The major kidney AE1 isoform does not bind ankyrin (Ank1) in vitro. An essential role for the 79 NH2-terminal amino acid residues of band 3

Identification of variants in 94 Chinese patients with hereditary spherocytosis by next-generation sequencing

Hereditary spherocytosis (HS) is the most common type of hereditary erythrocyte membrane disease and has varied phenotypic features and genetic patterns. We herein performed a retrospective study of 94 patients with HS and aimed to investigate the genetic variations and genotype-phenotype correlations using targeted next-generation sequencing. In 79/94 (84%) patients, 83 HS variants including 67 novel variants were identified. Pathogenic variants of SPTB, ANK1, SLC4A1, SPTA1, and EPB42 were found in 32/79(41%), 22/79(28%), 15/79 (19%), 8/79 (9%), and 3/79 (4%) of the patients respectively, revealing that SPTB is the most frequently mutated HS gene in Eastern China. Most SPTB and ANK1 gene variations were nonsense and frameshift variations. Missense variants were the main variant type of SLC4A1, SPTA1, and EPB42 genes. Interestingly, one SPTA1 variant (p. Arg1757Cys) showed an autosomal dominant inheritance pattern and one EPB42 variant (p. Gln377His) was apparent as a hotspot variation. Furthermore, genotype-phenotype analysis was performed among the five mutated gene groups. Besides the finding that patients with the SLC4A1 variant had the highest mean corpuscular hemoglobin levels, no clear correlations between genotype and phenotype were observed.

Architecture of the human erythrocyte ankyrin-1 complex

The stability and shape of the erythrocyte membrane is provided by the ankyrin-1 complex, but how it tethers the spectrin-actin cytoskeleton to the lipid bilayer and the nature of its association with the band 3 anion exchanger and the Rhesus glycoproteins remains unknown. Here we present structures of ankyrin-1 complexes purified from human erythrocytes. We reveal the architecture of a core complex of ankyrin-1, the Rhesus proteins RhAG and RhCE, the band 3 anion exchanger, protein 4.2, glycophorin A and glycophorin B. The distinct T-shaped conformation of membrane-bound ankyrin-1 facilitates recognition of RhCE and, unexpectedly, the water channel aquaporin-1. Together, our results uncover the molecular details of ankyrin-1 association with the erythrocyte membrane, and illustrate the mechanism of ankyrin-mediated membrane protein clustering.

The Complexity of Genotype-Phenotype Correlations in Hereditary Spherocytosis: A Cohort of 95 Patients: Genotype-Phenotype Correlation in Hereditary Spherocytosis

Hereditary spherocytosis (HS) is a phenotypically and genetically heterogeneous disease. With the increased use of Next Generation Sequencing (NGS) techniques in the diagnosis of red blood cell disorders, the list of unique pathogenic mutations underlying HS is growing rapidly. In this study, we aimed to explore genotype-phenotype correlation in 95 HS patients genotyped by targeted NGS as part of routine diagnostics (UMC Utrecht, Utrecht, The Netherlands). In 85/95 (89%) of patients a pathogenic mutation was identified, including 56 novel mutations. SPTA1 mutations were most frequently encountered (36%, 31/85 patients), primarily in patients with autosomal recessive forms of HS. Three SPTA1 (α-spectrin) mutations showed autosomal dominant inheritance. ANK1 (ankyrin1) mutations accounted for 27% (23/85 patients) and SPTB (β-spectrin) mutations for 20% (17/85 patients). Moderate or severe HS was more frequent in patients with SPTB or ANK1 mutations, reflected by lower hemoglobin concentrations and higher reticulocyte counts. Interestingly, mutations affecting spectrin association domains of ANK1, SPTA1 and SPTB resulted in more severe phenotypes. Additionally, we observed a clear association between phenotype and aspects of red cell deformability as determined by the Laser assisted Optical Rotational Cell Analyzer (LoRRca MaxSis). Both maximal deformability and area under the curve were negatively associated with disease severity (respectively r = -0.46, p < 0.01, and r = -0.39, p = 0.01). Genotype-phenotype prediction in HS facilitates insight in consequences of pathogenic mutations for the assembly and dynamic interactions of the red cell cytoskeleton. In addition, we show that measurements of red blood cell deformability are clearly correlated with HS severity.

Regulation of erythrocyte Na+/K+/2Cl- cotransport by an oxygen-switched kinase cascade

