Purification and characterization of band 3 protein

Half-Life Extension and Biodistribution Modulation of Biotherapeutics via Red Blood Cell Hitch-Hiking with Novel Anti-Band 3 Single-Domain Antibodies

Small therapeutic proteins are receiving increased interest as therapeutic drugs; however, their clinical success has been limited due to their rapid elimination. Here, we report a half-life extension strategy via strategy via red blood cell red blood cell (RBC) hitch-hiking. This manuscript details the development and characterization of novel anti-RBC single-domain antibodies (sdAbs), their genetic fusion to therapeutic antibody fragments (TAF) as bispecific fusion constructs, and their influence on TAF pharmacokinetics and biodistribution. Several sdAbs specific to the band 3 antigen were generated via phage-display technology. Binding affinity to RBCs was assessed via flow cytometry. Affinity maturation via random mutagenesis was carried out to improve the binding affinity of the sdAbs. Bi-specific constructs were generated by fusing the anti-RBC sdAbs with anti-tissue necrosis factor alpha (TNF-α) TAF via the use of a glycine-serine flexible linker, and assessments for binding were performed via enzyme-linked immunosorbent assay and flow cytometry. Pharmacokinetics of anti-RBC sdAbs and fusion constructs were evaluated following intravenous bolus dosing in mice at a 1 mg/kg dose. Two RBC-binding sdAbs, RB12 and RE8, were developed. These two clones showed high binding affinity to human RBC with an estimated KD of 17.7 nM and 23.6 nM and low binding affinity to mouse RBC with an estimated KD of 335 nM and 528 nM for RB12 and RE8, respectively. Two derivative sdAbs, RMA1, and RMC1, with higher affinities against mouse RBC, were generated via affinity maturation (KD of 66.9 nM and 30.3 nM, respectively). Pharmacokinetic investigations in mice demonstrated prolonged circulation half-life of an anti-RBC-TNF-α bispecific construct (75 h) compared to a non-RBC binding control (1.3 h). In summary, the developed anti-RBC sdAbs and fusion constructs have demonstrated high affinity in vitro, and sufficient half-life extension in vivo.

Half-Life Extension and Biodistribution Modulation of Biotherapeutics via Red Blood Cell Hitch-Hiking with Novel Anti-Band 3 Single-Domain Antibodies

Small therapeutic proteins are receiving increased interest as therapeutic drugs; however, their clinical success has been limited due to their rapid elimination. Here, we report a half-life extension strategy via strategy via red blood cell red blood cell (RBC) hitch-hiking. This manuscript details the development and characterization of novel anti-RBC single-domain antibodies (sdAbs), their genetic fusion to therapeutic antibody fragments (TAF) as bispecific fusion constructs, and their influence on TAF pharmacokinetics and biodistribution. Several sdAbs specific to the band 3 antigen were generated via phage-display technology. Binding affinity to RBCs was assessed via flow cytometry. Affinity maturation via random mutagenesis was carried out to improve the binding affinity of the sdAbs. Bi-specific constructs were generated by fusing the anti-RBC sdAbs with anti-tissue necrosis factor alpha (TNF-α) TAF via the use of a glycine-serine flexible linker, and assessments for binding were performed via enzyme-linked immunosorbent assay and flow cytometry. Pharmacokinetics of anti-RBC sdAbs and fusion constructs were evaluated following intravenous bolus dosing in mice at a 1 mg/kg dose. Two RBC-binding sdAbs, RB12 and RE8, were developed. These two clones showed high binding affinity to human RBC with an estimated K_D of 17.7 nM and 23.6 nM and low binding affinity to mouse RBC with an estimated K_D of 335 nM and 528 nM for RB12 and RE8, respectively. Two derivative sdAbs, RMA1, and RMC1, with higher affinities against mouse RBC, were generated via affinity maturation (K_D of 66.9 nM and 30.3 nM, respectively). Pharmacokinetic investigations in mice demonstrated prolonged circulation half-life of an anti-RBC-TNF-α bispecific construct (75 h) compared to a non-RBC binding control (1.3 h). In summary, the developed anti-RBC sdAbs and fusion constructs have demonstrated high affinity in vitro, and sufficient half-life extension in vivo.

Analytical Ultracentrifugation as a Matrix-Free Probe for the Study of Kinase Related Cellular and Bacterial Membrane Proteins and Glycans

Analytical ultracentrifugation is a versatile approach for analysing the molecular mass, molecular integrity (degradation/aggregation), oligomeric state and association/dissociation constants for self-association, and assay of ligand binding of kinase related membrane proteins and glycans. It has the great property of being matrix free-providing separation and analysis of macromolecular species without the need of a separation matrix or membrane or immobilisation onto a surface. This short review-designed for the non-hydrodynamic expert-examines the potential of modern sedimentation velocity and sedimentation equilibrium and the challenges posed for these molecules particularly those which have significant cytoplasmic or extracellular domains in addition to the transmembrane region. These different regions can generate different optimal requirements in terms of choice of the appropriate solvent (aqueous/detergent). We compare how analytical ultracentrifugation has contributed to our understanding of two kinase related cellular or bacterial protein/glycan systems (i) the membrane erythrocyte band 3 protein system-studied in aqueous and detergent based solvent systems-and (ii) what it has contributed so far to our understanding of the enterococcal VanS, the glycan ligand vancomycin and interactions of vancomycin with mucins from the gastrointestinal tract.

