of cdb3 during rabbit erythrocyte aging, we compared rabbit cdb3 with the corresponding protein from human erythrocyte membranes. We describe a purification procedure for rabbit cdb3 comparing rabbit and human cdb3 on sodium dodecyl sulphate-polyacrylamide electrophoresis, we found fragments of different molecular weights, implying different chymotryptic cuts in the two species. Anti-human antibodies did not bind to the rabbit cdb3; we also noticed structural differences in the protein between the two species, which may also play a role in the aging processes. Rabbit erythrocyte membranes have a higher capacity of binding hemichromes, due to the higher content of band 3. While in rabbit erythrocyte membranes only one binding site for hemichromes (corresponding to cdb3) was found, we confirmed the existence of two binding sites in human membranes. The second binding site probably corresponds to glycophorin, a protein not present in rabbit membranes.

Effect of band 3 subunit equilibrium on the kinetics and affinity of ankyrin binding to erythrocyte membrane vesicles

The membrane-spanning protein, band 3, anchors the spectrin-based membrane skeleton to the lipid bilayer via the bridging protein, ankyrin. To understand how band 3 subunit stoichiometry influences this membrane-skeletal junction, we have induced changes in the band 3 association equilibrium and assayed the kinetics and equilibrium properties of ankyrin binding. We observe that band 3 oligomers convert slowly to dimers and ultimately monomers following removal of ankyrin. Addition of excess ankyrin back to these membranes enriched in dissociated band 3 then shifts band 3 almost entirely to tetramers, confirming that the tetrameric form of band 3 constitutes the preferred oligomeric state of ankyrin binding. 4, 4'-Diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) labeling of band 3, which is shown to shift most of the band 3 population to dimers, eliminates the majority of ankyrin-binding sites on the membrane and greatly reduces retention of band 3 in detergent-extracted membrane skeletons. Furthermore, DIDS- modified membranes lack all low affinity ankyrin-binding sites and roughly half of all high affinity sites. Since labeled membranes lack the rapid kinetic phase of ankyrin binding and exhibit only half of the normal amplitude of the slow kinetic phase, it can be concluded that the rapid phase of ankyrin association involves low affinity sites and the slow phase involves high affinity sites. A model accounting for these data and most previous data on ankyrin-band 3 interactions is provided.

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.

Mapping of ankyrin binding determinants on the erythroid anion exchanger, AE1

The association of ankyrin with the AE1 anion exchanger contributes an essential function to the mechanical and viscoelastic properties of the erythrocyte and constitutes the best understood link between the plasma membrane and the underlying membrane skeleton. The AE1 binding domain of ankyrin consists of 24 tandem repeats of a 33-amino acid motif that is present on a wide variety of otherwise unrelated proteins. The experiments described in this paper are aimed at identifying the specific amino acid sequences in AE1 that comprise the ankyrin binding site. We have exploited a cell-free binding assay to quantify the binding affinity of anion exchangers and a recombinant fragment of ANK1, R13-H. Our previous study (Ding, Y., Casey, J. R. and Kopito, R. R. (1995) J. Biol. Chem. 269, 32201-32208) identified an essential role of the amino-terminal 79 AE1 residues in ankyrin binding. The present study extends these findings to show that these 79 amino acids, although necessary, are not sufficient for ankyrin binding. Using chimeras between AE1 and the closely related anion exchanger AE2, which does not bind ankyrin, we have defined a 40-residue region of AE1 between positions 155 and 195 that is also essential for ankyrin binding.

Anisotropy decay measurement of segmental dynamics of the anion binding domain in erythrocyte band 3

