Monoclonal antibodies to the membrane domain of the human erythrocyte anion transport protein. Localization of the C-terminus of the protein to the cytoplasmic side of the red cell membrane and distribution of the protein in some human tissues

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.

Large conformational dynamics in Band 3 protein: Significance for erythrocyte senescence signalling

Band 3 (Anion Exchanger 1, AE1), the predominant protein of erythrocyte membranes, facilitates Cl^-/HCO₃^- exchange and anchors the plasma membrane to the cytoskeleton. The Band 3 crystal structure revealed the amino acid 812-830 region as intracellular, conflicting with protein chemical data that suggested extracellular disposition. Further, circulating senescent cell auto-antibody that cannot enter erythrocytes, binds two regions of Band 3: residues 538-554 and 812-830. To reconcile this discrepancy, we assessed localization of residues 812-830 with Band 3 expressed in HEK293 cells and human erythrocytes, using chemical labeling probes and an antibody against residues 812-830. Antibody and chemical probes revealed reorientation of 812-830 region between extracellular and intracellular. This dramatic conformational change is an intrinsic property of the Band 3 molecule, occurring when expressed in HEK293 cells and without the damage that occurs during erythrocyte circulation. Conditions used to crystallize Band 3 for structural determination did not alter conformational dynamics. Collectively, these data reveal large Band 3 conformational dynamics localized to a region previously identified as an erythrocyte senescence epitope. Surface exposure of the senescence epitope (812-830), limited by conformational dynamics, may act as the "molecular clock" in erythrocyte senescence.

Molecular mechanism for the red blood cell senescence clock

Lacking protein synthesis machinery and organelles necessary for autophagy or apoptosis, aged red blood cells (RBCs) are marked by circulating auto-antibodies for macrophage-mediated clearance. The antigen recognized by these auto-antibodies is the major protein of the RBC membrane, Band 3. To ensure regulation and specificity in clearance, the molecular "clock" must mark senescent cells in a way that differentiates them from younger cells, to prevent premature clearance. Predominant models of Band 3 senescence signaling are reviewed, and merits are discussed in light of the recently published crystal structure of the Band 3 membrane domain. © 2017 IUBMB Life, 70(1):32-40, 2018.

Structure and Function of SLC4 Family [Formula: see text] Transporters

The solute carrier SLC4 family consists of 10 members, nine of which are [Formula: see text] transporters, including three Na(+)-independent Cl(-)/[Formula: see text] exchangers AE1, AE2, and AE3, five Na(+)-coupled [Formula: see text] transporters NBCe1, NBCe2, NBCn1, NBCn2, and NDCBE, as well as "AE4" whose Na(+)-dependence remains controversial. The SLC4 [Formula: see text] transporters play critical roles in pH regulation and transepithelial movement of electrolytes with a broad range of demonstrated physiological relevances. Dysfunctions of these transporters are associated with a series of human diseases. During the past decades, tremendous amount of effort has been undertaken to investigate the topological organization of the SLC4 transporters in the plasma membrane. Based upon the proposed topology models, mutational and functional studies have identified important structural elements likely involved in the ion translocation by the SLC4 transporters. In the present article, we review the advances during the past decades in understanding the structure and function of the SLC4 transporters.

Autophagic vesicles on mature human reticulocytes explain phosphatidylserine-positive red cells in sickle cell disease

During maturation to an erythrocyte, a reticulocyte must eliminate any residual organelles and reduce its surface area and volume. Here we show this involves a novel process whereby large, intact, inside-out phosphatidylserine (PS)-exposed autophagic vesicles are extruded. Cell surface PS is a well-characterized apoptotic signal initiating phagocytosis. In peripheral blood from patients after splenectomy or in patients with sickle cell disease (SCD), the number of circulating red cells exposing PS on their surface is elevated. We show that in these patients PS is present on the cell surface of red cells in large (∼1.4 µm) discrete areas corresponding to autophagic vesicles. The autophagic vesicles found on reticulocytes are identical to those observed on red cells from splenectomized individuals and patients with SCD. Our data suggest the increased thrombotic risk associated with splenectomy, and patients with hemoglobinopathies is a possible consequence of increased levels of circulating mature reticulocytes expressing inside-out PS-exposed autophagic vesicles because of asplenia.

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.

Anion exchanger 1 interacts with nephrin in podocytes

The central role of the multifunctional protein nephrin within the macromolecular complex forming the glomerular slit diaphragm is well established, but the mechanisms linking the slit diaphragm to the cytoskeleton and to the signaling pathways involved in maintaining the integrity of the glomerular filter remain incompletely understood. Here, we report that nephrin interacts with the bicarbonate/chloride transporter kidney anion exchanger 1 (kAE1), detected by yeast two-hybrid assay and confirmed by immunoprecipitation and co-localization studies. We confirmed low-level glomerular expression of kAE1 in human and mouse kidneys by immunoblotting and immunofluorescence microscopy. We observed less kAE1 in human glomeruli homozygous for the NPHS1(FinMaj) nephrin mutation, whereas kAE1 expression remained unchanged in the collecting duct. We could not detect endogenous kAE1 expression in NPHS1(FinMaj) podocytes in primary culture, but heterologous re-introduction of wild-type nephrin into these podocytes rescued kAE1 expression. In kidneys of Ae1(-/-) mice, nephrin abundance was normal but its distribution was altered along with the reported kAE1-binding protein integrin-linked kinase (ILK). Ae1(-/-) mice had increased albuminuria with glomerular enlargement, mesangial expansion, mesangiosclerosis, and expansion of the glomerular basement membrane. Glomeruli with ILK-deficient podocytes also demonstrated altered AE1 and nephrin expression, further supporting the functional interdependence of these proteins. These data suggest that the podocyte protein kAE1 interacts with nephrin and ILK to maintain the structure and function of the glomerular basement membrane.

Investigating the key membrane protein changes during in vitro erythropoiesis of protein 4.2 (-) cells (mutations Chartres 1 and 2)

BACKGROUND

Protein 4.2 deficiency caused by mutations in the EPB42 gene results in hereditary spherocytosis with characteristic alterations of CD47, CD44 and RhAG. We decided to investigate at which stage of erythropoiesis these hallmarks of protein 4.2 deficiency arise in a novel protein 4.2 patient and whether they cause disruption to the band 3 macrocomplex.

DESIGN AND METHODS

We used immunoprecipitations and detergent extractability to assess the strength of protein associations within the band 3 macrocomplex and with the cytoskeleton in erythrocytes. Patient erythroblasts were cultured from peripheral blood mononuclear cells to study the effects of protein 4.2 deficiency during erythropoiesis.

RESULTS

We report a patient with two novel mutations in EPB42 resulting in complete protein 4.2 deficiency. Immunoprecipitations revealed a weakened ankyrin-1-band 3 interaction in erythrocytes resulting in increased band 3 detergent extractability. CD44 abundance and its association with the cytoskeleton were increased. Erythroblast differentiation revealed that protein 4.2 and band 3 appear simultaneously and associate early in differentiation. Protein 4.2 deficiency results in lower CD47, higher CD44 expression and increased RhAG glycosylation starting from the basophilic stage. The normal downregulation of CD44 expression was not seen during protein 4.2(-) erythroblast differentiation. Knockdown of CD47 did not increase CD44 expression, arguing against a direct reciprocal relationship.

CONCLUSIONS

We have established that the characteristic changes caused by protein 4.2 deficiency occur early during erythropoiesis. We postulate that weakening of the ankyrin-1-band 3 association during protein 4.2 deficiency is compensated, in part, by increased CD44-cytoskeleton binding.

Structure of the membrane domain of human erythrocyte anion exchanger 1 revealed by electron crystallography

The membrane domain of human erythrocyte anion exchanger 1 (AE1) works as a Cl(-)/HCO(3)(-) antiporter. This exchange is a key step for CO(2)/O(2) circulation in the blood. In spite of their importance, structural information about AE1 and the AE (anion exchanger) family are still very limited. We used electron microscopy to solve the three-dimensional structure of the AE1 membrane domain, fixed in an outward-open conformation by cross-linking, at 7.5-A resolution. A dimer of AE1 membrane domains packed in two-dimensional array showed a projection map similar to that of the prokaryotic homolog of the ClC chloride channel, a Cl(-)/H(+) antiporter. In a three-dimensional map, there are V-shaped densities near the center of the dimer and slightly narrower V-shaped clusters at a greater distance from the center of the dimer. These appear to be inserted into the membrane from opposite sides. The structural motifs, two homologous pairs of helices in internal repeats of the ClC transporter (helices B+C and J+K), are well fitted to those AE1 densities after simple domain movement.