Human erythrocyte band 3 is a host receptor for Plasmodium falciparum glutamic acid-rich protein

Malaria remains a major global threat to human health and economic development. Microvascular lesions caused by Plasmodium falciparum-infected human erythrocytes/red blood cells are hallmarks of severe pathogenesis contributing to high mortality, particularly in children from sub-Saharan Africa. In this study, we used a phage display complementary DNA library screening strategy to identify P falciparum glutamic acid-rich protein (PfGARP) as a secreted ligand that recognizes an ectodomain of human erythrocyte anion-exchanger, band 3/AE1, as a host receptor. Domain mapping of PfGARP revealed distinct nonoverlapping repeats encoding the immune response epitopes and core erythrocyte-binding activity. Synthetic peptides derived from the erythrocyte-binding repeats of PfGARP induced erythrocyte aggregation reminiscent of the rosetting phenomenon. Using peptides derived from the immunogenic repeats, a quantitative immunoassay was developed to detect a selective immune response against PfGARP in human plasma samples obtained from patients in rural Mali, suggesting the feasibility of PfGARP as a potential biomarker of disease progression. Collectively, our results suggest that PfGARP may play a functional role in enhancing the adhesive properties of human erythrocytes by engaging band 3 as a host receptor. We propose that immunological and pharmacological inhibition of PfGARP may unveil new therapeutic options for mitigating lesions in cerebral and pregnancy-associated malaria.

Purification of bacterial membrane sensor kinases and biophysical methods for determination of their ligand and inhibitor interactions

This article reviews current methods for the reliable heterologous overexpression in Escherichia coli and purification of milligram quantities of bacterial membrane sensor kinase (MSK) proteins belonging to the two-component signal transduction family of integral membrane proteins. Many of these methods were developed at Leeds alongside Professor Steve Baldwin to whom this review is dedicated. It also reviews two biophysical methods that we have adapted successfully for studies of purified MSKs and other membrane proteins–synchrotron radiation circular dichroism (SRCD) spectroscopy and analytical ultracentrifugation (AUC), both of which are non-immobilization and matrix-free methods that require no labelling strategies. Other techniques such as isothermal titration calorimetry (ITC) also share these features but generally require high concentrations of material. In common with many other biophysical techniques, both of these biophysical methods provide information regarding membrane protein conformation, oligomerization state and ligand binding, but they possess the additional advantage of providing direct assessments of whether ligand binding interactions are accompanied by conformational changes. Therefore, both methods provide a powerful means by which to identify and characterize inhibitor binding and any associated protein conformational changes, thereby contributing valuable information for future drug intervention strategies directed towards bacterial MSKs.

Band 3, the human red cell chloride/bicarbonate anion exchanger (AE1, SLC4A1), in a structural context

The crystal structure of the dimeric membrane domain of human Band 3(1), the red cell chloride/bicarbonate anion exchanger 1 (AE1, SLC4A1), provides a structural context for over four decades of studies into this historic and important membrane glycoprotein. In this review, we highlight the key structural features responsible for anion binding and translocation and have integrated the following topological markers within the Band 3 structure: blood group antigens, N-glycosylation site, protease cleavage sites, inhibitor and chemical labeling sites, and the results of scanning cysteine and N-glycosylation mutagenesis. Locations of mutations linked to human disease, including those responsible for Southeast Asian ovalocytosis, hereditary stomatocytosis, hereditary spherocytosis, and distal renal tubular acidosis, provide molecular insights into their effect on Band 3 folding. Finally, molecular dynamics simulations of phosphatidylcholine self-assembled around Band 3 provide a view of this membrane protein within a lipid bilayer.

Interaction of Plasmodium vivax Tryptophan-rich Antigen PvTRAg38 with Band 3 on Human Erythrocyte Surface Facilitates Parasite Growth

Plasmodium tryptophan-rich proteins are involved in host-parasite interaction and thus potential drug/vaccine targets. Recently, we have described several P. vivax tryptophan-rich antigens (PvTRAgs), including merozoite expressed PvTRAg38, from this noncultivable human malaria parasite. PvTRAg38 is highly immunogenic in humans and binds to host erythrocytes, and this binding is inhibited by the patient sera. This binding is also affected if host erythrocytes were pretreated with chymotrypsin. Here, Band 3 has been identified as the chymotrypsin-sensitive erythrocyte receptor for this parasite protein. Interaction of PvTRAg38 with Band 3 has been mapped to its three different ectodomains (loops 1, 3, and 6) exposed at the surface of the erythrocyte. The binding region of PvTRAg38 to Band3 has been mapped to its sequence, KWVQWKNDKIRSWLSSEW, present at amino acid positions 197-214. The recombinant PvTRAg38 was able to inhibit the parasite growth in in vitro Plasmodium falciparum culture probably by competing with the ligand(s) of this heterologous parasite for the erythrocyte Band 3 receptor. In conclusion, the host-parasite interaction at the molecular level is much more complicated than known so far and should be considered during the development of anti-malarial therapeutics.

Three-dimensional model for the human Cl-/HCO3- exchanger, AE1, by homology to the E. coli ClC protein

AE1 mediates electroneutral 1:1 exchange of bicarbonate for chloride across the plasma membrane of erythrocytes and type A cells of the renal collecting duct. No high-resolution structure is available for the AE1 membrane domain, which alone is required for its transport activity. A recent electron microscopy structure of the AE1 membrane domain was proposed to have a similar protein fold to ClC chloride channels. We developed a three-dimensional homology model of the AE1 membrane domain, using the Escherichia coli ClC channel structure as a template. This model agrees well with a long list of biochemically established spatial constraints for AE1. To investigate the AE1 transport mechanism, we created point mutations in regions corresponding to E. coli ClC transport mechanism residues. When expressed in HEK293 cells, several mutants had Cl(-)/HCO3(-) exchange rates significantly different from that of wild-type AE1. When further assessed in Xenopus laevis oocytes, there were significant changes in the transport activity of several AE1 point mutants as assessed by changes in pH. None of the mutants, however, added an electrogenic component to AE1 transport activity. This indicates that the AE1 point mutants altered the transport activity of AE1, without changing its electrogenicity and stoichiometry. The homology model successfully identified residues in AE1 that are critical to AE1 transport activity. Thus, we conclude that AE1 has a similar protein fold to ClC chloride channels.