Many erythrocyte processes and pathways, including glycolysis, the pentose phosphate pathway (PPP), KCl cotransport, ATP release, Na⁺/K⁺-ATPase activity, ankyrin-band 3 interactions, and nitric oxide (NO) release, are regulated by changes in O₂ pressure that occur as a red blood cell (RBC) transits between the lungs and tissues. The O₂ dependence of glycolysis, PPP, and ankyrin-band 3 interactions (affecting RBC rheology) are controlled by O₂-dependent competition between deoxyhemoglobin (deoxyHb), but not oxyhemoglobin (oxyHb), and other proteins for band 3. We undertook the present study to determine whether the O₂ dependence of Na⁺/K⁺/2Cl^- cotransport (catalyzed by Na⁺/K⁺/2Cl^- cotransporter 1 [NKCC1]) might similarly originate from competition between deoxyHb and a protein involved in NKCC1 regulation for a common binding site on band 3. Using three transgenic mouse strains having mutated deoxyhemoglobin-binding sites on band 3, we found that docking of deoxyhemoglobin at the N terminus of band 3 displaces the protein with no lysine kinase 1 (WNK1) from its overlapping binding site on band 3. This displacement enabled WNK1 to phosphorylate oxidative stress-responsive kinase 1 (OSR1), which, in turn, phosphorylated and activated NKCC1. Under normal solution conditions, the NKCC1 activation increased RBC volume and thereby induced changes in RBC rheology. Because the deoxyhemoglobin-mediated WNK1 displacement from band 3 in this O₂ regulation pathway may also occur in the regulation of other O₂-regulated ion transporters, we hypothesize that the NKCC1-mediated regulatory mechanism may represent a general pattern of O₂ modulation of ion transporters in erythrocytes.

Mouse models of SLC4-linked disorders of HCO3--transporter dysfunction

The SLC4 family Cl^-/[Formula: see text] cotransporters (NBCe1, NBCe2, NBCn1, and NBCn2) contribute to a variety of vital physiological processes including pH regulation and epithelial fluid secretion. Accordingly, their dysfunction can have devastating effects. Disorders such as epilepsy, hemolytic anemia, glaucoma, hearing loss, osteopetrosis, and renal tubular acidosis are all genetically linked to SLC4-family gene loci. This review summarizes how studies of Slc4-modified mice have enhanced our understanding of the etiology of SLC4-linked pathologies and the interpretation of genetic linkage studies. The review also surveys the novel disease signs exhibited by Slc4-modified mice which could either be considered to presage their description in humans, or to highlight interspecific differences. Finally, novel Slc4-modified mouse models are proposed, the study of which may further our understanding of the basis and treatment of SLC4-linked disorders of [Formula: see text]-transporter dysfunction.

Stomatin modulates the activity of the Anion Exchanger 1 (AE1, SLC4A1)

Anion Exchanger 1 (AE1) and stomatin are integral proteins of the red blood cell (RBC) membrane. Erythroid and kidney AE1 play a major role in HCO₃^- and Cl^- exchange. Stomatins down-regulate the activity of many channels and transporters. Biochemical studies suggested an interaction of erythroid AE1 with stomatin. Moreover, we previously reported normal AE1 expression level in stomatin-deficient RBCs. Here, the ability of stomatin to modulate AE1-dependent Cl^-/HCO₃^- exchange was evaluated using stopped-flow methods. In HEK293 cells expressing recombinant AE1 and stomatin, the permeabilities associated with AE1 activity were 30% higher in cells overexpressing stomatin, compared to cells with only endogenous stomatin expression. Ghosts from stomatin-deficient RBCs and controls were resealed in the presence of pH- or chloride-sensitive fluorescent probes and submitted to inward HCO₃^- and outward Cl^- gradients. From alkalinization rate constants, we deduced a 47% decreased permeability to HCO₃^- for stomatin-deficient patients. Similarly, kinetics of Cl^- efflux, followed by the probe dequenching, revealed a significant 42% decrease in patients. In situ Proximity Ligation Assays confirmed an interaction of AE1 with stomatin, in both HEK recombinant cells and RBCs. Here we show that stomatin modulates the transport activity of AE1 through a direct protein-protein interaction.

PDLIM5 links kidney anion exchanger 1 (kAE1) to ILK and is required for membrane targeting of kAE1

Anion exchanger 1 (AE1) mediates Cl⁻/HCO₃⁻ exchange in erythrocytes and kidney intercalated cells where it functions to maintain normal bodily acid-base homeostasis. AE1’s C-terminal tail (AE1C) contains multiple potential membrane targeting/retention determinants, including a predicted PDZ binding motif, which are critical for its normal membrane residency. Here we identify PDLIM5 as a direct binding partner for AE1 in human kidney, via PDLIM5’s PDZ domain and the PDZ binding motif in AE1C. Kidney AE1 (kAE1), PDLIM5 and integrin-linked kinase (ILK) form a multiprotein complex in which PDLIM5 provides a bridge between ILK and AE1C. Depletion of PDLIM5 resulted in significant reduction in kAE1 at the cell membrane, whereas over-expression of kAE1 was accompanied by increased PDLIM5 levels, underscoring the functional importance of PDLIM5 for proper kAE1 membrane residency, as a crucial linker between kAE1 and actin cytoskeleton-associated proteins in polarized cells.

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.