Helical image reconstruction of the outward-open human erythrocyte band 3 membrane domain in tubular crystals

The C-terminal membrane domain of erythrocyte band 3 functions as an anion exchanger. Here, we report the three-dimensional (3D) structure of the membrane domain in an inhibitor-stabilized, outward-open conformation at 18A resolution. Unstained, frozen-hydrated tubular crystals containing the membrane domain of band 3 purified from human red blood cells (hB3MD) were examined using cryo-electron microscopy and iterative helical real-space reconstruction (IHRSR). The 3D image reconstruction of the tubular crystals showed the molecular packing of hB3MD dimers with dimensions of 60 x 110 A in the membrane plane and a thickness of 70A across the membrane. Immunoelectron microscopy and carboxyl-terminal digestion demonstrated that the intracellular surface of hB3MD was exposed on the outer surface of the tubular crystal. A 3D density map revealed that hB3MD consists of at least two subdomains and that the outward-open form is characterized by a large hollow area on the extracellular surface and continuous density on the intracellular surface.

Evidence against a direct interaction between intracellular carbonic anhydrase II and pure C-terminal domains of SLC4 bicarbonate transporters

Based on solid-phase binding assays with enzyme-linked immunosorbent assay detection, previous investigators suggested that intracellular carbonic anhydrase II (CA II) interacts at high affinity with the C-terminal (Ct) domains of SLC4 bicarbonate-transport proteins, expressed as glutathione S-transferase (GST) fusion proteins, to form functional HCO3- metabolons. Here we re-evaluated this protein-protein interaction using two solid-phase binding assays. We first compared the ability of the Ct domain of three SLC4 transporters, SLC4-A1 (AE1), SLC4-A4 (NBCe1), and SLC4-A8 (NDCBE), to bind immobilized CA II, using enzyme-linked immunosorbent assay detection. We found that when expressed as GST fusion proteins, all three bind to CA II (Kd 300-600 nM) better than does pure GST. However, we detected no binding of pure SLC4-Ct peptides to immobilized CA II. Second, we reversed assay orientation by immobilizing the SLC4-Ct fusion proteins or peptides. We found that more CA II binds to GST than to any of the three GST-SLC4-Ct fusion proteins. Furthermore, we detected no binding of CA II to any of the immobilized pure SLC4-Ct peptides. Finally, we used surface plasmon resonance to detect possible rapid interactions between CA II and the pure peptides. Although we detected acetazolamide binding to immobilized CA II and specific antibodies binding to immobilized SLC4-Ct peptides, we detected no binding of CA II to immobilized SLC4-Ct or vice versa. Thus, although an HCO3 metabolon may exist, CA II cannot bind directly to pure SLC4-Ct peptides and can bind to GST-SLC4-Ct fusion proteins only when the CA II is immobilized and the fusion protein is soluble, and not vice versa.

The human NBCe1-A mutant R881C, associated with proximal renal tubular acidosis, retains function but is mistargeted in polarized renal epithelia

The human electrogenic renal Na-HCO(3) cotransporter (NBCe1-A; SLC4A4) is localized to the basolateral membrane of proximal tubule cells. Mutations in the SLC4A4 gene cause an autosomal recessive proximal renal tubular acidosis (pRTA), a disease characterized by impaired ability of the proximal tubule to reabsorb HCO(3)(-) from the glomerular filtrate. Other symptoms can include mental retardation and ocular abnormalities. Recently, a novel homozygous missense mutant (R881C) of NBCe1-A was reported from a patient with a severe pRTA phenotype. The mutant protein was described as having a lower than normal activity when expressed in Xenopus oocytes, despite having normal Na(+) affinity. However, without trafficking data, it is impossible to determine the molecular basis for the phenotype. In the present study, we expressed wild-type NBCe1-A (WT) and mutant NBCe1-A (R881C), tagged at the COOH terminus with enhanced green fluorescent protein (EGFP). This approach permitted semiquantification of surface expression in individual Xenopus oocytes before assay by two-electrode voltage clamp or measurements of intracellular pH. These data show that the mutation reduces the surface expression rather than the activity of the individual protein molecules. Confocal microscopy on polarized mammalian epithelial kidney cells [Madin-Darby canine kidney (MDCK)I] expressing nontagged WT or R881C demonstrates that WT is expressed at the basolateral membrane of these cells, whereas R881C is retained in the endoplasmic reticulum. In summary, the pathophysiology of pRTA caused by the R881C mutation is likely due to a deficit of NBCe1-A at the proximal tubule basolateral membrane, rather than a defect in the transport activity of individual molecules.

Topology of transmembrane segments 1-4 in the human chloride/bicarbonate anion exchanger 1 (AE1) by scanning N-glycosylation mutagenesis

Human AE1 (anion exchanger 1), or Band 3, is an abundant membrane glycoprotein found in the plasma membrane of erythrocytes. The physiological role of the protein is to carry out chloride/bicarbonate exchange across the plasma membrane, a process that increases the carbon-dioxide-carrying capacity of blood. To study the topology of TMs (transmembrane segments) 1-4, a series of scanning N-glycosylation mutants were created spanning the region from EC (extracellular loop) 1 to EC2 in full-length AE1. These constructs were expressed in HEK-293 (human embryonic kidney) cells, and their N-glycosylation efficiencies were determined. Unexpectedly, positions within putative TMs 2 and 3 could be efficiently glycosylated. In contrast, the same positions were very poorly glycosylated when present in mutant AE1 with the SAO (Southeast Asian ovalocytosis) deletion (DeltaA400-A408) in TM1. These results suggest that the TM2-3 region of AE1 may become transiently exposed to the endoplasmic reticulum lumen during biosynthesis, and that there is a competition between proper folding of the region into the membrane and N-glycosylation at introduced sites. The SAO deletion disrupts the proper integration of TMs 1-2, probably leaving the region exposed to the cytosol. As a result, engineered N-glycosylation acceptor sites in TM2-3 could not be utilized by the oligosaccharyltransferase in this mutant form of AE1. The properties of TM2-3 suggest that these segments form a re-entrant loop in human AE1.

Protein-4.2 association with band 3 (AE1, SLCA4) in Xenopus oocytes: effects of three natural protein-4.2 mutations associated with hemolytic anemia

We have investigated the effects of coexpression of protein 4.2 and three protein-4.2 variants with band 3 in the Xenopus oocyte expression system. Normal protein 4.2 increased band-3–specific chloride transport in the oocytes. Protein 4.2 also coimmunoprecipitated with band 3 and colocalized with band 3 at the oocyte plasma membrane. The increase in band-3–mediated chloride transport and coimmunoprecipitation of protein 4.2 required the presence of the N-terminal cytoplasmic domain of band 3. Protein 4.2 also localized to the oocyte plasma membrane in the absence of band 3. The protein-4.2 variants 4.2 Tozeur (R310Q) and 4.2 Komatsu (D175Y) had impaired ability to bind to band 3 and these variants did not localize to the oocyte plasma membrane when expressed on their own or when coexpressed with band 3. Unexpectedly, 4.2 Nippon (A142T) behaved similarly to normal protein 4.2. In the absence of a crystal structure of protein 4.2, we propose a homology model of protein 4.2 based on the structure of the sequence-related protein transglutaminase. Using our results in oocytes and this homology model we speculate how these mutations affect protein 4.2 and result in hereditary spherocytosis.