Purification of functional human Cl(-)/HCO(3)(-) exchanger, AE1, over-expressed in Saccharomyces cerevisiae

There is no high-resolution structure for the membrane domain of the human erythrocyte anion exchanger, AE1 (Band 3). In this report, we have developed an expression and purification strategy for AE1 to be used in crystallization trials. Saccharomyces cerevisiae strain BJ5457 was transformed with an expression vector encoding the AE1 membrane domain (AE1MD, amino acids 388-911), fused C-terminally to an epitope tag, corresponding to the nine C-terminal amino acids of rhodopsin. The fusion protein, AE1MD-Rho, was expressed at a concentration of 0.3 mg/l of culture. Confocal immunofluorescence microscopy and sucrose gradient ultracentrifugation revealed that AE1MD-Rho did not process to the plasma membrane of S. cerevisiae, but was retained in an intracellular membrane fraction. Treatment with the endoglycosidase, PNGase F, showed that AE1MD-Rho is not N-glycosylated. AE1MD-Rho solubilized from yeast membranes, with Fos-choline detergent, was purified to 93% homogeneity in a single-step, using a 1D4 antibody affinity resin, in amounts up to 2.5 mg from 18 l of culture. The ability of purified AE1MD-Rho to transport sulfate was examined in reconstituted vesicles. The rate of sulfate efflux mediated by vesicles reconstituted with AE1MD-Rho was indistinguishable from vesicles with purified erythrocyte-source AE1. Using this purification strategy, sufficient amounts of functional, homogeneous AE1MD-Rho can be purified to enable crystallization trials.

Congenital dyserythropoietic anemia type II (CDAII/HEMPAS): where are we now?

Congenital diserythropoietic anemias (CDA) were classified according to bone marrow changes and biochemical features 40 years ago. A consistent finding in CDA type II, the most frequent subgroup of CDAs is a relevant hypoglycosylation of erythrocyte membrane proteins. It is a matter of debate if the hypoglycosylation is the primary cause of the disorder or a phenomenon secondary to other pathomechanisms. The molecular cause of the disorder is still unknown although some enzyme deficiencies have been proposed to cause CDA II in the last 2 decades and a linkage analysis locating the CDA II gene in a 5 cM region on chromosome 20 was done in 1997. In this review biochemical and genetic data are discussed and diagnostic methods based on biochemical observations of the recent years are reviewed.

Effects of phosphatidylethanolamine glycation on lipid-protein interactions and membrane protein thermal stability

Non-enzymatic glycation of biomolecules has been implicated in the pathophysiology of aging and diabetes. Among the potential targets for glycation are biological membranes, characterized by a complex organization of lipids and proteins interacting and forming domains of different size and stability. In the present study, we analyse the effects of glycation on the interactions between membrane proteins and lipids. The phospholipid affinity for the transmembrane surface of the PMCA (plasma-membrane Ca(2+)-ATPase) was determined after incubating the protein or the phospholipids with glucose. Results show that the affinity between PMCA and the surrounding phospholipids decreases significantly after phosphospholipid glycation, but remains unmodified after glycation of the protein. Furthermore, phosphatidylethanolamine glycation decreases by approximately 30% the stability of PMCA against thermal denaturation, suggesting that glycated aminophospholipids induce a structural rearrangement in the protein that makes it more sensitive to thermal unfolding. We also verified that lipid glycation decreases the affinity of lipids for two other membrane proteins, suggesting that this effect might be common to membrane proteins. Extending these results to the in vivo situation, we can hypothesize that, under hyperglycaemic conditions, glycation of membrane lipids may cause a significant change in the structure and stability of membrane proteins, which may affect the normal functioning of membranes and therefore of cells.

Chronic lymphocytic leukemia cells bind and present the erythrocyte protein band 3: possible role as initiators of autoimmune hemolytic anemia

The mechanisms underlying the frequent association between chronic lymphocytic leukemia (CLL) and autoimmune hemolytic anemia are currently unclear. The erythrocyte protein band 3 (B3) is one of the most frequently targeted Ags in autoimmune hemolytic anemia. In this study, we show that CLL cells specifically recognize B3 through a still unidentified receptor. B3 interaction with CLL cells involves the recognition of its N-terminal domain and leads to its internalization. Interestingly, when binding of erythrocyte-derived vesicles as found physiologically in blood was assessed, we observed that CLL cells could only interact with inside-out vesicles, being this interaction strongly dependent on the recognition of the N-terminal portion of B3. We then examined T cell responses to B3 using circulating CLL cells as APCs. Resting B3-pulsed CLL cells were unable to induce T cell proliferation. However, when deficient costimulation was overcome by CD40 engagement, B3-pulsed CLL cells were capable of activating CD4(+) T cells in a HLA-DR-dependent fashion. Therefore, our work shows that CLL cells can specifically bind, capture, and present B3 to T cells when in an activated state, an ability that could allow the neoplastic clone to trigger the autoaggressive process against erythrocytes.