Renal peroxiredoxin 6 interacts with anion exchanger 1 and plays a novel role in pH homeostasis

Peroxiredoxin 6 (PRDX6) is one of six members of the PRDX family, which have peroxidase and antioxidant activity. PRDX6 is unique, containing only one conserved cysteine residue (C47) rather than the two found in other PRDXs. A yeast two-hybrid screen found PRDX6 to be a potential binding partner of the C-terminal tail of anion exchanger 1 (AE1), a Cl⁻/HCO_3⁻ exchanger basolaterally expressed in renal α-intercalated cells. PRDX6 immunostaining in human kidney was both cytoplasmic and peripheral and co-localized with AE1. Analysis of native protein showed it was largely monomeric, whereas expressed tagged protein was more dimeric. Two methionine oxidation sites were identified. In vitro and ex vivo pulldowns and immunoprecipitation assays confirmed interaction with AE1, but mutation of the conserved cysteine resulted in loss of interaction. Prdx6 knockout mice had a baseline acidosis with a major respiratory component and greater AE1 expression than wild type animals. After an oral acid challenge, PRDX6 expression increased in wild type mice, with preservation of AE1. However, AE1 expression was significantly decreased in knockout animals. Kidneys from acidified mice showed widespread proximal tubular vacuolation in wild type but not knockout animals. Knockdown of PRDX6 by siRNA in mammalian cells reduced both total and cell membrane AE1 levels. Thus, PRDX6-AE1 interaction contributes to maintenance of AE1 during cellular stress such as during metabolic acidosis.

Evidence of a structural and functional ammonium transporter RhBG·anion exchanger 1·ankyrin-G complex in kidney epithelial cells

The renal ammonium transporter RhBG and anion exchanger 1 kAE1 colocalize in the basolateral domain of α-intercalated cells in the distal nephron. Although we have previously shown that RhBG is linked to the spectrin-based skeleton through ankyrin-G and that its NH3 transport activity is dependent on this association, there is no evidence for an interaction of kAE1 with this adaptor protein. We report here that the kAE1 cytoplasmic N terminus actually binds to ankyrin-G, both in yeast two-hybrid analysis and by coimmunoprecipitation in situ in HEK293 cells expressing recombinant kAE1. A site-directed mutagenesis study allowed the identification of three dispersed regions on kAE1 molecule linking the third and fourth repeat domains of ankyrin-G. One secondary docking site corresponds to a major interacting loop of the erythroid anion exchanger 1 (eAE1) with ankyrin-R, whereas the main binding region of kAE1 does not encompass any eAE1 determinant. Stopped flow spectrofluorometry analysis of recombinant HEK293 cells revealed that the Cl(-)/HCO3 (-) exchange activity of a kAE1 protein mutated on the ankyrin-G binding site was abolished. This disruption impaired plasma membrane expression of kAE1 leading to total retention on cytoplasmic structures in polarized epithelial Madin-Darby canine kidney cell transfectants. kAE1 also directly interacts with RhBG without affecting its surface expression and NH3 transport function. This is the first description of a structural and functional RhBG·kAE1·ankyrin-G complex at the plasma membrane of kidney epithelial cells, comparable with the well known Rh·eAE1·ankyrin-R complex in the red blood cell membrane. This renal complex could participate in the regulation of acid-base homeostasis.

Physical and functional links between anion exchanger-1 and sodium pump

Anion exchanger-1 (AE1) mediates chloride-bicarbonate exchange across the plasma membranes of erythrocytes and, via a slightly shorter transcript, kidney epithelial cells. On an omnivorous human diet, kidney AE1 is mainly active basolaterally in α-intercalated cells of the collecting duct, where it is functionally coupled with apical proton pumps to maintain normal acid-base homeostasis. The C-terminal tail of AE1 has an important role in its polarized membrane residency. We have identified the β1 subunit of Na(+),K(+)-ATPase (sodium pump) as a binding partner for AE1 in the human kidney. Kidney AE1 and β1 colocalized in renal α-intercalated cells and coimmunoprecipitated (together with the catalytic α1 subunit of the sodium pump) from human kidney membrane fractions. ELISA and fluorescence titration assays confirmed that AE1 and β1 interact directly, with a Kd value of 0.81 μM. GST-AE1 pull-down assays using human kidney membrane proteins showed that the last 11 residues of AE1 are important for β1 binding. siRNA-induced knockdown of β1 in cell culture resulted in a significant reduction in kidney AE1 levels at the cell membrane, whereas overexpression of kidney AE1 increased cell surface sodium pump levels. Notably, membrane staining of β1 was reduced throughout collecting ducts of AE1-null mouse kidney, where increased fractional excretion of sodium has been reported. These data suggest a requirement of β1 for proper kidney AE1 membrane residency, and that activities of AE1 and the sodium pump are coregulated in kidney.

Membrane peroxidation and methemoglobin formation are both necessary for band 3 clustering: mechanistic insights into human erythrocyte senescence