Regions of human kidney anion exchanger 1 (kAE1) required for basolateral targeting of kAE1 in polarised kidney cells: mis-targeting explains dominant renal tubular acidosis (dRTA)

Distal renal tubular acidosis (dRTA) is characterised by defective acid secretion by kidney α-intercalated cells. Some dominantly inherited forms of dRTA result from anion exchanger 1 (AE1) mutations. We have developed a stably transfected cell model for the expression of human kidney AE1 (kAE1) and mutant kAE1 proteins in MDCKI cells. Normal kAE1 was delivered to the plasma membrane of non-polarised cells and to the basolateral membrane of polarised cells. The AE1 N-glycan was processed to a complex form. Surprisingly, expression of kAE1 increased the permeability of the paracellular barrier of polarised MDCKI monolayers. All dominant dRTA mutations examined altered the targeting of kAE1 in MDCKI cells. The mutant proteins kAE1(R589H), kAE1(S613F) and kAE1(R901Stop) were retained in the ER in non-polarised cells, but the kAE1(R901Stop) protein was also present in late endosomes/lysosomes. The complex N-glycan of kAE1(R901Stop) was larger than that of normal kAE1. In polarised cells, the mutant kAE1(R901Stop) was mis-targeted to the apical membrane, while the kAE1(R589H) and kAE1(S613F) mutants did not reach the cell surface. These results demonstrate that dominant dRTA mutations cause aberrant targeting of kAE1 in polarised kidney cells and provide an explanation for the origin of dominant dRTA. Our data also demonstrate that the 11 C-terminal residues of kAE1 contain a tyrosine-dependent basolateral targeting signal that is not recognised by μ1B-containing AP-1 adaptor complexes. In the absence of the N-terminus of kAE1, the C-terminus was not sufficient to localise kAE1 to the basolateral membrane. These results suggest that a determinant within the kAE1 N-terminus co-operates with the C-terminus for kAE1 basolateral localisation.

Altered Structure and Anion Transport Properties of Band 3 (AE1, SLC4A1) in Human Red Cells Lacking Glycophorin A

We have studied the properties of band 3 in different glycophorin A (GPA)-deficient red cells. These red cells lack either both GPA and glycophorin B (GPB) (M(k)M(k) cells) or GPA (En(a-) cells) or contain a hybrid of GPA and GPB (MiV cells). Sulfate transport was reduced in all three red cell types to approximately 60% of that in normal control red cells as a result of an increased apparent K(m) for sulfate. Transport of the monovalent anions iodide and chloride was also reduced. The reduced iodide transport resulted from a reduction in the V(max) for iodide transport. The anion transport site was investigated by measuring iodide fluorescence quenching of eosin-5-maleimide (EMA)-labeled band 3. The GPA-deficient cells had a normal K(d) for iodide binding, in agreement with the unchanged K(m) found in transport studies. However, the apparent diffusion quenching constant (K(q)) was increased, and the fluorescence polarization of band 3-bound EMA decreased in the variant cells, suggesting increased flexibility of the protein in the region of the EMA-binding site. This increased flexibility is probably associated with the decrease in V(max) observed for iodide transport. Our results suggest that band 3 in the red cell can take up two different structures: one with high anion transport activity when GPA is present and one with lower anion transport activity when GPA is absent.

Novel topology in C-terminal region of the human plasma membrane anion exchanger, AE1

Human AE1 performs electroneutral exchange of Cl(-) for HCO(3)(-) across the erythrocyte membrane. We examined the topology of the AE1 C-terminal region using cysteine-scanning mutagenesis and sulfhydryl-specific chemistry. Eighty individual cysteine residues, introduced into an otherwise cysteine-less mutant between Phe(806) and Cys(885), were expressed by transient transfection of HEK293 cells. Topology of the region was determined by comparing cysteine labeling with the membrane-permeant cysteine-directed reagent biotin maleimide, with or without prior labeling with the membrane-impermeant reagents, bromotrimethylammoniumbimane bromide (qBBr) and lucifer yellow iodoacetamide (LYIA). Phe(806)-Leu(835), Ser(852)-Ala(855), and Ile(872)-Cys(885) were labeled by biotin maleimide, suggesting their location in an aqueous environment. In contrast, Phe(836)-Lys(851) and Ser(856)-Arg(871) were not labeled by biotin maleimide and therefore localize to the plane of the bilayer, as transmembrane segments (TM). Labeling by qBBr revealed that Pro(815)-Lys(829) and Ser(852)-Ala(855) are accessible to the extracellular medium. Pro(815)-Lys(829) mutants were also labeled with LYIA. Mutants Ile(872)-Cys(885) were inaccessible to the extracellular medium and thus localized to the intracellular surface of AE1. Functional assays revealed that one face of each of two AE1 TMs was sensitive to mutation. Based on these results, we propose a topology model for the C-terminal region of the membrane domain of human AE1.

Topology of the anion exchange protein AE1: the controversial sidedness of lysine 743

The topology of the band 3 (AE1) polypeptide of the erythrocyte membrane is not fully established despite extensive study. Residues near lysine 743 (K743) have been reported to be extracellular in some studies and cytoplasmic in others. In the work presented here, we have attempted to establish the sidedness of K743 using in situ proteolysis. Trypsin, papain, and proteinase K do not cleave band 3 at or near K743 in intact red cells, even under conditions that cause cleavage on the C-terminal side of the glycosylation site (N642) in extracellular loop 4. In contrast, trypsin sealed inside red cell ghosts cleaves at K743, as does trypsin treatment of inside-out vesicles (IOVs). The transport inhibitor 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulfonate (H(2)DIDS), acting from the extracellular side, blocks trypsin cleavage at K743 in unsealed membranes by inducing a protease-resistant conformation. H(2)DIDS added to IOVs does not prevent cleavage at K743; therefore, trypsin cleavage at K743 in IOVs is not a consequence of cleavage of right-side-out or leaky vesicles. Finally, microsomes were prepared from HEK293 cells expressing the membrane domain of AE1 lacking the normal glycosylation site. This polypeptide does not traffic to the surface membrane; trypsin treatment of microsomes containing this polypeptide produces the 20 kDa fragment, providing further evidence that K743 is exposed at the cytoplasmic surface. Therefore, the actions of trypsin on intact cells, resealed ghosts, unsealed ghosts, inside-out vesicles, and microsomes from HEK293 cells all indicate that K743 is cytoplasmic and not extracellular.

Band 3 Walton, a C-terminal deletion associated with distal renal tubular acidosis, is expressed in the red cell membrane but retained internally in kidney cells

Human band 3 Walton is an AE1 mutation that results in the deletion of the 11 COOH-terminal amino acids of the protein and is associated with dominant distal renal tubular acidosis. The properties of band 3 Walton expressed with normal band 3 in the heterozygous mutant erythrocytes and the kidney isoform expressed in Xenopus oocytes and in the Madin-Darby canine kidney cell line were examined. The mutant erythrocytes have normal hematology but have reduced band 3 Walton content. Transport studies showed that erythrocyte band 3 Walton has normal sulfate transport activity, and kidney band 3 Walton has normal chloride transport activity when expressed in Xenopus oocytes. The mutant protein is clearly able to reach the cell surface of erythrocytes and oocytes. In contrast, while normal kidney band 3 was expressed at the cell surface in the kidney cell line, the Walton mutant protein was retained intracellularly within the kidney cells. The results demonstrate that band 3 Walton is targeted differently in erythrocytes and kidney cells and indicate that the COOH-terminal tail of band 3 is required to allow movement to the cell surface in kidney cells. It is proposed here that the mutant band 3 gives rise to dominant distal renal tubular acidosis by inhibiting the movement of normal band 3 to the cell surface. It is suggested that this results from the association of the normal and mutant proteins in band 3 hetero-oligomers, which causes the intracellular retention of normal band 3 with the mutant protein.

A transport metabolon. Functional interaction of carbonic anhydrase II and chloride/bicarbonate exchangers

The cytoplasmic carboxyl-terminal domain of AE1, the plasma membrane chloride/bicarbonate exchanger of erythrocytes, contains a binding site for carbonic anhydrase II (CAII). To examine the physiological role of the AE1/CAII interaction, anion exchange activity of transfected HEK293 cells was monitored by following the changes in intracellular pH associated with AE1-mediated bicarbonate transport. AE1-mediated chloride/bicarbonate exchange was reduced 50-60% by inhibition of endogenous carbonic anhydrase with acetazolamide, which indicates that CAII activity is required for full anion transport activity. AE1 mutants, unable to bind CAII, had significantly lower transport activity than wild-type AE1 (10% of wild-type activity), suggesting that a direct interaction was required. To determine the effect of displacement of endogenous wild-type CAII from its binding site on AE1, AE1-transfected HEK293 cells were co-transfected with cDNA for a functionally inactive CAII mutant, V143Y. AE1 activity was maximally inhibited 61 +/- 4% in the presence of V143Y CAII. A similar effect of V143Y CAII was found for AE2 and AE3cardiac anion exchanger isoforms. We conclude that the binding of CAII to the AE1 carboxyl-terminus potentiates anion transport activity and allows for maximal transport. The interaction of CAII with AE1 forms a transport metabolon, a membrane protein complex involved in regulation of bicarbonate metabolism and transport.