Hsa, an adhesin of Streptococcus gordonii DL1, binds to alpha2-3-linked sialic acid on glycophorin A of the erythrocyte membrane

Bacterial recognition of host sialic acid-containing receptors plays an important role in microbial colonization of the human oral cavity. The aggregation of human platelets by Streptococcus gordonii DL1 is implicated in the pathogenesis of infective endocarditis. In addition, we consider that hemagglutination of this organism may act as an additive factor to increase the severity of this disease. We previously reported that this interaction requires the bacterial expression of a 203-kDa protein (Hsa), which has sialic acid-binding activity. In the present study, we confirmed that erythrocyte surface sialoglycoproteins are the receptors for Hsa. We examined the effects of proteinase K, chymotrypsin, phospholipase C, and alpha(2-3) or alpha(2-3, 6, 8) neuraminidase on hemagglutination activity and found that the interaction occurs between Hsa and alpha2-3-linked sialic acid-containing proteins of erythrocytes. We expressed recombinant NR2, which is the putative binding domain of Hsa, fused with GST in Escherichia coli BL21. Dot-blot analysis demonstrated that GST-HsaNR2 binds both glycophorin A (GPA) and band 3. Moreover, GPA and a small amount of band 3 were detected by GST pull-down assays. These findings indicate that S. gordonii Hsa specifically binds to GPA and band 3, alpha2-3-linked sialic acid membrane glycoproteins.

Oligomeric structure and minimal functional unit of the electrogenic sodium bicarbonate cotransporter NBCe1-A

The electrogenic sodium bicarbonate cotransporter NBCe1-A mediates the basolateral absorption of sodium and bicarbonate in the proximal tubule. In this study the oligomeric state and minimal functional unit of NBCe1-A were investigated. Wild-type (wt) NBCe1-A isolated from mouse kidney or heterologously expressed in HEK293 cells was predominantly in a dimeric state as was shown using fluorescence energy transfer, pulldown, immunoprecipitation, cross-linking experiments, and nondenaturing perfluorooctanoate-PAGE. NBCe1-A monomers were found to be covalently linked by S-S bonds. When each of the 15 native cysteine residues were individually removed on a wt-NBCe1-A backbone, dimerization of the cotransporter was not affected. In experiments involving multiple native cysteine residue removal, both Cys(630) and Cys(642) in extracellular loop 3 were shown to mediate S-S bond formation between NBCe1-A monomers. When native NBCe1-A cysteine residues were individually reintroduced into a cysteineless NBCe1-A mutant backbone, the finding that a Cys(992) construct that lacked S-S bonds functioned normally indicated that stable covalent linkage of NBCe1-A monomers was not a necessary requirement for functional activity of the cotransporter. Studies using concatameric constructs of wt-NBCe1-A, whose activity is resistant to methanesulfonate reagents, and an NBCe1-A(T442C) mutant, whose activity is completely inhibited by methanesulfonate reagents, confirmed that NBCe1-A monomers are functional. Our results demonstrate that wt-NBCe1-A is predominantly a homodimer, dependent on S-S bond formation that is composed of functionally active monomers.

Characterization of the N-glycosylation phenotype of erythrocyte membrane proteins in congenital dyserythropoietic anemia type II (CDA II/HEMPAS)

Congenital dyserythropoetic anemia type II (CDA II) is characterized by bi- and multinucleated erythroblasts and an impaired N-glycosylation of erythrocyte membrane proteins. Several enzyme defects have been proposed to cause CDA II based on the investigation of erythrocyte membrane glycans pinpointing to defects of early Golgi processing steps. Hitherto no molecular defect could be elucidated. In the present study, N-glycosylation of erythrocyte membrane proteins of CDA II patients and controls was investigated by SDS-Page, lectin binding studies, and MALDI-TOF/MS mapping in order to allow an embracing view on the glycosylation defect in CDA II. Decreased binding of tomato lectin was a consistent finding in all typical CDA II patients. New insights into tomato lectin binding properties were found indicating that branched polylactosamines are the main target. The binding of Aleuria aurantia, a lectin preferentially binding to alpha1-6 core-fucose, was reduced in western blots of CDA II erythrocyte membranes. MALDI-TOF analysis of band 3 derived N-glycans revealed a broad spectrum of truncated structures showing the presence of high mannose and hybrid glycans and mainly a strong decrease of large N-glycans suggesting impairment of cis, medial and trans Golgi processing.

CONCLUSION

Truncation of N-glycans is a consistent finding in CDA II erythrocytes indicating the diagnostic value of tomato-lectin studies. However, structural data of erythrocyte N-glycans implicate that CDA II is not a distinct glycosylation disorder but caused by a defect disturbing Golgi processing in erythroblasts.

Effect of block deletions in the C-terminus on the functional expression of human anion exchanger 1 (AE1)

The human anion exchanger 1 (AE1) is the most abundant integral membrane protein in red cells and is responsible for the exchange of Cl(-) for HCO(3)(-). However, the detailed role played by the AE1 C-terminal region in the anion translocation and membrane trafficking process remains unclear. In this paper, we created four mutants in the human AE1 C-terminus by deletion of the residues Ala(891)-Phe(895), Asp(896)-Glu(899), Asp(902)-Glu(906) and Val(907)-Val(911), to investigate the role of these sequences in functional expression of AE1. WT AE1 and its deletion mutant constructs were expressed in HEK 293 cells. Western blotting showed that deletions of Ala(891)-Phe(895), Asp(896)-Glu(899), and Val(907)-Val(911) induced high expression of AE1, whereas loss of Asp(902)-Glu(906) results in stable low expression. Pulse chase assays of WT AE1 and its mutants showed that the stability of protein is unaffected by the levels of expression of the AE1 and its mutants. Ala(891)-Phe(895), Asp(902)-Glu(906) and Val(907)-Val(911) mutants exhibited lower levels of trafficking to the plasma membrane compared with WT AE1, while the Asp(896)-Glu(899) mutant was more highly expressed at the plasma membrane. The decreased ability of the mutants to mediate Cl(-)/HCO(3)(-) exchange in transfected cells revealed that the deletion sequences have an important role in transport activity. These results demonstrate that the studied residues in the AE1 C-terminus differently affect the expression, membrane trafficking and functional folding of AE1.