Time-resolved anisotropy was utilized to detect nanosecond segmental motions of the band 3 intramembrane domain. Band 3 at lysine 430 was fluorescently labeled in ghost membranes by fluorescein or eosin maleimide treatment of intact human erythrocytes followed by hypotonic lysis. Single lifetimes for fluorescein (3.8-4.1 ns) and eosin (3.2-3.4 ns) were observed. Phase-modulation measurement of anisotropy decay indicated a segmental motion model, r(t) = exp(-t/tau 1c)[r infinity + (ro-r infinity) exp(-t/tau 2c)], defined by rotational correlation times corresponding to band 3 segmental motion (tau 1c, 30-70 ns) and rapid fluorescein motion in its binding pocket (tau 2c, 200-400 ps), and a residual anisotropy (r infinity, 0.23-0.28) describing hindered fluorescein motion. In PBS at pH 7.4, tau 1c, tau 2c, and r infinity were 44 ns, 307 ps, and 0.24, respectively, predicting a steady-state anisotropy of 0.24, in agreement with the measured value of 0.23. Factors that might influence band 3 structure/dynamics were examined. Whereas pH (range 5-10) had little effect on r(t), [NaCl] addition (0-150 mM) remarkably decreased tau 1c from 68 to 44 ns. The decrease in tau 1c correlated with solution ionic strength, and did not depend on osmolality (studied by mannitol addition), or specific anion interactions (comparing Cl, Br, F, SO4, citrate). The ionic strength effect was not observed in fluorescein-labeled carbonic anhydrase and trypsin-cleaved band 3, suggesting a specific effect on intact band 3. Anisotropy decay was relatively insensitive to external lectin or internal 2,3-DPG binding, but was sensitive to temperature, membrane fluidity, urea denaturation, fluid-phase viscosity, and aldehyde fixation.(ABSTRACT TRUNCATED AT 250 WORDS)

Partial characterization of the cytoplasmic domain of human kidney band 3

The major anion exchanger in type A intercalated cells of the cortical and medullary collecting ducts of the human kidney is a truncated isoform of erythrocyte band 3 (AE1) that lacks the N-terminal 65 residues. Because this missing sequence has been implicated in the binding of ankyrin, protein 4.1, several glycolytic enzymes, hemoglobin, and hemichromes in erythrocytes, we have undertaken examination of the structure and peripheral protein interactions of this kidney isoform. The cytoplasmic domain of kidney band 3, kidney CDB3, was expressed in Escherichia coli and purified to homogeneity. The kidney isoform exhibited a circular dichroism spectrum and Stokes radius similar to its larger erythrocyte counterpart. Kidney CDB3 was also observed to engage in the same conformational equilibrium characteristic of erythrocyte CDB3. In contrast, the tryptophan and cysteine clusters of kidney CDB3 behaved very differently from erythrocyte CDB3 in response to pH changes and oxidizing conditions. Furthermore, kidney CDB3 did not bind ankyrin, protein 4.1, or aldolase, and expression of erythrocyte CDB3 was toxic to its bacterial host, whereas expression of kidney CDB3 was not. Taken together, these data suggest that the absence of the N-terminal 65 amino acids in kidney CDB3 eliminates the major function currently ascribed to CDB3 in erythrocytes, i.e. that of peripheral protein binding. The primary function of residues 66-379 found in kidney CDB3 thus remains to be elucidated.

Crystallization and preliminary X-ray analysis of the cytoplasmic domain of human erythrocyte band 3

A cytoplasmic domain of the human erythrocyte membrane protein band 3 (M(r) = 42,500), residues 1-379, expressed in and purified from E. coli, has been crystallized by the method of vapor diffusion in sitting drops with subsequent streak-seeding at room temperature. Initial crystals were grown from solutions containing 65-68% saturated ammonium sulfate at pH 4.9 and 2 mg/ml protein. Subsequent streak-seeding into solutions of 50-53% ammonium sulfate at pH 4.9 and 7 mg/ml protein produced single crystals suitable for X-ray analysis, which contained pure protein as revealed by gel electrophoresis. The crystals belong to the monoclinic space group C2 with cell dimensions of a = 178.8 A, b = 90.5 A, c = 122.1 A, and beta = 131.3 degrees and diffract at least to 2.7 A resolution (at 100 K). A self-rotation function shows the presence of approximate 222 local symmetry.

ABO blood grouping of bloodstains by sandwich ELISA using monoclonal antibody specific for human red cell band 3

ABO blood grouping of human bloodstains was performed by a sandwich ELISA using a species-specific monoclonal antibody to the amino-terminal cytoplasmic domain of human red cell membrane band 3. In a blind trial, all A, B and O bloodstains (a 1 cm long thread) and AB bloodstains (a 1.5 cm long thread) were accurately typed by this method. Even when bloodstains were contaminated by other body fluids (e.g., semen and saliva), only the ABO blood group epitopes on band 3 of the red cell membrane were detected. Thus, identification of human blood and ABO blood grouping of bloodstains which were contaminated by other body fluids could be simultaneously performed by this method.