Oxidative damage and clustering of band 3 in the membrane have been implicated in the removal of senescent human erythrocytes from the circulation at the end of their 120 day life span. However, the biochemical and mechanistic events leading to band 3 cluster formation have yet to be fully defined. Here we show that while neither membrane peroxidation nor methemoglobin (MetHb) formation on their own can induce band 3 clustering in the human erythrocytes, they can do so when acting in combination. We further show that binding of MetHb to the cytoplasmic domain of band 3 in peroxidized, but not in untreated, erythrocyte membranes induces cluster formation. Age-fractionated populations of erythrocytes from normal human blood, obtained by a density gradient procedure, have allowed us to examine a subpopulation, highly enriched in senescent cells. We have found that band 3 clustering is a feature of only this small fraction, amounting to ∼0.1% of total circulating erythrocytes. These senescent cells are characterized by an increased proportion of MetHb as a result of reduced nicotinamide adenine dinucleotide-dependent reductase activity and accumulated oxidative membrane damage. These findings have allowed us to establish that the combined effects of membrane peroxidation and MetHb formation are necessary for band 3 clustering, and this is a very late event in erythrocyte life. A plausible mechanism for the combined effects of membrane peroxidation and MetHb is proposed, involving high-affinity cooperative binding of MetHb to the cytoplasmic domain of oxidized band 3, probably because of its carbonylation, rather than other forms of oxidative damage. This modification leads to dissociation of ankyrin from band 3, allowing the tetrameric MetHb to cross-link the resulting freely diffusible band 3 dimers, with formation of clusters.

The divergence, actions, roles, and relatives of sodium-coupled bicarbonate transporters

The mammalian Slc4 (Solute carrier 4) family of transporters is a functionally diverse group of 10 multi-spanning membrane proteins that includes three Cl-HCO3 exchangers (AE1-3), five Na(+)-coupled HCO3(-) transporters (NCBTs), and two other unusual members (AE4, BTR1). In this review, we mainly focus on the five mammalian NCBTs-NBCe1, NBCe2, NBCn1, NDCBE, and NBCn2. Each plays a specialized role in maintaining intracellular pH and, by contributing to the movement of HCO3(-) across epithelia, in maintaining whole-body pH and otherwise contributing to epithelial transport. Disruptions involving NCBT genes are linked to blindness, deafness, proximal renal tubular acidosis, mental retardation, and epilepsy. We also review AE1-3, AE4, and BTR1, addressing their relevance to the study of NCBTs. This review draws together recent advances in our understanding of the phylogenetic origins and physiological relevance of NCBTs and their progenitors. Underlying these advances is progress in such diverse disciplines as physiology, molecular biology, genetics, immunocytochemistry, proteomics, and structural biology. This review highlights the key similarities and differences between individual NCBTs and the genes that encode them and also clarifies the sometimes confusing NCBT nomenclature.

Anion exchanger 1 in red blood cells and kidney: Band 3's in a pod

The bicarbonate/chloride exchanger 1 (AE1, Band 3) is abundantly expressed in the red blood cell membrane, where it is involved in gas exchange and functions as a major site of cytoskeletal attachment to the erythrocyte membrane. A truncated kidney isoform (kAE1) is highly expressed in type A intercalated cells of the distal tubules, where it is vital for urinary acidification. Recently, kAE1 has emerged as a novel physiologically significant protein in the kidney glomerulus. This minireview will discuss the known interactions of kAE1 in the podocytes and the possible mechanisms whereby this important multispanning membrane protein may contribute to the function of the glomerular filtration barrier and prevent proteinuria.

Determination of structural models of the complex between the cytoplasmic domain of erythrocyte band 3 and ankyrin-R repeats 13-24

The adaptor protein ankyrin-R interacts via its membrane binding domain with the cytoplasmic domain of the anion exchange protein (AE1) and via its spectrin binding domain with the spectrin-based membrane skeleton in human erythrocytes. This set of interactions provides a bridge between the lipid bilayer and the membrane skeleton, thereby stabilizing the membrane. Crystal structures for the dimeric cytoplasmic domain of AE1 (cdb3) and for a 12-ankyrin repeat segment (repeats 13-24) from the membrane binding domain of ankyrin-R (AnkD34) have been reported. However, structural data on how these proteins assemble to form a stable complex have not been reported. In the current studies, site-directed spin labeling, in combination with electron paramagnetic resonance (EPR) and double electron-electron resonance, has been utilized to map the binding interfaces of the two proteins in the complex and to obtain inter-protein distance constraints. These data have been utilized to construct a family of structural models that are consistent with the full range of experimental data. These models indicate that an extensive area on the peripheral domain of cdb3 binds to ankyrin repeats 18-20 on the top loop surface of AnkD34 primarily through hydrophobic interactions. This is a previously uncharacterized surface for binding of cdb3 to AnkD34. Because a second dimer of cdb3 is known to bind to ankyrin repeats 7-12 of the membrane binding domain of ankyrin-R, the current models have significant implications regarding the structural nature of a tetrameric form of AE1 that is hypothesized to be involved in binding to full-length ankyrin-R in the erythrocyte membrane.