Cystic fibrosis transmembrane conductance regulator in human and mouse red blood cell membranes and its interaction with ecto-apyrase

Elevated blood ATP and increased red blood cell (RBC) ATP transport is associated with cystic fibrosis (CF). In this report, we demonstrate the presence of the wild-type and the DeltaF508 mutant form of the CF transmembrane conductance regulator protein in RBC membranes and its putative interaction with ecto-apyrase, an ATP hydrolyzing enzyme also present in the RBC membrane. RBC membranes of control and DeltaF508 individuals and of wild-type and CF transmembrane conductance regulator-knockout mice were examined by immunoblot using several antibodies directed against different epitopes of this protein. These experiments indicated that human RBC membranes contain comparable amounts of the wild-type CF transmembrane conductance regulator protein and the DeltaF508 mutant form of the protein, respectively. CF transmembrane conductance regulator protein was also detected in wild-type mouse RBC membranes but not in the gene knockout mouse RBC membranes. Antibodies directed against ecto-apyrase co-immunoprecipitated CF transmembrane conductance regulator protein of human RBC membranes indicating a physical interaction between these two membrane proteins consistent with ATP transport and extracellular hydrolysis. We conclude that RBCs are a significant repository of CF transmembrane conductance regulator protein and should provide a novel system for evaluating its expression and function.

T-cell specificity in murine autoimmune haemolytic anaemia induced by rat red blood cells

Autoimmune haemolytic anaemia (AIHA) can be induced in mice by repeated injections with rat red blood cells (RBC). Here we describe the identification of rat and murine RBC antigens recognized by T-cells from mice with this disease. Splenic T-cells from mice with AIHA proliferated in response to multiple murine RBC membrane components, each of which is recognized by rat RBC induced autoantibodies. Thus, there were responses to murine autoantigen fractions that correspond in apparent molecular mass with the anion channel Band 3, with spectrin from the membrane skeleton and with the high and low molecular mass glycophorins, and the equivalent fractions from rat RBC also stimulated proliferation by T-cells. It was confirmed that purified Band 3 from murine and rat RBC also elicited responses. In contrast with the results in AIHA, T-cells from healthy control mice failed to respond to the antigens from either species, with the exception of proliferation induced by murine spectrin in one experiment and weak responses elicited by rat Band 3. It is suggested that T-cells activated by multiple cross-reactions between rat and murine RBC proteins, and by epitope spreading, are necessary to drive autoantibody production in this model of AIHA.

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.

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.

Erythrocyte membrane ATP binding cassette (ABC) proteins: MRP1 and CFTR as well as CD39 (ecto-apyrase) involved in RBC ATP transport and elevated blood plasma ATP of cystic fibrosis

In addition to the better-known roles of the erythrocyte in the transport of oxygen and carbon dioxide, the concept that the red blood cell is involved in the transport and release of ATP has been evolving (J. Luthje, Blut 59, 367, 1989; G. R. Bergfeld and T. Forrester, Cardiovasc. Res. 26, 40, 1992; M. L. Ellsworth et al., Am. J. Physiol. 269, H2155, 1995; R. S. Sprague et al., Am. J. Physiol. 275, H1726, 1998). Membrane proteins involved in the release of ATP from erythrocytes now appear to include members of the ATP binding cassette (ABC) family (C. F. Higgins, Annu. Rev. Cell Biol. 8, 67, 1992; C. F. Higgins, Cell 82, 693, 1995). In addition to defining physiologically the presence of ABC proteins in RBCs, accumulating gel electrophoretic evidence suggests that the cystic fibrosis transmembrane conductance regulator (CFTR) and the multidrug resistance-associated protein (MRP1), respectively, constitute significant proteins in the red blood cell membrane. As such, this finding makes the mature erythrocyte compartment a major mammalian repository of these important ABC proteins. Because of its relative structural simplicity and ready accessibility, the erythrocyte offers an ideal system to explore details of the physiological functions of ABC proteins. Moreover, the presence of different ABC proteins in a single membrane implies that interaction among these proteins and with other membrane proteins may be the norm and not the exception in terms of modulation of their functions.

Purification and characterization of human erythrocyte glucose transporter in decylmaltoside detergent solution

The facilitative glucose transporter from human erythrocyte membrane, Glut1, was purified by a novel method. The nonionic detergent decylmaltoside was selected for solubilization on the basis of its efficiency to extract Glut1 from the erythrocyte membrane and its ability to maintain the protein in a monodisperse state. A positive, anion-exchange chromatography protocol produced a Glut1 preparation of 95% purity with little copurified lipid. This protein preparation exhibited cytochalasin B binding in detergent solution, as measured by tryptophan fluorescence quenching. The transporter existed as a monomer in decylmaltoside, with a Stokes radius of 50 A and a molecular mass of 147 kDa for the protein-detergent complex. We screened detergent, pH, additive, and lipid and have found conditions to maintain Glut1 monodispersity for 8 days at 25 degrees C or over 5 weeks at 4 degrees C. This Glut1 preparation represents the best available material for two- and three-dimensional crystallization trials of the human glucose transporter protein.