Human erythrocyte band 3 (EPB3) polymorphism: analysis of blood and bloodstains by an immunodetection method and frequency of the EPB3* Memphis variant

The erythrocyte band 3 (EPB3) variant, band 3 Memphis (EPB3*Memphis), was detected by immunoblotting with a monoclonal antibody to the 41 kDa cytoplasmic N-terminal domain of band 3 without protease treatment of erythrocytes. EPB3*Memphis was also detected by immunoblotting from 3-month-old bloodstains subjected to alpha-chymotrypsin treatment. A population genetic study using this method indicated that the EPB3 variant would be useful for forensic work in Japan, since the frequency of this variant in Japanese (Wakayama prefecture) is relatively high (0.159).

Expression, purification, and characterization of the functional dimeric cytoplasmic domain of human erythrocyte band 3 in Escherichia coli

The cytoplasmic domain of the human erythrocyte membrane protein, band 3 (cdb3), contains binding sites for hemoglobin, several glycolytic enzymes, band 4.1, band 4.2, and ankyrin, and constitutes the major linkage between the membrane skeleton and the membrane. Although erythrocyte cdb3 has been partially purified from proteolyzed red blood cells, further separation of the water-soluble 43-kDa and 41-kDa proteolytic fragments has never been achieved. In order to obtain pure cdb3 for crystallization and site-directed mutagenesis studies, we constructed an expression plasmid that has a tandemly linked T7 promoter placed upstream of the N-terminal 379 amino acids of the erythrocyte band 3 gene. Comparison of several Escherichia coli strains led to the selection of the BL21 (DE3) strain containing the pLysS plasmid as the best host for efficient production of cdb3. About 10 mg of recombinant cdb3 can be easily purified from 4 L of E. coli culture in two simple steps. Comparison of cdb3 released from the red blood cell by proteolysis with recombinant cdb3 reveals that both have the same N-terminal sequence, secondary structure, and pH-dependent conformational change. The purified recombinant cdb3 is also a soluble stable dimer with the same Stokes radius as erythrocyte cdb3. The affinities of the two forms of cdb3 for ankyrin are essentially identical; however, recombinant cdb3 with its unblocked N-terminus exhibits a slightly lower affinity for aldolase.

Mediation of inorganic anion transport by the hydrophobic domain of mouse erythroid band 3 protein expressed in oocytes of Xenopus laevis

A cDNA clone of the mouse erythroid band 3 protein encoding the 556 amino acid residues of the hydrophobic domain from Thr-374 to the C-terminal Val-929 is shown by immunoprecipitation to be expressed in Xenopus oocytes. Measurements of 36Cl- efflux indicate that the translation product mediates Cl- transport, which is inhibitable reversibly by DNDS or H2DIDS, specific inhibitors of band 3-mediated transport. The apparent KI values are 3.6 microM and 0.094 microM, respectively, and hence similar to those found in the wild type band 3-mediated anion transport. The rapid reversible inhibition by H2DIDS slowly changes to irreversible inhibition. The rate of change increases with increasing pH, again similar as to the wild-type band 3. It is concluded that the hydrophobic domain of band 3 is capable of executing anion transport essentially similar to the full-length band 3, although minor differences with respect to transport and inhibition kinetics cannot be ruled out.

Rotational diffusion of the erythrocyte integral membrane protein band 3: effect of hemichrome binding

Human erythrocyte band 3 was covalently labeled within the integral membrane domain by incubating intact erythrocytes with the phosphorescent probe eosinyl-5-maleimide. The rotational diffusion of band 3 in membranes prepared from these labeled cells was measured using the technique of time-resolved phosphorescence anisotropy. Three rotational correlation times ranging from 16 to 3800 microseconds were observed, suggesting that band 3 exists in different aggregate states within the plane of the membrane. The oxidizing agent phenylhydrazine was used to induce hemichrome formation within intact erythrocytes. The immobilization of band 3 in membranes prepared from these erythrocytes suggests that the binding of hemichromes induces clustering of band 3. The addition of purified hemichromes to erythrocyte ghosts leads to a similar effect. We have also examined the mobility of the cytoplasmic domain of band 3. This region was labeled indirectly using a phosphorescently labeled antibody which binds to an epitope within the cytoplasmic domain. We observed very rapid motion of the cytoplasmic region of band 3, which was only partially restricted upon hemichrome binding. This suggests that the integral and cytoplasmic domains of band 3 may be independently mobile.