Localization of endogenous and recombinant Na(+)-driven anion exchanger protein NDAE1 from Drosophila melanogaster

Na(+)-dependent Cl(-)/HCO exchange activity helps maintain intracellular pH (pH(i)) homeostasis in many invertebrate and vertebrate cell types. Our laboratory cloned and characterized a Na(+)-dependent Cl(-)/HCO exchanger (NDAE1) from Drosophila melanogaster (Romero MF, Henry D, Nelson S, Harte PJ, and Sciortino CM. J Biol Chem 275: 24552--24559, 2000). In the present study we used immunohistochemical and Western blot techniques to characterize the developmental expression, subcellular localization, and tissue distribution of NDAE1 protein in D. melanogaster. We have shown that a polyclonal antibody raised against the NH(2) terminus of NDAE1 (alpha CWR57) recognizes NDAE1 electrophysiologically characterized in Xenopus oocytes. Moreover, our results begin to delineate the NDAE1 topology, i.e., both the NH(2) and COOH termini are intracellular. NDAE1 is expressed throughout Drosophila development in the central and peripheral nervous systems, sensilla, and the alimentary tract (Malpighian tubules, gut, and salivary glands). Coimmunolabeling of larval tissues with NDAE1 antibody and a monoclonal antibody to the Na(+)-K(+)-ATPase alpha-subunit revealed that the majority of NDAE1 is located at the basolateral membranes of Malpighian tubule cells. These results suggest that NDAE1 may be a key pH(i) regulatory protein and may contribute to basolateral ion transport in epithelia and nervous system of Drosophila.

Coexpression of band 3 mutants and Rh polypeptides: differential effects of band 3 on the expression of the Rh complex containing D polypeptide and the Rh complex containing CcEe polypeptide

K562 cells were stably transfected with cDNAs encoding the band 3 found in Southeast Asian ovalocytosis (B3SAO, deletion of residues 400-408), band 3 with a transport-inactivating E681Q point mutation (B3EQ), or normal band 3 (B3). Flow cytometric analysis and quantitative immunoblotting revealed that B3SAO expressed alone was translocated to the plasma membrane, at levels similar to B3 or B3EQ. Nine monoclonal antibodies that reacted with extracellular loops of B3 also reacted with B3SAO, although the affinity of most antibodies for the mutant protein was reduced. Both known Wr(b) epitopes were expressed on K562/B3SAO cells, demonstrating that B3SAO interacts with glycophorin A. The growth rates of K562 clones expressing equivalent amounts of B3 and B3EQ were the same, suggesting that the potentially toxic transport function of band 3 may be regulated in K562 cells. The band 3-mediated enhancement of Rh antigen reactivity and the depression of Rh epitopes on SAO erythrocytes were investigated by comparing the coexpression of B3, B3SAO, or B3EQ in K562 clones expressing exogenous RhcE or RhD polypeptides. The results are consistent with an interaction between band 3 and the Rh polypeptide-Rh glycoprotein (RhAG) complex, which may enhance translocation of the complex or affect its conformation in the plasma membrane. The data suggest that the interaction between band 3 and the RhD-RhAG complex is weaker than it is between band 3 and the RhCcEe-RhAG complex.

Localization of the Cl-/HCO3- anion exchanger binding site to the amino-terminal region of carbonic anhydrase II

Human carbonic anhydrase II (CAII) possesses a binding site for an acidic motif (D887ADD) within the carboxyl-terminal region (Ct) of the human erythrocyte chloride/bicarbonate anion exchanger, AE1. In this study, the amino acid sequence comprising this AE1 binding site was localized to the first 17 residues of CAII, which form a basic patch on the surface of the protein. Truncation of the amino terminal of CAII by five residues resulted in a 3-fold reduction in the apparent affinity of the interaction with a GST fusion protein of the Ct of AE1 (GST-Ct) measured by a sensitive microtiter plate binding assay. Further amino-terminal truncation of CAII by 17 or 24 residues caused a loss of binding. The homologous isoform CAI does not bind AE1, despite having 60% sequence identity to CAII. One major difference between the two CA isoforms, within the amino-terminal region, is a high content of histidine residues in CAII (His3, -4, -10, -15, -17) not found in CAI. Mutation of pairs of these histidines (and one lysine) in CAII to the analogous residues in CAI (H3P/H4D or K9D/H10K or H15Q/H17S), or combinations of these various double mutants, did not greatly affect binding between GST-Ct and the mutant CAII. However, when all six of the targeted CAII residues were mutated to the corresponding sequence in CAI, binding of GST-Ct was lost. These results indicate that the AE1 binding site is located within the first 17 residues of CAII, and that the interaction is mediated by electrostatic interactions involving histidine and/or lysine residues. Further specificity for the interaction of AE1 and CAII is provided by a conserved leucine residue (L886) in AE1 that, when mutated to alanine, resulted in loss of GST-Ct binding to immobilized CAII. The binding of the basic amino-terminal region of CAII to an acidic Ct in AE1 provides a structural basis for linking bicarbonate transport across the cell membrane to intracellular bicarbonate metabolism.

Band 3 mutations, renal tubular acidosis and South-East Asian ovalocytosis in Malaysia and Papua New Guinea: loss of up to 95% band 3 transport in red cells

We describe three mutations of the red-cell anion exchangerband 3 (AE1, SLC4A1) gene associated with distalrenal tubular acidosis (dRTA) in families from Malaysia and Papua NewGuinea: Gly(701)-->Asp (G701D), Ala(858)-->Asp(A858D) and deletion of Val(850) (DeltaV850). The mutationsA858D and DeltaV850 are novel; all three mutations seem to berestricted to South-East Asian populations. South-East Asianovalocytosis (SAO), resulting from the band 3 deletion of residues400-408, occurred in many of the families but did not itselfresult in dRTA. Compound heterozygotes of each of the dRTA mutationswith SAO all had dRTA, evidence of haemolytic anaemia and abnormal red-cell properties. The A858D mutation showed dominant inheritance and therecessive DeltaV850 and G701D mutations showed a pseudo-dominantphenotype when the transport-inactive SAO allele was also present. Red-cell and Xenopus oocyte expression studies showed that theDeltaV850 and A858D mutant proteins have greatly decreased aniontransport when present as compound heterozygotes (DeltaV850/A858D,DeltaV850/SAO or A858D/SAO). Red cells with A858D/SAO had only 3% ofthe SO(4)(2-) efflux of normal cells, thelowest anion transport activity so far reported for human red cells. The results suggest dRTA might arise by a different mechanism for eachmutation. We confirm that the G701D mutant protein has an absoluterequirement for glycophorin A for movement to the cell surface. Wesuggest that the dominant A858D mutant protein is possibly mis-targetedto an inappropriate plasma membrane domain in the renal tubular cell,and that the recessive DeltaV850 mutation might give dRTA because ofits decreased anion transport activity.

Identification of the carbonic anhydrase II binding site in the Cl(-)/HCO(3)(-) anion exchanger AE1

The human Cl(-)/HCO(3)(-) anion exchanger (AE1) possesses a binding site within its 33 residue carboxyl-terminal region (Ct) for carbonic anhydrase II (CAII). The amino acid sequence comprising this CAII binding site was determined by peptide competition and by testing the ability of truncation and point mutants of the Ct sequence to bind CAII with a sensitive microtiter plate binding assay. A synthetic peptide consisting of the entire 33 residues of the Ct (residues 879-911) could compete with a GST fusion protein of the Ct (GST-Ct) for binding to immobilized CAII, while a peptide consisting of the last 16 residues (896-911) could not. A series of truncation mutants of the GST-Ct showed that the terminal 21 residues of AE1 were not required for binding CAII. Removal of four additional residues (887-890) from the Ct resulted in loss of CAII binding. Acidic residues in this region (D887ADD) were critical for binding since mutating this sequence in the GST-Ct to DAAA, AAAA, or NANN caused loss of CAII binding. A GST-Ct construct mutated to D887ANE, the homologous sequence in AE2, could bind CAII. AE2 is a widely expressed anion exchanger and has a homologous Ct region with 60% sequence identity to AE1. A GST fusion protein of the 33 residue Ct of AE2 could bind to CAII similarly to the Ct of AE1. Tethering of CAII to an acidic motif within the Ct of anion exchangers may be a general mechanism for promoting bicarbonate transport across cell membranes.