Hydrodynamic properties of human erythrocyte band 3 solubilized in reduced Triton X-100

The oligomeric state and function of band 3, purified by sulfhydryl affinity chromatography in reduced Triton X-100, was investigated. Size exclusion high-performance liquid chromatography showed that a homogeneous population of band 3 dimers could be purified from whole erythrocyte membranes. The elution profile of band 3 purified from membranes that had been stripped of its cytoskeleton before solubilization was a broad single peak describing a heterogeneous population of oligomers with a mean Stokes radius of 100 A. Sedimentation velocity ultracentrifugation analysis confirmed particle heterogeneity and further showed monomer/dimer/tetramer equilibrium self-association. Whether the conversion of dimer to the form described by a Stokes radius of 100 A was initiated by removal of cytoskeletal components, alkali-induced changes in band 3 conformation, or alkali-induced loss of copurifying ligands remains unclear. After incubation at 20 degrees C for 24 h, both preparations of band 3 converted to a common form characterized by a mean Stokes radius of 114 A. This form of the protein, examined by equilibrium sedimentation ultracentrifugation, is able to self-associate reversibly, and the self-association can be described by a dimer/tetramer/hexamer model, although the presence of higher oligomers cannot be discounted. The ability of the different forms of the protein to bind stilbene disulfonates revealed that the dimer had the highest inhibitor binding affinity, and the form characterized by a mean Stokes radius of 114 A to have the lowest.

Regulation of intracellular pH in capacitated human spermatozoa by a Na+/H+ exchanger

We previously demonstrated that the progesterone-(P) initiated human sperm acrosome reaction (AR) was dependent on the presence of extracellular Na+ (Na(-)0). Moreover, Na(-)0 depletion resulted in a decreased cytosolic pH (pHi), suggesting involvement of a Na(+)-dependent pHi regulatory mechanism during the P-initiated AR. We now report that the decreased pHi resulting from Na(+)0 depletion is reversible and mediated by a Na+/H+ exchange (NHE) mechanism. To determine the role of an NHE in the regulation of pHi, capacitated spermatozoa were incubated in Na(+)-deficient, bicarbonate/CO2-buffered (ONaB) medium for 15-30 min, which resulted in an intracellular acidification as previously reported. These spermatozoa were then transferred to Na(+)-containing, bicarbonate/CO2-buffered (NaB) medium; Na(+)-containing, Hepes-buffered (NaH) medium; or maintained in the ONaB medium. Included in the NaH medium was the NHE inhibitor 5-(N-ethyl-N-isopropyl) amiloride (EIPA). The steady-state pHi was then determined by spectrofluorometric measurement of bis(carboxyethyl)5(6)-carboxyfluoroscein (BCECF) fluorescence. EIPA (0.1 microM) significantly (P < 0.05) inhibited the pHi recovery produced by NaH medium. Moreover, the pHi in NaH medium was not significantly (P < 0.05) different than NaB medium. These results indicate that a Na(+)-dependent, bicarbonate-independent pHi regulatory mechanism, with a pharmacological characteristic consistent with an NHE, is present in capacitated spermatozoa. In support of the involvement of a sperm NHE, we also demonstrated specific immunoreactivity for a 100 kDa porcine sperm protein using an NHE-1 specific monoclonal antibody. Interestingly, no significant (P = 0.79) effect was seen on the P-initiated AR when EIPA was included in either the NaH or NaB medium. While these findings suggest that inhibition of NHE-dependent pHi regulation in capacitated spermatozoa is not sufficient to block initiation of the AR by P, they do not preclude the possibility that an NHE mediates the regulation of capacitation or sperm motility.

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.

Oligomeric state of human erythrocyte band 3 measured by fluorescence resonance energy homotransfer

The oligomeric state of the erythrocyte anion exchange protein, band 3, has been assayed by resonance energy homotransfer. Homotransfer between oligomeric subunits, labeled with eosin-5-maleimide at Lys430 in the transmembrane domain, has been demonstrated by steady-state and time-resolved fluorescence spectroscopy, and is readily observed by its depolarization of the eosin fluorescence. Polarized fluorescence measurements of HPLC-purified band 3 oligomers indicate that eosin homotransfer increases progressively with increasing species size. This shows that homotransfer also occurs between labeled band 3 dimers as well as within the dimers, making fluorescence anisotropy measurements sensitive to band 3 self-association. Treatment of ghost membranes with either Zn2+ or melittin, agents that cluster band 3, significantly decreases the anisotropy as a result of the increased homotransfer within the band 3 clusters. By comparison with the anisotropy of species of known oligomeric state, the anisotropy of erythrocyte ghost membranes at 37 degrees C is consistent with dimeric and/or tetrameric band 3, and does not require postulation of a fraction of large clusters. Proteolytic removal of the cytoplasmic domain of band 3, which significantly increases the rotational mobility of the transmembrane domain, does not affect its oligomeric state, as reported by eosin homotransfer. These results support a model in which interaction with the membrane skeleton restricts the mobility of band 3 without significantly altering its self-association state.

Two glycoprotein populations of band 3 dimers are present in human erythrocytes

The human erythrocyte Band 3 anion exchanger contains a single site of N-glycosylation that contains either a short complex oligosaccharide or an extended polylactosaminyl oligosaccharide. Approximately equal amounts of the different glycosylated forms of Band 3 are found in human red cells. As Band 3 exists predominantly as dimers, they may be uniform and consist of a subunit containing an extended oligosaccharide paired with a subunit containing a short oligosaccharide chain. Alternatively, Band 3 dimers may be comprised of subunits that either contain polylactosaminyl or short oligosaccharide chains. To distinguish between these two extremes, the ability of Band 3 membrane domain dimers to bind to immobilized tomato lectin, which specifically binds polylactosaminyl oligosaccharide, was tested. The dimeric membrane domain of Band 3 could be resolved into two fractions by tomato lectin chromatography. This shows that Band 3 dimers are not homogeneous and that two pools exist in red cells, some with a long polylactosaminyl oligosaccharide and the other with a short complex type. The amount of short chain form recovered in the unbound fraction was higher than expected for a random distribution of oligosaccharide chains on Band 3 dimers. Detergent extraction experiments showed that Band 3 glycoforms did not display a differential interaction with the cytoskeleton. The ability to separate Band 3 dimers into two glycoform populations suggests that subunit exchange between dimers does not occur in the membrane or in detergent solution. Furthermore, the results show that while one population of Band 3 dimers is processed to contain polylactosaminyl oligosaccharide, the other largely escapes this processing step.