Structural characterization of the cytosolic domain of kidney chloride/bicarbonate anion exchanger 1 (kAE1)

Kidney anion exchanger 1 (kAE1) is a membrane glycoprotein expressed in alpha-intercalated cells in the collecting ducts of the kidney where it mediates electroneutral chloride/bicarbonate exchange. Human kAE1 is a truncated form of erythroid AE1 missing the first 65 residues of the N-terminal cytosolic domain, which includes a disordered acidic region (residues 1-54) and the first beta-strand (residues 55-65) of the folded region. Unlike erythroid AE1, kAE1 does not bind deoxyhemoglobin, glycolytic enzymes, or cytoskeletal components. To understand the effect of the N-terminal deletion on the structure of the cytosolic domain, we performed an extensive biophysical analysis on His 6 tagged cytosolic domains of erythroid AE1 (cdAE1), kidney AE1 (cdkAE1), and a novel truncation mutant (cdDelta54AE1) missing the first 54 residues, but retaining the beta-strand. Circular dichroism did not detect any major differences in secondary structure, and sedimentation analyses showed that all three proteins were dimeric. Differential scanning calorimetry revealed that cdAE1 and cdDelta54AE1 had similar thermal stabilities with midpoints of transition higher than cdkAE1. cdAE1 and cdDelta54AE1 underwent similar pH-dependent fluorescence changes, while cdkAE1 exhibited a higher intrinsic fluorescence at neutral and acidic pH. Urea denaturation resulted in dequenching of tryptophan fluorescence in cdAE1, while tryptophans in cdkAE1 were already dequenched in the native state. We conclude that the absence of the central beta-strand in cdkAE1 results in a less stable and more open structure than cdAE1. This structural change, in addition to the loss of the acidic amino-terminal region, may account for the altered protein binding properties of kAE1.

Glycophorin A: Band 3 aid

Band 3 (B3) is a major site of cytoskeletal attachment to the erythrocyte membrane and is important for gas exchange. A truncated isoform of B3 (kB3) is expressed in the alpha-intercalated cells of the kidney and its functional activity and basolateral localization are essential for acid secretion. B3 mutations generally lead to red blood cell (RBC) specific disease (hereditary spherocytosis (HS), Southeast Asian Ovalocytosis or hereditary stomatocytosis) or kidney disease (distal Renal Tubular Acidosis--dRTA). It is rare for both the RBC and kidney disease phenotypes to co-exist, but this does occur in knockout mice, and also in humans (B3 Coimbra and B3 Courcouronne) or cattle with homozygous HS mutations. This is because RBCs express a B3 chaperone-like molecule in the form of Glycophorin A that can rescue the majority of B3 mutations that cause dRTA but probably not the majority of HS mutations. The study of naturally occurring B3 variant blood and expression of B3 or kB3 mutants in heterologous expression systems has provided valuable information concerning B3 trafficking and interactions in the RBC and kidney. This article will review these studies and comment on our current understanding of the interaction between GPA with B3 and also on the proposed B3 centred macrocomplex.

Interaction of integrin-linked kinase with the kidney chloride/bicarbonate exchanger, kAE1

Kidney anion exchanger 1 (kAE1) mediates chloride/bicarbonate exchange at the basolateral membrane of kidney alpha-intercalated cells, thereby facilitating bicarbonate reabsorption into the blood. Human kAE1 lacks the N-terminal 65 residues of the erythroid form (AE1, band 3), which are essential for binding of cytoskeletal and cytosolic proteins. Yeast two-hybrid screening identified integrin-linked kinase (ILK), a serine/threonine kinase, and an actin-binding protein as an interacting partner with the N-terminal domain of kAE1. Interaction between kAE1 and ILK was confirmed in co-expression experiments in HEK 293 cells and is mediated by a previously unidentified calponin homology domain in the kAE1 N-terminal region. The calponin homology domain of kAE1 binds the C-terminal catalytic domain of ILK to enhance association of kAE1 with the actin cytoskeleton. Overexpression of ILK increased kAE1 levels at the cell surface as shown by flow cytometry, cell surface biotinylation, and anion transport activity assays. Pulse-chase experiments revealed that ILK associates with kAE1 early in biosynthesis, likely in the endoplasmic reticulum. ILK co-localized with kAE1 at the basolateral membrane of polarized Madin-Darby canine kidney cells and in alpha-intercalated cells of human kidneys. Taken together these results suggest that ILK and kAE1 traffic together from the endoplasmic reticulum to the basolateral membrane. ILK may provide a linkage between kAE1 and the underlying actin cytoskeleton to stabilize kAE1 at the basolateral membrane, resulting in higher levels of cell surface expression.

Targeting of the AE2 anion exchanger to the Golgi apparatus is cell type-dependent and correlates with the expression of Ank(195), a Golgi membrane skeletal protein

Sodium-independent anion exchangers (AE1-4) show remarkable variability in their tissue-specific expression and subcellular localization. Currently, isoform-specific targeting mechanisms are considered to be responsible for this variable localization. Here, we report that targeting can also be cell type-specific. We show that the full-length AE2 protein and its green fluorescent protein- or DsRed-tagged variants localize predominantly either to the Golgi apparatus in COS-7 cells, or to the plasma membrane in HeLa cells. This alternative targeting did not seem to result from either translational or post-translational differences, but rather from differential expression of at least one of the Golgi membrane skeletal proteins, ankyrin(195) (Ank(195)), between the two cell types. Comparative studies with several different cell lines revealed that the Golgi localization of the AE2 protein correlated strictly with the expression of Ank(195) in the cells. The two Golgi-associated proteins also co-localized well and similarly resisted detergent extraction in the cold, whereas the plasma membrane-localized AE2 in Ank(195)-deficient cells was mostly detergent-soluble. Collectively, our results suggest that Ank(195) expression is a key determinant for the variable and cell type-dependent localization of the AE2 protein in the Golgi apparatus in mammalian cells.