Cellular and subcellular distribution of polycystin-2, the protein product of the PKD2 gene

Mutations in the PKD1 and PKD2 genes account for 85 and 15% of cases of autosomal dominant polycystic kidney disease, respectively. Polycystin-2, the product of the PKD2 gene, is predicted to be an integral membrane protein with homology to a family of voltage-activated Ca(2+) channels. In vitro studies suggest that it may interact with polycystin-1, the PKD1 gene product, via coiled-coil domains present in their C-terminal domains. In this study, the cellular and subcellular distribution of polycystin-2 is defined and compared with polycystin-1. A panel of rabbit polyclonal antisera was raised against polycystin-2 and shown to recognize a single band consistent with polycystin-2 in multiple tissues and cell lines by immunoprecipitation and Western blotting. Immunostaining of human and murine renal tissues demonstrated widespread and developmentally regulated expression of polycytin-2, with highest levels in the kidney in the thick ascending limbs of the loop of Henle and the distal convoluted tubule. In contrast, polycystin-1 expression, while localizing to the same tubular segments, was highest in the collecting ducts. Immunohistochemical staining and immunofluorescence microscopy localized polycystin-2 to the basolateral plasma membrane of kidney tubular epithelial cells compared with the junctional localization of polycystin-1. Differences in the developmental, cellular, and subcellular expression of polycystin-1 and polycystin-2 suggest that they may be able to function independently of each other in addition to a potential in vivo interaction via their C-termini.

Structural model for the organization of the transmembrane spans of the human red-cell anion exchanger (band 3; AE1)

We have examined the functional co-assembly of non-complementary pairs of N- and C-terminal polypeptide fragments of the anion transport domain (b3mem) of human red-cell band 3. cDNA clones encoding non-contiguous pairs of fragments with one transmembrane (TM) region omitted, or overlapping pairs of fragments with between one and ten TM regions duplicated, were co-expressed in Xenopus oocytes and a cell-free translation system. Stilbene disulphonate-sensitive chloride uptake assays in oocytes revealed that the omission of any single TM region of b3mem except spans 6 and 7 caused a complete loss of functional expression. In contrast, co-expressed pairs of fragments overlapping a single TM region 5, 6, 7, 8, 9-10 or 11-12 retained a high level of functionality, whereas fragments overlapping the clusters of TM regions 2-5, 4-5, 5-8 and 8-10 also mediated some stilbene disulphonate-sensitive uptake. The co-assembly of N- or C-terminal fragments with intact band 3, b3mem or other fragments was examined by co-immunoprecipitation in non-denaturing detergent solutions by using monoclonal antibodies against the termini of b3mem. All the fragments, except for TM spans 13-14, co-immunoprecipitated with b3mem. The medium-sized N-terminal fragments comprising spans 1-6, 1-7 or 1-8 co-immunoprecipitated particularly strongly with the C-terminal fragments containing spans 8-14 or 9-14. The fragments comprising spans 1-4 or 1-12 co-immunoprecipitated less extensively than the other N-terminal fragments with either b3mem or C-terminal fragments. There is sufficient flexibility in the structure of b3mem to allow the inclusion of at least one duplicated TM span without a loss of function. We propose a working model for the organization of TM spans of dimeric band 3 based on current evidence.

Autosomal recessive distal renal tubular acidosis associated with Southeast Asian ovalocytosis

BACKGROUND

A defect in the anion exchanger 1 (AE1) of the basolateral membrane of type A intercalated cells in the renal collecting duct may result in a failure to maintain a cell-to-lumen H+ gradient, leading to distal renal tubular acidosis (dRTA). Thus, dRTA may occur in Southeast Asian ovalocytosis (SAO), a common AE1 gene abnormality observed in Southeast Asia and Melanesia. Our study investigated whether or not this renal acidification defect exists in individuals with SAO.

METHODS

Short and three-day NH4Cl loading tests were performed in 20 individuals with SAO and in two subjects, including their families, with both SAO and dRTA. Mutations of AE1 gene in individuals with SAO and members of the two families were also studied.

RESULTS

Renal acidification in the 20 individuals with SAO and in the parents of the two families was normal. However, the two clinically affected individuals with SAO and dRTA had compound heterozygosity of 27 bp deletion in exon 11 and missense mutation G701D resulting from a CGG-->CAG substitution in exon 17 of the AE1 gene. Red cells of the two subjects with dRTA and SAO and the family members with SAO showed an approximate 40% reduction in sulfate influx with normal 4,4'-di-isothiocyanato-stilbene-2,2'-disulfonic acid sensitivity and pH dependence.

CONCLUSION

These findings suggest that compound heterozygosity of abnormal AE1 genes causes autosomal recessive dRTA in SAO.

Chloride-bicarbonate exchangers in the human fetal pancreas

Cystic fibrosis (CF) is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene that encodes a small conductance cAMP-activated chloride ion channel. In the CF pancreatic duct, mutations in CFTR cause a reduction in bicarbonate secretion. This is thought to result from CFTR operating in parallel with a chloride-bicarbonate (Cl(-)/HCO(-)(3)) exchanger, located in the apical membrane of pancreatic duct cells. The molecular basis of this Cl(-)/HCO(-)(3) exchanger has not been identified. A combination of screening cDNA libraries, RNase protection, and 5' RACE analysis was used to identify Cl(-)/HCO(-)(3) exchangers in human fetal pancreas. An AE2 Cl(-)/HCO(-)(3) exchanger was shown to be expressed in human fetal pancreas from the midtrimester of gestation, at a time when CF-associated pathology commences. In addition, an AE1 Cl(-)/HCO(3) was identified in fetal pancreas but was absent from the adult pancreas and cultured ductal epithelial cells from fetal and adult pancreas.

Glycophorin A mutation Ala65 --> Pro gives rise to a novel pair of MNS alleles ENEP (MNS39) and HAG (MNS41) and altered Wrb expression: direct evidence for GPA/band 3 interaction necessary for normal Wrb expression

We report here a novel Glycophorin A (GPA) mutation Ala65 --> Pro which gives rise to a low-incidence antigen HAG, lack of a high-incidence antigen ENEP and aberrant expression of the high-incidence Wrb antigen. Anti-ENEP was identified in the serum of a transfused male patient (E.H.) who was homozygous for a GPA Ala65 --> Pro mutation and possessed a novel low-incidence antigen which we have called HAG. An unrelated HAG-positive individual, heterozygous for the Ala65 --> Pro mutation, has also been identified. Anti-HAG was present in several multispecific antisera to low-incidence antigens and in one monospecific serum. Normal expression of the Wrb antigen depends on the presence of amino acid Glu658 of band 3 and on the presence of GPA. However, a specific epitope on GPA has not previously been implicated. DNA sequence analysis of band 3 from patient E.H. was normal in the region of Wra/Wrb polymorphism with homozygous presence of Glu658 and therefore the abnormal Wrb expression results from the Ala65 --> Pro mutation in GPA. The ENEP and HAG antigens have been assigned the MNS blood group system numbers 002.039 and 002.041, respectively, by the ISBT Working Party on Terminology for Red Cell Surface Antigens.

Transmembrane folding of the human erythrocyte anion exchanger (AE1, Band 3) determined by scanning and insertional N-glycosylation mutagenesis

The human erythrocyte anion exchanger (AE1, Band 3) contains up to 14 transmembrane segments, with a single site of N-glycosylation at Asn642 in extracellular (EC) loop 4. Scanning and insertional N-glycosylation mutagenesis were used to determine the folding pattern of AE1 in the membrane. Full-length AE1, when expressed in transfected human embryonic kidney (HEK)-293 or COS-7 cells, retained a high-mannose oligosaccharide structure. Scanning N-glycosylation mutagenesis of EC loop 4 showed that N-glycosylation acceptor sites (Asn-Xaa-Ser/Thr) spaced 12 residues from the ends of adjacent transmembrane segments could be N-glycosylated. An acceptor site introduced at position 743 in intracellular (IC) loop 5 that could be N-glycosylated in a cell-free translation system was not N-glycosylated in transfected cells. Mutations designed to disrupt the folding of this loop enhanced the level of N-glycosylation at Asn743 in vitro. The results suggest that this loop might be transiently exposed to the lumen of the endoplasmic reticulum during biosynthesis but normally folds rapidly, precluding N-glycosylation. EC loop 4 insertions into positions 428, 484, 754 and 854 in EC loops 1, 2, 6 and 7 respectively were efficiently N-glycosylated, showing that these regions were extracellular. EC loop 4 insertions into positions 731 or 785 were poorly N-glycosylated, which was inconsistent with an extracellular disposition for these regions of AE1. Insertion of EC loop 4 into positions 599 and 820 in IC loops 3 and 6 respectively were not N-glycosylated in cells, which was consistent with a cytosolic disposition for these loops. Inhibitor-affinity chromatography with 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonate (SITS)-Affi-Gel was used to assess whether the AE1 mutants were in a native state. Mutants with insertions at positions 428, 484, 599, 731 and 785 showed impaired inhibitor binding, whereas insertions at positions 754, 820 and 854 retained binding. The results indicate that the folding of the C-terminal region of AE1 is more complex than originally proposed and that this region of the transporter might have a dynamic aspect.