The effect of apical and basolateral lipids on the function of the band 3 anion exchange protein

Although many polarized proteins are sorted to the same membrane domain in all epithelial tissues, there are some that exhibit a cell type-specific polarity. We recently found that band 3 (the anion exchanger AE1) was present in the apical membrane of a renal intercalated cell line when these cells were seeded at low density, but its targeting was reversed to the basolateral membrane under the influence of an extracellular matrix protein secreted when the cells were seeded at high density. Because apical and basolateral lipids differ in epithelia, we asked what effect might these lipids have on band 3 function. This question is especially interesting since apical anion exchange in these cells is resistant to disulfonic stilbene inhibitors while basolateral anion exchange is quite sensitive. Furthermore, the apical anion exchanger cannot be stained by antibodies that readily identify the basolateral protein. We used short chain sphingolipid analogues and found that sphingomyelin was preferentially targeted to the basolateral domain in the intercalated cell line. The ganglioside GM1 (Gal 1beta1, 3GalNAcbeta1, 4Gal-NeuAcalpha2, 3Galbeta1, 4Glc ceramide) was confined to the apical membrane as visualized by confocal microscopy after addition of fluorescent cholera toxin to filter grown cells. We reconstituted erythrocyte band 3 into liposomes using apical and basolateral types of lipids and examined the inhibitory potency of 4, 4'-dinitorsostilbene-2,2'-disulfonic acid (DNDS; a reversible stilbene) on 35SO4/SO4 exchange. Although anion exchange in sphingomyelin liposomes was sensitive to inhibition, the addition of increasing amounts of the ganglioside GM1 reduced the potency of the inhibitor drastically. Because these polarized lipids are present in the exofacial surface of the bilayer, we propose that the lipid structure might influence the packing of the transmembrane domains of band 3 in that region, altering the binding of the stilbenes to these chains. These results highlight the role of polarized lipids in changing the function of unpolarized proteins or of proteins whose locations differ in different epithelia.

Self-association of Band 3, the human erythrocyte anion exchanger, in detergent solution

Dimeric Band 3 purified in n-dodecyl octaethyleneglycol (C12E8) underwent an irreversible, temperature-dependent association, resulting in a complex with a Stokes radius slightly larger than a native tetramer, before forming a higher molecular weight aggregate. Self-association occurred with a half-time of about 1 h at 37 degrees C but did not occur at 0 degrees C after several days. No change in the secondary structure of Band 3, as observed by circular dichroism, occurred during the association process. However, self-association of Band 3 was accompanied by loss of the stilbene disulfonate inhibitor binding site. No association or loss of inhibitor binding occurred with the dimeric membrane domain under similar incubation conditions. The membrane domain dimer was also stable over a wide range of pH (5.5-9.5) and buffer conditions, while Band 3 aggregated below pH 6.5. Inhibitors of anion transport, which stabilize the membrane domain, slowed the association. Band 3, depleted of phospholipids by extensive washing of resin-bound protein with detergent or, incubated with excess detergent, was more prone to aggregation. The membrane domain also showed some aggregation when depleted of lipids. Preparations could be stabilized by adding dimyristoylphosphatidylcholine (DMPC) prior to the 37 degrees C incubation. The effect of inhibitors and DMPC was additive, with a combination of 1 mM 4,4'-dinitrostilbene-2,2'-disulfonate (DNDS) and 1:1 (wt/wt) DMPC:Band 3 stabilizing 90% of the protein to a 24-h incubation at 37 degrees C. The results suggest that self-association of Band 3 dimers is promoted by the cytoplasmic domain but results in alterations to the membrane domain involving the loss of essential phospholipids. Addition of phospholipid or inhibitors to Band 3 results in a stable preparation of the intact protein that may be suitable for crystallization studies.

Reaction of ozone with protein tryptophans: band III, serum albumin, and cytochrome C

Treatment of red cell ghosts with ozone inhibited both AChE (marking the outside of the membrane) and G3PDH (marking the inside of the membrane). There was no change in tryptophan fluorescence of the ghosts after the ozone treatment. Band 3 protein was isolated from the ozone-treated ghosts. The protein was digested with trypsin to obtain water soluble peptides from the cytoplasmic N-terminal tail and the interhelical loops. Fluorescent peptides included GWVIHPLGLR from the outer loop between helices 7 and 8, and peptide WMEAAR from the N-terminal cytoplasmic tail. Neither one of these peptides was oxidized by ozone. This was true whether or not the ghosts were sealed. We conclude that the position of these tryptophans either in the membrane structure, or because of binding to other proteins in the cytoplasmic tail, protects them from oxidation by ozone. Treatment of horse heart cytochrome c with ozone did not change the absorbance spectrum in the heme region or the tryptophan absorbing region. HPLC of the ozone-treated cytochrome c showed that cytochrome c was being modified, indicated by a change in the elution time. Treatment of cytochrome c with ozone did not change the activity in the NADH-cytochrome c reductase assay. Digestion of the ozone-treated cytochrome c with trypsin gave peptides which demonstrated normal fluorescence. (Cytochrome c has abnormally low fluorescence, which is not changed by ozone exposure.) The peptides were separated by HPLC. The fluorescence of the tryptophan-containing peptide (GITWK) was not decreased by treatment of the cytochrome c by ozone. Amino acid analysis of the ozone-treated cytochrome c indicated that methionine was oxidized. We conclude that tryptophan in cytochrome c is protected from oxidation by ozone because of the interaction with the porphyrin ring. Bovine serum albumin and human serum albumin were treated with ozone. There was a monotonic decrease in tryptophan fluorescence in both cases. Digestion of BSA with trypsin produced two fluorescent peptides. The peptide FWGK was identified by coelution with the authentic peptide. The putative peptide AWSVAR was not the same as the chemically synthesized peptide. The peptide sequences FWGK and "AWSVAR" were both oxidized in ozone-treated bovine serum albumin, with no detectable discrimination. Tryptic digestion of the ozone-treated human serum albumin produced a single fluorescent peptide, which was oxidized by ozone. The putative peptide AWAVAR in the tryptic digest of HSA was distinct from chemically synthesized peptide. The oxidation of tryptophans in proteins by ozone is markedly influenced by position in tertiary structure, position in membrane structure, and by chemical interactions within the protein.