Frequencies of Band 3 variants of human red cell membranes in some different populations

A variant of Band 3, the major protein of the erythrocyte membrane, was observed by Mueller and Morrison in 1977 in 6-7% of healthy blood donors on sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) of erythrocyte membranes treated with pronase. Pronase treated red cells containing this first recognized variant [here designated 'Band 3-Memphis (m)'] section had two bands of about 63,000 and 60,000 Mr while pronase treated normal cells had only the lighter Mr band. The present study includes data on the frequency of variants resembling Band 3-Memphis in patients of different ethnic groups and on random donors obtained earlier in Memphis. These variants were detected by the original method of Mueller and Morrison and were not associated with recognized clinical or haematological abnormalities. Significantly higher gene frequencies for the variants of the (m) type were observed in American Indians, African Americans and Filipinos than in Caucasians; putative heterozygotes and homozygotes were identified among each of these groups. The frequency of silent Band 3 polymorphisms in different populations should be considered in the interpretation of clinical findings associated with the presence of Band 3 variants.

Regulation of intracellular pH by a neuronal homolog of the erythrocyte anion exchanger

We have isolated AE3, a novel gene expressed primarily in brain neurons and in heart. The predicted AE3 polypeptide shares a high degree of identity with the anion exchange and cytoskeletal binding domains of the erythrocyte band 3 protein. Expression of AE3 cDNA in COS cells leads to chronic cytoplasmic acidification and to chloride- and bicarbonate-dependent changes in intracellular pH, confirming that this gene product is an anion exchanger. Characterization of an AE3 mutant lacking the NH2-terminal 645 amino acids demonstrates that the COOH-terminal half of the polypeptide is both necessary and sufficient for correct insertion into the plasma membrane and for anion exchange activity. The NH2-terminal domain may play a role in regulating the activity of the exchanger and may be involved in the structural organization of the cytoskeleton in neurons.

Abnormalities in erythrocyte membrane band 3 in chronic myelogenous leukemia

The anion transport activities of erythrocytes from patients with chronic myelogenous leukemia (CML) and normal donors were comparable. In CML erythrocytes, significant reduction in the number of ankyrin-binding sites, present in the cytoplasmic domain of band 3, may lead to partial loss of cytoskeletal anchorage to the bilayer and account for their increased Con-A agglutinability and heat-sensitivity (Basu, J., Kundu, M., Rakshit, M.M. and Chakrabarti, P. (1988) Biochim. Biophys. Acta 945, 121-126).

Antigenic determinants of the cytoplasmic domain of band 3 from bovine erythrocyte membrane

Rabbit antibodies were prepared against the cytoplasmic 38K-Da fragment of bovine band 3 and the immunological cross-reactivity with human, murine, rat, and chicken band 3 was examined. The antibodies cross-reacted with human and rodent band 3, indicating that there is an antigenic determinant(s) common to primate and nonprimate species. However, the antibodies did not recognize chicken band 3. Antigenic sites on the 38K-Da fragment were determined via amino acid sequence and immunoblotting analyses of proteolytic peptides of the fragment. Positions of antigenic determinants which were assumed to be common to primate and nonprimate species were mapped to the areas of residues 127-160 and of residues 259-304 in the primary structure of human band 3. Another epitope(s), which is absent in human band 3, existed in a region having a bovine-specific amino acid sequence. In addition, comparison of sequence data from different species showed that a proposed hinge region and a tryptophan-rich region on the cytoplasmic domain of band 3 [P. S. Low et al. (1984) J. Biol. Chem. 259, 13,070-13,076; R. R. Kopito and H. F. Lodish (1985) Nature (London) 316, 234-238] are also conserved in the bovine case.

Cytoskeletal restraints of band 3 rotational mobility in human erythrocyte membranes

The interaction of band 3 with cytoskeletal proteins was investigated in erythrocyte membranes by measuring the rotational mobility of band 3 using the method of transient dichroism. It was found that selective proteolysis of ankyrin, a protein known to link band 3 to the spectrin-actin network, had no significant effect on band 3 rotation. Incubating ghosts to 70 degrees C, at which temperature ankyrin is expected to be denatured, also had no effect. It thus appears probable that linkage of band 3 to the cytoskeleton via ankyrin does not act as a restraint on band 3 rotational motion. It is suggested that this is a consequence of flexibility in the cytoskeletal structure. In further investigations of the effect of heat treatment, a large enhancement of band 3 rotational mobility was found to result from incubation of intact cells for 1 h at 50 degrees C. This effect was not observed if ghosts were subjected to the same treatment, nor did it occur if the incubation of cells was performed at 47 degrees C. These findings, in combination with previous studies of band 3 rotational mobility, indicate that the interactions which restrain band 3 are likely to be more complex than commonly envisaged.