Topology of the membrane domain of human erythrocyte anion exchange protein, AE1

Anion exchanger 1 (AE1) is the chloride/bicarbonate exchange protein of the erythrocyte membrane. By using a combination of introduced cysteine mutants and sulfhydryl-specific chemistry, we have mapped the topology of the human AE1 membrane domain. Twenty-seven single cysteines were introduced throughout the Leu708-Val911 region of human AE1, and these mutants were expressed by transient transfection of human embryonic kidney cells. On the basis of cysteine accessibility to membrane-permeant biotin maleimide and to membrane-impermeant lucifer yellow iodoacetamide, we have proposed a model for the topology of AE1 membrane domain. In this model, AE1 is composed of 13 typical transmembrane segments, and the Asp807-His834 region is membrane-embedded but does not have the usual alpha-helical conformation. To identify amino acids that are important for anion transport, we analyzed the anion exchange activity for all introduced cysteine mutants, using a whole cell fluorescence assay. We found that mutants G714C, S725C, and S731C have very low transport activity, implying that this region has a structurally and/or catalytically important role. We measured the residual anion transport activity after mutant treatment with the membrane-impermeant, cysteine-directed compound, sodium (2-sulfonatoethyl)methanethiosulfonate) (MTSES). Only two mutants, S852C and A858C, were inhibited by MTSES, indicating that these residues may be located in a pore-lining region.

Functional aspects of red cell antigens

Over 250 blood group determinants are known and most of these are located on integral red cell proteins and glycoproteins. The functions of some of these structures are known: Diego (band 3) is the red cell anion exchanger; Kidd, a urea transporter; Colton (aquaporin 1), a water channel; Cromer (DAF) and Knops (CRI), complement regulators; Diego (band 3) and Gerbich (glycophorin C/D) link the red cell membrane and the membrane skeleton. The Duffy glycoprotein is a chemokine receptor that may act as a scavenger for inflammatory mediators in the peripheral blood, but is also exploited as a receptor by Plasmodium vivax merozoites. The functions of some blood group antigens can be speculated upon because of structural similarity to proteins and glycoproteins of known function. For example, the Lutheran, LW, and Ok glycoproteins are members of the immunoglobulin superfamily of receptors and signal transducers, the Rh proteins and related glycoproteins show homology to ammonium transporters, and the Kell glycoprotein resembles a family of endopeptidases. Yet most blood groups systems contain null phenotypes associated with no apparent pathology. If these blood group antigens have important functions, other structures must be able to carry out those functions in their absence. Almost nothing is known of the biological significance of blood group polymorphism.

Functional Cell Surface Expression of Band 3, the Human Red Blood Cell Anion Exchange Protein (AE1), in K562 Erythroleukemia Cells: Band 3 Enhances the Cell Surface Reactivity of Rh Antigens

Human K562 erythroleukemia cells were transfected with human band 3 (anion exchanger 1 [AE1]) cDNA, using the pBabe retroviral vector. Stable K562 clones expressing band 3 were isolated by flow cytometry, and surface expression was quantified by immunoblotting. The function of band 3 expressed at the cell surface was demonstrated in chloride transport assays. K562 cells expressing band 3 also displayed high levels of the Wrb blood group antigen, confirming the role of band 3 in Wrb expression, and an increase in the low levels of endogenous Rh antigen activity. We also performed coexpression experiments with K562 clones that had previously been transduced with cDNAs encoding RhD or RhcE polypeptides. The transfection and expression of band 3 in these clones substantially increased the levels of RhD and cE antigen activity expressed on the cells and also increased the reactivity of the cells with antibody to the endogenous Rh glycoprotein (RhGP, Rh50). The increased reactivity of Rh antigens may result from cell surface or intracellular interactions of band 3 with the protein complex which contains the Rh polypeptides and RhGP, or from indirect effects of band 3 on the membrane environment. This work establishes a system for cell surface expression of band 3 in a mammalian cell line, which will enable further studies of the protein and its interactions with other membrane components.

Lysine-691 of the anion exchanger from human erythrocytes is located on its cytoplasmic surface

A combination of vectorial modification and site-directed immunochemistry has been used to determine the disposition, with respect to the membrane, of Lys-691 of the anion exchanger from human erythrocytes. Intact erythrocytes and inside-out vesicles were vectorially modified in the same container with pyridoxal phosphate and sodium [3H]borohydride. The modified inside-out vesicles were separated from erythrocytes by differential centrifugation and the vesicles and erythrocyte membranes were treated with alkali and digested with trypsin and thermolysin to liberate the peptides IVSKPER and IVSK¿Nepsilon-[4'-(5'-phospho-[4-3H]pyridoxyl)]¿PER. These peptides, containing the unmodified and modified versions of Lys-691, were retrieved from the digests by site-directed immunochemistry and were identified by HPLC and liquid scintillation spectroscopy. Both the inside-out vesicles and the intact erythrocytes contained the peptide IVSKPER, however, the 3H-label from the phosphopyridoxylated peptide could be detected only in the inside-out vesicles. The incorporation of 3H into Lys-691 of the anion exchanger from inside-out vesicles was at least 30-fold greater than the incorporation into Lys-691 of the anion exchanger from intact erythrocytes. It follows that Lys-691 of the anion exchanger is located on the cytoplasmic surface of the plasma membrane.

Functional Cell Surface Expression of Band 3, the Human Red Blood Cell Anion Exchange Protein (AE1), in K562 Erythroleukemia Cells: Band 3 Enhances the Cell Surface Reactivity of Rh Antigens

Human K562 erythroleukemia cells were transfected with human band 3 (anion exchanger 1 [AE1]) cDNA, using the pBabe retroviral vector. Stable K562 clones expressing band 3 were isolated by flow cytometry, and surface expression was quantified by immunoblotting. The function of band 3 expressed at the cell surface was demonstrated in chloride transport assays. K562 cells expressing band 3 also displayed high levels of the Wrb blood group antigen, confirming the role of band 3 in Wrb expression, and an increase in the low levels of endogenous Rh antigen activity. We also performed coexpression experiments with K562 clones that had previously been transduced with cDNAs encoding RhD or RhcE polypeptides. The transfection and expression of band 3 in these clones substantially increased the levels of RhD and cE antigen activity expressed on the cells and also increased the reactivity of the cells with antibody to the endogenous Rh glycoprotein (RhGP, Rh50). The increased reactivity of Rh antigens may result from cell surface or intracellular interactions of band 3 with the protein complex which contains the Rh polypeptides and RhGP, or from indirect effects of band 3 on the membrane environment. This work establishes a system for cell surface expression of band 3 in a mammalian cell line, which will enable further studies of the protein and its interactions with other membrane components.

Carbonic anhydrase II binds to the carboxyl terminus of human band 3, the erythrocyte C1-/HCO3- exchanger

In this study, we provide evidence that the 33-residue carboxyl-terminal (Ct) region of the human erythrocyte chloride/bicarbonate exchanger, band 3, binds carbonic anhydrase II (CAII). Immunofluorescence showed that tomato lectin-mediated clustering of band 3 in ghost membranes caused a similar clustering of CAII, indicating an in situ association. CAII cosolubilized and coimmunoprecipitated with band 3, suggesting that the two proteins form a complex. Band 3 (K1/2 = 70 nM) or the membrane domain of band 3 (K1/2 = 100 nM) bound saturably to immobilized CAII in a solid phase binding assay. The interaction with CAII was specifically blocked by an antibody to the Ct of band 3. Affinity blotting showed that a glutathione S-transferase (GST)-fusion protein (GST-Ct) containing the last 33 residues of human band 3 bound to CAII. The solid phase binding assay showed that binding of GST-Ct to immobilized CAII was saturable (K1/2 = 20 nM). The binding rate was slow (t1/2 = 12 h) at physiological ionic strength and pH but was enhanced at low ionic strength or acidic pH. Intact band 3 (Ki = 15 nM), the membrane domain of band 3 (Ki = 100 nM), or antibodies to the Ct of band 3 were able to block GST-Ct binding to CAII, confirming the specificity of the interaction. Affinity chromatography showed that CAII bound to immobilized GST-Ct with a 1:1 stoichiometry. This work indicates that CAII, the bicarbonate supplier, is directly coupled to band 3, the chloride/bicarbonate exchanger in red blood cells.