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.

Dynamics of ankyrin-containing complexes in chicken embryonic erythroid cells: role of phosphorylation

Chicken erythroid ankyrin undergoes a fairly rapid cycle of cytoskeletal association, dissociation, and turnover. In addition, the cytoskeletal association of ankyrin is regulated by phosphorylation. Treatment of erythroid cells with serine and threonine phosphatase inhibitors stimulated the hyperphosphorylation of the 225- and 205-kDa ankyrin isoforms, and dissociated the bulk of these isoforms from cytoskeletal spectrin. In vitro binding studies have shown that this dissociation of ankyrin from spectrin in vivo can be attributed to a reduced ability of hyperphosphorylated ankyrin to bind spectrin. Interestingly, a significant fraction of detergent insoluble ankyrin accumulates in a spectrin-independent pool. At least some of this spectrin-independent pool of ankyrin is complexed with the AE1 anion exchanger, and the solubility properties of this pool are also regulated by phosphorylation. Treatment of cells with serine and threonine phosphatase inhibitors had no effect on ankyrin/AE1 complex formation. However, these inhibitors were sufficient to shift ankyrin/AE1 complexes from the detergent insoluble to the soluble pool. These analyses, which are the first to document the in vivo consequences of ankyrin phosphorylation, indicate that erythroid ankyrin-containing complexes can undergo dynamic rearrangements in response to changes in phosphorylation.

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.

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.

Intracellular trafficking of variant chicken kidney AE1 anion exchangers: role Of alternative NH(2) termini in polarized sorting and Golgi recycling

The variant chicken kidney AE1 anion exchangers differ only at the NH(2) terminus of their cytoplasmic domains. Transfection studies have indicated that the variant chicken AE1-4 anion exchanger accumulates in the basolateral membrane of polarized MDCK kidney epithelial cells, while the AE1-3 variant, which lacks the NH(2)-terminal 63 amino acids of AE1-4, primarily accumulates in the apical membrane. Mutagenesis studies have shown that the basolateral accumulation of AE1-4 is dependent upon two tyrosine residues at amino acids 44 and 47 of the polypeptide. Interestingly, either of these tyrosines is sufficient to direct efficient basolateral sorting of AE1-4. However, in the absence of both tyrosine residues, AE1-4 accumulates in the apical membrane of MDCK cells. Pulse-chase studies have shown that after delivery to the cell surface, newly synthesized AE1-4 is recycled to the Golgi where it acquires additional N-linked sugar modifications. This Golgi recycling activity is dependent upon the same cytoplasmic tyrosine residues that are required for the basolateral sorting of this variant transporter. Furthermore, mutants of AE1-4 that are defective in Golgi recycling are unable to associate with the detergent insoluble actin cytoskeleton and are rapidly turned over. These studies, which represent the first description of tyrosine-dependent cytoplasmic sorting signal for a type III membrane protein, have suggested a critical role for the actin cytoskeleton in regulating AE1 anion exchanger localization and stability in this epithelial cell type.

A spliced variant of AE1 gene encodes a truncated form of Band 3 in heart: the predominant anion exchanger in ventricular myocytes

The anion exchangers (AE) are encoded by a multigenic family that comprises at least three genes, AE1, AE2 and AE3, and numerous splicoforms. Besides regulating intracellular pH (pHi) via the Cl-/HCO3- exchange, the AEs exert various cellular functions including generation of a senescent antigen, anchorage of the cytoskeleton to the membrane and regulation of metabolism. Most cells express several AE isoforms. Despite the key role of this family of proteins, little is known about the function of specific AE isoforms in any tissue, including the heart. We therefore chose isolated cardiac cells, in which a tight control of pHi is mandatory for the excitation-contraction coupling process, to thoroughly investigate the expression of the AE genes at both the mRNA and protein levels. RT-PCR revealed the presence of AE1, AE2 and AE3 mRNAs in both neonatal and adult rat cardiomyocytes. AE1 is expressed both as the erythroid form (Band 3 or eAE1) and a novel alternate transcript (nAE1), which was more specifically characterized using a PCR mapping strategy. Two variants of AE2 (AE2a and AE2c) were found at the mRNA level. Cardiac as well as brain AE3 mRNAs were expressed in both neonatal and adult rat cardiomyocytes. Several AE protein isoforms were found, including a truncated form of AE1 and two AE3s, but there was no evidence of AE2 protein in adult rat cardiomyocytes. In cardiomyocytes transfected with an AE3 oligodeoxynucleotide antisense, AE3 immunoreactivity was dramatically decreased but the activity of the Cl-/HCO3- exchange was unchanged. In contrast, intracellular microinjection of blocking anti-AE1 antibodies inhibited the AE activity. Altogether, our findings suggest that a specific and novel AE1 splicoform (nAE1) mediates the cardiac Cl-/HCO3- exchange. The multiple gene and protein expression within the same cell type suggest numerous functions for this protein family.