Spectrin and related molecules

This review begins with a complete discussion of the erythrocyte spectrin membrane skeleton. Particular attention is given to our current knowledge of the structure of the RBC spectrin molecule, its synthesis, assembly, and turnover, and its interactions with spectrin-binding proteins (ankyrin, protein 4.1, and actin). We then give a historical account of the discovery of nonerythroid spectrin. Since the chicken intestinal form of spectrin (TW260/240) and the brain form of spectrin (fodrin) are the best characterized of the nonerythroid spectrins, we compare these molecules to RBC spectrin. Studies establishing the existence of two brain spectrin isoforms are discussed, including a description of the location of these spectrin isoforms at the light- and electron-microscope level of resolution; a comparison of their structure and interactions with spectrin-binding proteins (ankyrin, actin, synapsin I, amelin, and calmodulin); a description of their expression during brain development; and hypotheses concerning their potential roles in axonal transport and synaptic transmission.

Partial characterization of the copolymerization reaction of erythrocyte membrane band 3 with hemichromes

Early intermediates in the denaturation of hemoglobin, termed hemichromes, have been found previously to associate with the cytoplasmic domain of erythrocyte membrane band 3 in a manner which rapidly propagates into an insoluble, macroscopic copolymer. Because this interaction is thought to force a redistribution of band 3 in situ, the properties of the copolymerization reaction were investigated in greater detail. The band 3-hemichrome coaggregate was found to be stabilized largely by ionic interactions since elevation of either ionic strength or pH led to dissolution of the complex. The pH dependence, however, shifted to a more alkaline pH with increasing hemichrome concentration, suggesting a strong linkage between band 3 or hemichrome protonation and copolymer formation. The stoichiometry of the copolymer was measured at five globin chains per band 3 chain whenever underivatized dimer-tetramer hemichrome mixtures were employed. However, cross-linking of the hemichromes at either the alpha or the beta chains to form the stabilized tetramer yielded a copolymer stoichiometry of approximately eight globin chains per band 3 chain, i.e., two hemichrome sites per band 3 subunit. While underivatized hemichromes exhibited both a fast and slow phase of copolymerization, the cross-link-stabilized tetrameric hemichromes displayed predominantly the fast phase kinetics. Naturally occurring disulfide cross-linked hemichromes also reacted more avidly with band 3 than their reduced counterparts; however, the copolymerization process also proceeded to completion with totally reduced components. It is concluded that copolymerization of band 3 with hemichromes should occur under normal cellular conditions and at an accelerated velocity when the intracellular reducing power is low.

Target molecular weights for red cell band 3 stilbene and mercurial binding sites

Radiation inactivation was used to measure the target sizes for binding of disulfonic stilbene anion transport inhibitor 4,4'-dibenzamido-2,2'-disulfonic stilbene (DBDS) and mercurial water transport inhibitor p-chloromercuribenzene sulfonate (pCMBS) to human erythrocytes. The measured target size for erythrocyte ghost acetylcholinesterase was 78 +/- 3 kDa. DBDS binding to ghost membranes was measured by a fluorescence enhancement technique. Radiation (0-26 Mrad) had no effect on total membrane protein and DBDS binding affinity, whereas DBDS binding stoichiometry decreased exponentially with radiation dose, giving a target size of 59 +/- 4 kDa. H2-4,4'-diisothiocyano-2,2'-disulfonic stilbene (H2-DIDS, 5 microM) blocked greater than 95% of DBDS binding at all radiation doses. pCMBS binding was measured from the time course of tryptophan fluorescence quenching in ghosts treated with the sulfhydryl reagent N-ethylmaleimide (NEM). Radiation did not affect the kinetics of tryptophan quenching, whereas the total amplitude of the fluorescence signal inactivated with radiation with a target size of 31 +/- 6 kDa. These results support the notion that DBDS and pCMBS bind to the transmembrane domain of erythrocyte band 3 in NEM-treated ghosts and demonstrate that radiation inactivation may probe a target significantly smaller than a covalently linked protein subunit. The small target size for the band 3 stilbene binding site may correspond to the intramembrane domain of the band 3 monomer (52 kDa), which is physically distinct from the cytoplasmic domain (42 kDa).