Complementation studies with co-expressed fragments of human red cell band 3 (AE1): the assembly of the anion-transport domain in xenopus oocytes and a cell-free translation system

We examined the assembly of the membrane domain of the human red cell anion transporter (band 3; AE1) by co-expression of recombinant N- and C-terminal fragments in Xenopus oocytes and in cell-free translation with canine pancreatic microsomes. Co-immunoprecipitation was performed in non-denaturing detergent solutions using antibodies directed against the N- and C-termini of the membrane domain. Eleven of the twelve fragments were expressed stably in oocytes in the presence or absence of their respective partners. However, the fragment containing from putative span nine to the C-terminus could be detected in oocytes only when co-expressed with its complementary partner containing the first eight spans. Co-expression of pairs of fragments divided in the first, second, third and fourth exofacial loops and in the fourth cytoplasmic loop resulted in a concentration-dependent association, but a pair of fragments divided in the sixth cytoplasmic loop did not co-immunoprecipitate. When two complementary fragments were translated separately in the cell-free system and the purified microsomes were then mixed, co-immunoprecipitation was observed only if the membranes were first fused using polyethylene glycol. This shows that co-immunoprecipitation results from specific interactions within the membrane and is not an artefact of detergent solubilization or immunoprecipitation. We demonstrate that band 3 assembly can occur within the membrane after translation, insertion and initial folding of the individual fragments have been completed. We conclude that most band 3 fragments contain the necessary information to fold in the membrane and adopt a structure that is sufficiently similar to the native protein that it permits correct assembly with its complementary partner.

Complete Deficiency of Glycophorin A in Red Blood Cells From Mice With Targeted Inactivation of the Band 3 (AE1) Gene

Glycophorin A is the major transmembrane sialoglycoprotein of red blood cells. It has been shown to contribute to the expression of the MN and Wright blood group antigens, to act as a receptor for the malaria parasite Plasmodium falciparum and Sendai virus, and along with the anion transporter, band 3, may contribute to the mechanical properties of the red blood cell membrane. Several lines of evidence suggest a close interaction between glycophorin A and band 3 during their biosynthesis. Recently, we have generated mice where the band 3 expression was completely eliminated by selective inactivation of the AE1 anion exchanger gene, thus allowing us to study the effect of band 3 on the expression of red blood cell membrane proteins. In this report, we show that the band 3 −/− red blood cells contain protein 4.1, adducin, dematin, p55, and glycophorin C. In contrast, the band 3 −/− red blood cells are completely devoid of glycophorin A (GPA), as assessed by Western blot and immunocytochemistry techniques, whereas the polymerase chain reaction (PCR) confirmed the presence of GPA mRNA. Pulse-label and pulse-chase experiments show that GPA is not incorporated in the membrane and is rapidly degraded in the cytoplasm. Based on these findings and other published evidence, we propose that band 3 plays a chaperone-like role, which is necessary for the recruitment of GPA to the red blood cell plasma membrane.

Complete Deficiency of Glycophorin A in Red Blood Cells From Mice With Targeted Inactivation of the Band 3 (AE1) Gene

Glycophorin A is the major transmembrane sialoglycoprotein of red blood cells. It has been shown to contribute to the expression of the MN and Wright blood group antigens, to act as a receptor for the malaria parasite Plasmodium falciparum and Sendai virus, and along with the anion transporter, band 3, may contribute to the mechanical properties of the red blood cell membrane. Several lines of evidence suggest a close interaction between glycophorin A and band 3 during their biosynthesis. Recently, we have generated mice where the band 3 expression was completely eliminated by selective inactivation of the AE1 anion exchanger gene, thus allowing us to study the effect of band 3 on the expression of red blood cell membrane proteins. In this report, we show that the band 3 −/− red blood cells contain protein 4.1, adducin, dematin, p55, and glycophorin C. In contrast, the band 3 −/− red blood cells are completely devoid of glycophorin A (GPA), as assessed by Western blot and immunocytochemistry techniques, whereas the polymerase chain reaction (PCR) confirmed the presence of GPA mRNA. Pulse-label and pulse-chase experiments show that GPA is not incorporated in the membrane and is rapidly degraded in the cytoplasm. Based on these findings and other published evidence, we propose that band 3 plays a chaperone-like role, which is necessary for the recruitment of GPA to the red blood cell plasma membrane.

Familial distal renal tubular acidosis is associated with mutations in the red cell anion exchanger (Band 3, AE1) gene

All affected patients in four families with autosomal dominant familial renal tubular acidosis (dRTA) were heterozygous for mutations in their red cell HCO3-/Cl- exchanger, band 3 (AE1, SLC4A1) genes, and these mutations were not found in any of the nine normal family members studied. The mutation Arg589--> His was present in two families, while Arg589--> Cys and Ser613--> Phe changes were found in the other families. Linkage studies confirmed the co-segregation of the disease with a genetic marker close to AE1. The affected individuals with the Arg589 mutations had reduced red cell sulfate transport and altered glycosylation of the red cell band 3 N-glycan chain. The red cells of individuals with the Ser613--> Phe mutation had markedly increased red cell sulfate transport but almost normal red cell iodide transport. The erythroid and kidney isoforms of the mutant band 3 proteins were expressed in Xenopus oocytes and all showed significant chloride transport activity. We conclude that dominantly inherited dRTA is associated with mutations in band 3; but both the disease and its autosomal dominant inheritance are not related simply to the anion transport activity of the mutant proteins.

Cytoplasmic disposition of aspartate 821 in anion exchanger from human erythrocytes

The location with respect to the plasma membrane of aspartate 821 in erythrocytic anion exchanger has been determined by labeling inside-out vesicles and intact erythrocytes with impermeant reagents and following the outcome by site-directed immunochemistry. Intact erythrocytes and inside-out vesicles in the same container were vectorially modified with 1-ethyl-3-[3-(trimethylammonio)propyl]carbodiimide and [35S]sulfanilic acid. The inside-out vesicles were separated from the erythrocytes by differential centrifugation, and both the vesicles and membranes made from the erythrocytes were stripped with alkali and digested with trypsin to liberate from each sample the peptide YHPDVPYVK containing aspartate 821. The tryptic digests were passed over an immunoadsorbent specific for peptides with the amino-and carboxy-terminal sequences YHPD- and -PYVK. Specifically bound peptides were eluted with acid, and the eluates were pooled and submitted to high-pressure liquid chromatography. A peak of absorbance at 229 nm corresponding to the peptide YHPDVPYVK was present in chromatograms of samples from both the inside-out vesicles and the intact erythrocytes. Another peak that displayed absorbance at 229 and 250 nm, corresponding to the peptide YHP(p-[35S]sulfo-beta-aspartanilide)VPYVK, was observed in the chromatogram of the sample from the inside-out vesicles but not in the chromatogram of the sample from the erythrocytes. This peak had associated with it a large number of counts per minute of [35S]sulfur, whereas no counts per minute of [35S]sulfur above background were detected on the chromatogram of the sample from the erythrocytes. The incorporation of [35S]sulfanilic acid into aspartate 821 of anion exchanger in inside-out vesicles was at least 10-fold greater than the incorporation of [35S]sulfanilic acid into aspartate 821 of anion exchanger in erythrocytes when the two preparations were labeled in the same solution. These results demonstrate that aspartate 821, found between two hydrophobic segments in the sequence of anion exchanger, is located on the cytoplasmic surface of this membrane-spanning protein.