Chicken erythroid AE1 anion exchangers associate with the cytoskeleton during recycling to the Golgi

Chicken erythroid AE1 anion exchangers receive endoglycosidase F (endo F)-sensitive sugar modifications in their initial transit through the secretory pathway. After delivery to the plasma membrane, anion exchangers are internalized and recycled to the Golgi where they acquire additional N-linked modifications that are resistant to endo F. During recycling, some of the anion exchangers become detergent insoluble. The acquisition of detergent insolubility correlates with the association of the anion exchanger with cytoskeletal ankyrin. Reagents that inhibit different steps in the endocytic pathway, including 0.4 M sucrose, ammonium chloride, and brefeldin A, block the acquisition of endo F-resistant sugars and the acquisition of detergent insolubility by newly synthesized anion exchangers. The inhibitory effects of ammonium chloride on anion exchanger processing are rapidly reversible. Furthermore, AE1 anion exchangers become detergent insoluble more rapidly than they acquire endo F-resistant modifications in cells recovering from an ammonium chloride block. This suggests that the cytoskeletal association of the recycling anion exchangers occurs after release from the compartment where they accumulate due to ammonium chloride treatment, and prior to their transit through the Golgi. The recycling pool of newly synthesized anion exchangers is reflected in the steady-state distribution of the polypeptide. In addition to plasma membrane staining, anion exchanger antibodies stain a perinuclear compartment in erythroid cells. This perinuclear AE1-containing compartment is also stained by ankyrin antibodies and partially overlaps the membrane compartment stained by NBD C6-ceramide, a Golgi marker. Detergent extraction of erythroid cells in situ has suggested that a substantial fraction of the perinuclear pool of AE1 is cytoskeletal associated. The demonstration that erythroid anion exchangers interact with elements of the cytoskeleton during recycling to the Golgi suggests the cytoskeleton may be involved in the post-Golgi trafficking of this membrane transporter.

Phenotypic plasticity in the intercalated cell: the hensin pathway

The collecting duct of the renal tubule contains two cell types, one of which, the intercalated cell, is responsible for acidification and alkalinization of urine. These cells exist in a multiplicity of morphological forms, with two extreme types, alpha and beta. The former acidifies the urine by an apical proton-translocating ATPase and a basolateral Cl/HCO3 exchanger, which is an alternately spliced form of band 3. This kidney form of band 3, kAE1, is present in the apical membrane of the beta-cell, which has the H+-ATPase on the basolateral membrane. We had suggested previously that metabolic acidosis leads to conversion of beta-types to alpha-types. To study the biochemical basis of this plasticity, we used an immortalized cell line of the beta-cell and showed that these cells convert to the alpha-phenotype when plated at superconfluent density. At high density these cells localize a new protein, which we term "hensin," to the extracellular matrix, and hensin acts as a molecular switch capable of changing the phenotype of these cells in vitro. Hensin induces new cytoskeletal proteins, makes the cells assume a more columnar shape and retargets kAE1 and the H+-ATPase. These recent studies suggest that the conversion of beta- to alpha-cells, at least in vitro, bears many of the hallmarks of terminal differentiation.

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.

Kanadaptin is a protein that interacts with the kidney but not the erythroid form of band 3

Although epithelial membrane proteins are separately targeted to apical or basolateral domains, some are apically located in one cell type but are basolateral in others. More dramatically, the anion exchanger of a clonal cell line of intercalated cells derived from the kidney can be retargeted from the apical to basolateral domain. This Cl:HCO3 exchanger, kAE1, is an alternately spliced form of the erythroid anion exchanger (AE1, band 3), but unlike band 3 it does not bind ankyrin. Here we identify a new protein (kanadaptin) that binds to the cytoplasmic domain of kAE1 in vitro and in vivo but not to the erythroid AE1 or to ankyrin. No significant homologous proteins have been reported so far. Kanadaptin is widely expressed in epithelial (kidney, lung, and liver) and non-epithelial cells (brain and skeletal and cardiac muscle). In kidney, we found by immunocytochemistry that kanadaptin was only expressed in the collecting tubule. In the intercalated cells of this segment, it colocalized with kAE1 in cytoplasmic vesicles but not when the exchanger was in the basolateral membrane. These results raised the possibility that this protein is involved in the targeting of kAE1 vesicles to their final destination.

The structure and function of band 3 (AE1): recent developments (review)

This review discusses recent advances in our understanding of the structure, function and molecular genetics of the membrane domain of red cell anion exchanger, band 3 (AE1), and its role in red cell and kidney disease. A new model for the topology of band 3 has been proposed, which suggests the membrane domain has 12 membrane spans, rather than the 14 membrane spans of earlier models. The major difference between the models is in the topology of the region on the C-terminal side of membrane spans 1-7. Two dimensional crystals of the deglycosylated membrane domain of band 3 have yielded two and three dimensional projection maps of the membrane domain dimer at low resolution. The human band 3 gene has been completely sequenced and this has facilitated the study of natural band 3 mutations and their involvement in disease. About 20% of hereditary spherocytosis cases arise from heterozygosity for band 3 mutations, and result in the absence or decrease of the mutant protein in the red cell membrane. Several other natural band 3 mutations are known that appear to be clinically benign, but alter red cell phenotype or are associated with altered red cell blood group antigens. These include the mutant band 3 present in Southeast Asian ovalocytosis, a condition which provides protection against cerebral malaria in children. Familial distal renal tubular acidosis, a condition associated with kidney stones, has been shown to result from a novel group of band 3 mutations. The total absence of band 3 has been described in animals-occurring naturally in cattle and after targeted disruption in mice. Some of these severely anaemic animals survive, so band 3 is not strictly essential for life. Although the band 3-negative red cells were very unstable, they contained a normally-assembled red cell skeleton, suggesting that the bilayer of the normal red cell membrane is stabilized by band 3 interactions with membrane lipids, rather than by interactions with the spectrin skeleton.