Mapping the ends of transmembrane segments in a polytopic membrane protein. Scanning N-glycosylation mutagenesis of extracytosolic loops in the anion exchanger, band 3

Band 3, the anion exchanger of human erythrocytes, contains up to 14 transmembrane (TM) segments and has a single endogenous site of N-glycosylation at Asn642 in extracellular (EC) loop 4. The requirements for N-glycosylation of EC loops and the topology of this polytopic membrane protein were determined by scanning N-glycosylation mutagenesis and cell-free translation in a reticulocyte lysate supplemented with microsomal membranes. The endogenous and novel acceptor sites located near the middle of the 35 residue EC loop 4 were efficiently N-glycosylated; however, no N-glycosylation occurred at sites located within sharply defined regions close to the adjacent TM segments. Acceptor sites located in the center of EC loop 3, which contains 25 residues, were poorly N-glycosylated. Expansion of this loop with a 4-residue insert containing an acceptor site increased N-glycosylation. Acceptor sites located in short (<10 residues) loops (putative EC loops 1, 2, 6, and 7) were not N-glycosylated; however, insertion of EC loop 4 into EC loops 1, 2, or 7, but not 6, resulted in efficient N-glycosylation. Acceptor sites in putative intracellular (IC) loop 5 exhibited a similar pattern of N-glycosylation as EC loop 4, indicating a lumenal disposition during biosynthesis. To be efficiently N-glycosylated, EC loops in polytopic membrane proteins must be larger than 25 residues in size, with acceptor sites located greater than 12 residues away from the preceding TM segment and greater than 14 residues away from the following TM segment. Application of this requirement allowed a significant refinement of the topology of Band 3 including a more accurate mapping of the ends of TM segments. The strict distance dependence for N-glycosylation of loops suggests that TM segments in polytopic membrane proteins are held quite precisely within the translocation machinery during the N-glycosylation process.

Infection of C3HeB/FeJ mice with the docile strain of lymphocytic choriomeningitis virus induces autoantibodies specific for erythrocyte Band 3

C3HeB/FeJ mice infected with the docile strain of lymphocytic choriomeningitis virus (LCMV-d) develop a persistent infection with a transient haemolytic anaemia. Immunoglobulin can be eluted from the red blood cells (RBC) of these mice but it cannot be detected on the RBC by a conventional antiglobulin test. The present study demonstrates that RBC from such mice bear erythrocyte autoantibodies which are predominantly of the IgG2a subclass, with lower levels of autoantibodies of the IgG1, IgG2b and IgG3 subclasses. To identify the target antigen the autoantibodies were eluted from the RBC of LCMV-infected mice. The eluted autoantibody bound to intact normal RBC and precipitated a 105000 MW component that corresponds to murine Band 3 protein. A monoclonal antibody derived from mice infected with LCMV-d also precipitated mouse Band 3, and reacted specifically by enzyme-linked immunosorbent assay against a purified preparation of Band 3. This study has shown that in C3H mice infected with LCMV-d which develop autoimmune haemolytic anaemia, the target autoantigen is erythrocyte membrane Band 3.

Functional cell surface expression of the anion transport domain of human red cell band 3 (AE1) in the yeast Saccharomyces cerevisiae

We expressed the 52-kDa integral membrane domain (B3mem) of the human erythrocyte anion transporter (band 3; AE1) in a protease-deficient strain of the yeast Saccharomyces cerevisiae under the control of the inducible GAL10-CYC1 promoter. Immunoblots of total protein from transformed yeast cells confirmed that the B3mem polypeptide was overexpressed shortly after induction with galactose. Cell surface expression of the functional anion transporter was detected by using a simple transport assay to measure stilbene disulfonate-inhibitable chloride influx into intact yeast cells. The B3mem polypeptide was recycled and degraded by the cells with a half-life of approximately 1-3 hr, which led to a steady-state level of expression in exponentially growing cultures. Our data suggest that 5-10% of total B3mem is functionally active at the cell surface at any one time and that overexpression of this anion transport protein does not interfere with cell growth or survival. This is one of only a few reports of the functional expression of a plasma membrane transport protein in the plasma membrane of yeast cells and to our knowledge is the first report of red cell band 3-mediated anion transport at the plasma membrane of cDNA-transformed cells. The cell surface expression system we describe will provide a simple means for future study of the functional properties of band 3 by using site-directed mutagenesis.

Monoclonal antibodies that react with human band 3 residues 542-555 recognize different conformations of this protein in uninfected and Plasmodium falciparum infected erythrocytes

A monoclonal antibody generated against synthetic peptides patterned on amino acids 542-555 of human band 3, designated 1F4, specifically immunostained Plasmodium falciparum-infected erythrocytes and inhibited the cytoadherence of P. falciparum-infected erythrocytes to C32 amelanotic melanoma cells. 1F4 did not recognize intact band 3 protein on immunoblots, however it was reactive towards proteolytic fragments of band 3. The binding region of another murine monoclonal antibody previously reported to recognize the membrane spanning domain of human band 3, designated B6, was found to also recognize residues 542-555, however its properties differed from 1F4. Mab B6 recognized both infected and uninfected red cells, and reacted only with intact band 3 on immunoblots. Mab B6 was without effect on cytoadherence. These results demonstrate that monoclonal antibodies reactive against a common peptide sequence may bind to different conformations of the peptide sequence and suggest that the adherent competency of P. falciparum-infected erythrocytes may result from a change in the surface topography of human band 3 protein.

Isolation and characterization of CD47 glycoprotein: a multispanning membrane protein which is the same as integrin-associated protein (IAP) and the ovarian tumour marker OA3

The CD47 glycoprotein was isolated from human erythrocytes by immunoprecipitation using monoclonal antibody (mAb) BRIC-125. Enzymic deglycosylation of the protein showed it contained N-linked oligosaccharides, and trypsin proteolysis of the protein in situ in the erythrocyte membrane cleaved it into two portions, one of which was glycosylated. Both the intact protein and the glycosylated fragment had blocked N-termini. Amino acid sequence was obtained from several proteolytic fragments of CD47. Comparison with the sequence database showed the protein to be very similar to or identical with OA3, a multispanning membrane protein. The protein also appears to be the same as the integrin-associated protein, which has a role in cell adhesion in non-erythroid cells. CD47 has six potential N-glycosylation sites, five of which are in an Ig superfamily domain. We show that three of these sites carry N-glycans in erythrocytes. Immunocytochemical staining of human tissues showed that CD47 was broadly distributed on mesenchyme and epithelia at multiple sites. Reactivity was particularly prominent in surface and ductular epithelia, and in the brain. The possible roles of the CD47 glycoprotein are discussed.

Comparative immunochemical analysis of Wra and Wrb red cell antigens

This paper describes immunoblotting and immunoprecipitation studies using monoclonal anti-Wra and anti-Wrb antibodies to investigate the nature of the low-incidence blood group antigen, Wra and its high-incidence allelic antigen Wrb. No membrane components were identified by the immunoblotting experiments. Immunoprecipitation studies confirmed that the Wrb antigen involves both glycophorin A and band 3. The monoclonal anti-Wra, BGU1-WR, failed to immunoprecipitate these or any othe red cell membrane component. The significance of these findings is discussed.

Immunohistochemical detection of chloride/bicarbonate anion exchangers in human liver

Sodium-independent Cl-/HCO3- exchange activity has been observed in isolated rat hepatocytes and intrahepatic bile duct epithelial cells, where it is involved in intracellular pH regulation and, possibly, biliary bicarbonate secretion. Monoclonal antibodies to the membrane domain of human chloride/bicarbonate anion exchanger proteins, AE1 and AE2, were prepared so that we might determine by immunohistochemical methods the presence and location of these antiporters in the human liver. To obtain the antibody against AE1, we immunized mice with injections of washed human erythrocytes. The selected monoclonal antibody was found to be specific for the 17-kD proteolytic membrane fragment of AE1 protein. The antibody to AE2 was produced with a 14-mer synthetic peptide, whose sequence corresponds specifically to amino acid residues 871 to 884 in the deduced primary structure of human kidney AE2 protein. When the monoclonal antibody to AE2 peptide was employed for the immunohistochemical study of liver specimens (by both immunofluorescence and immunoperoxidase), a clearly defined staining was present at the canalicular membrane of hepatocytes, as well as the luminal side of the membrane of bile duct epithelial cells from small and medium-sized bile ducts. No staining was observed in the liver parenchyma with the monoclonal antibody to AE1, which instead strongly decorated the erythrocytes in liver blood vessels. We conclude that AE2 immunoreactivity is present in human liver, where it localizes very specifically to the membrane regions, which appear most probably involved in the transport of bicarbonate to bile (i.e., the canalicular membrane of hepatocytes and the apical side of epithelial cells of small and medium bile ducts).