Anion exchanger 1 (band 3) is required to prevent erythrocyte membrane surface loss but not to form the membrane skeleton

The red blood cell (RBC) membrane protein AE1 provides high affinity binding sites for the membrane skeleton, a structure critical to RBC integrity. AE1 biosynthesis is postulated to be required for terminal erythropoiesis and membrane skeleton assembly. We used targeted mutagenesis to assess AE1 function in vivo. RBCs lacking AE1 spontaneously shed membrane vesicles and tubules, leading to severe spherocytosis and hemolysis, but the levels of the major skeleton components, the synthesis of spectrin in mutant erythroblasts, and skeletal architecture are normal or nearly normal. The results indicate that AE1 does not regulate RBC membrane skeleton assembly in vivo but is essential for membrane stability. We postulate that stabilization is achieved through AE1-lipid interactions and that loss of these interactions is a key pathogenic event in hereditary spherocytosis.

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.

Differential expression of Na(+)-K(+)-ATPase, ankyrin, fodrin, and E-cadherin along the kidney nephron

Ionic homeostasis in vertebrates is maintained by epithelial cells that line kidney nephrons. Transport of ions and solutes is coupled to Na+ reabsorption from the ultrafiltrate and requires specific subcellular distribution and activity of Na(+)-K(+)-ATPase along the nephron. Studies using cell culture models of renal epithelia indicate that the subcellular distribution of Na(+)-K(+)-ATPase is regulated by interactions with the submembrane cytoskeleton and E-cadherin-mediated adherens junctions. We have now examined the relevance of these in vitro observations to the subcellular organization of these proteins in different nephron segments of the adult mouse kidney using immunofluorescence microscopy. Our results demonstrate that segmental and subcellular distributions of Na(+)-K(+)-ATPase and the membrane-cytoskeletal proteins, ankyrin and fodrin, vary in parallel along the nephron and do not parallel variations in expression of the tight junction protein ZO-1 or E-cadherin. These data indicate that a mechanism for restricting Na(+)-K(+)-ATPase subcellular distributions through interactions with the membrane cytoskeleton is likely to be relevant in vivo.

The ANK repeats of erythrocyte ankyrin form two distinct but cooperative binding sites for the erythrocyte anion exchanger

The 24 ANK repeats of the membrane-binding domain of ankyrin form four folded subdomains of six ANK repeats each. These four repeat subdomains mediate interactions with at least seven different families of membrane proteins. In the erythrocyte, the main membrane target of ankyrin is the Cl-/HCO3- anion exchanger. This report presents the first evidence that ankyrin contains two separate binding sites for anion exchanger dimers. One site utilizes repeat subdomain two (repeats 7-12) while the other requires both repeat subdomains three and four (repeats 13-24). The two sites are positively coupled with a Hill coefficient of 1.4. Since the anion exchanger exists as a dimer in the membrane, the presence of two binding sites on ankyrin allows ankyrin to interact with four anion exchangers simultaneously. These findings provide a direct demonstration of the versatility of ANK repeats in protein recognition, and have important implications for the organization of ankyrin-linked integral membrane proteins in erythrocytes as well as other cells.

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.

The role of cysteine residues in the erythrocyte plasma membrane anion exchange protein, AE1

AE1 (Band 3), a congruent to 110-kDa integral plasma membrane protein, facilitates the electroneutral movement of Cl- and HCO3- across the erythrocyte membrane and serves as the primary attachment site for the erythrocyte spectrin-actin cytoskeleton. In this investigation, we have characterized the role of native cysteines in the function of AE1. We have constructed a mutant version of human AE1 (AE1C-) in which all five cysteines of AE1 were replaced with serines. Wild-type and AE1C- cDNAs were expressed by transient transfection of human embryonic kidney cells. Two of the mutated cysteines in AE1C- are in a region involved in ankyrin binding, and ankyrin binding has previously been shown to be sensitive to the oxidation state of these cysteines. However, the KD values for ankyrin binding by AE1 and AE1C- were indistinguishable, suggesting that AE1 cysteines are not essential components of the ankyrin-binding site. Using size exclusion chromatography, both AE1 and AE1C- were found to associate as a mixture of dimers and high molecular mass complexes. The rate of anion exchange by AE1C-, as measured in a reconstituted microsome sulfate transport assay, was indistinguishable from that by AE1 and was inhibited by 4,4'-diisothiocyanodihydrostilbene-2,2'-disulfonate. We conclude that the cysteines of AE1 are not required for the anion exchange or cytoskeletal binding roles of the protein.