Hydrodynamic examination of the dimeric cytoplasmic domain of the human erythrocyte anion transporter, band 3

Solution studies of the cytoplasmic domain (molecular mass approximately 40kDa) of band 3, the anion exchanger from human erythrocyte membranes, previously suggested a dimeric molecule on the basis of the relative techniques of calibrated gel filtration and calibrated preparative ultracentrifugation. This dimeric behavior is firmly established on an absolute basis by a combination of calibrated gel chromatography and absolute ultracentrifugation techniques. Sedimentation velocity in the analytical ultracentrifuge combined with calibrated gel chromatography give a molecular mass M of (77 +/- 4) kDa, a value confirmed by low-speed sedimentation equilibrium. Velocity sedimentation in the analytical ultracentrifuge gave a single sedimenting species with an s o 20,w of (3.74 +/- 0.07)S. Sedimentation equilibrium analysis was also used to establish the strength of the binding via the dissociation constant Kd, with a value from direct fitting of the concentration distribution curves of (2.8 +/- 0.5) microM, confirmed by a value of approximately 3 microM obtained from fitting a plot of molecular weight Mw,app versus cell loading concentration. Hydrodynamic calculations based on the classical translational frictional ratio showed that the protein was highly asymmetric, with an axial ratio of approximately 10:1, consistent with observations from electron microscopy.

Characterization of the stilbenedisulphonate binding site on band 3 Memphis variant II (Pro-854-->Leu)

Band 3 Memphis variant II is a mutant anion-exchange protein associated with the Diego a+ blood group antigen. There are two mutations in this transporter: Lys-56-->Glu within the cytoplasmic domain, and Pro-854-->Leu within the membrane-bound domain. The Pro-854 mutation, which is thought to give rise to the antigenicity, is located within the C-terminal subdomain of the membrane-bound domain. Yet, there is an apparent enhancement in the rate of covalent binding of H2DIDS (4,4'-di-isothiocyanatodihydro-2, 2'-stilbenedisulphonate) to 'lysine A' (Lys-539) in the N-terminal subdomain, suggesting widespread conformational changes. In this report, we have used various kinetic assays which differentiate between conformational changes in the two subdomains, to characterize the stilbenedisulphonate site on band 3 Memphis variant II. We have found a significantly higher H2DIDS (a C-terminal-sensitive inhibitor) affinity for band 3 Memphis variant II, due to a lower H2DIDS 'off' rate constant, but no difference was found between mutant and control when DBDS (4,4'-dibenzamido-2,2'-stilbenedisulphonate) (a C-terminal-insensitive inhibitor) 'off' rates were measured. Furthermore, there were no differences in the rates of covalent binding to lysine A, for either DIDS (4,4'-di-isothiocyanato-2,2'-stilbenedisulphonate) or H2DIDS. However, the rate of covalent intrasubunit cross-linking of Lys-539 and Lys-851 by H2DIDS was abnormally low for band 3 Memphis variant II. These results suggest that the Pro-854-->Leu mutation causes a localized conformational change in the C-terminal subdomain of band 3.

Effects of intracellular pH on high pressure-induced hemolysis of anion transport inhibitor-treated erythrocytes

Effects of anion transport inhibitors such as 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS) and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonate on hemolysis of human erythrocytes at 200 MPa were examined by changing intracellular pH (7.2-7.9). These inhibitors suppressed the hemolysis at neutral pH but enhanced it at alkaline pH. However, such an enhancement was suppressed by cross-linking of membrane proteins using diamide. From the near-UV CD spectra of band 3 and the relation between hemolysis and anion transport in intact or trypsin-treated erythrocytes, it was found that such hemolytic properties were characterized by the binding of inhibitors to band 3. In addition, spectrin detachment from the erythrocyte membrane by high pressure was considerably suppressed by DIDS treatment at neutral pH, but not by DIDS labeling at alkaline pH. These results suggest that the interaction of the cytoplasmic domain of band 3 with the cytoskeleton, which is induced by the binding of ligands to the exofacial domain of band 3, is dependent on the intracellular pH, i.e., the linking is tightened at neutral pH but relaxed at alkaline pH.

Asymmetric and functional reconstitution of band 3 into pre-formed phosphatidylcholine vesicles

Human erythrocyte band 3 protein was purified in 0.1% Triton X-100 and reconstituted into pre-formed phosphatidylcholine vesicles by a Triton X-100-mediated procedure [1]. Band 3 (and its transmembrane domain) could be asymmetrically reconstituted into phosphatidylcholine vesicles with retention of sulfate transport activity which showed behaviour characteristic of red cell anion transport in response to pH, H2DIDS and temperature. Successful reconstitution was also possible using high mol ratios of band 3/phosphatidylcholine (1:200), which are not achieved by any other method.

Autoreactive T cell specificity in autoimmune hemolytic anemia of the NZB mouse

Splenic T cells from Coombs'-positive New Zealand Black (NZB) mice proliferated consistently in vitro in response to the integral red blood cell (RBC) membrane protein Band 3, the antigen previously shown to be the target for the pathogenic RBC autoantibodies. The responding cells predominantly express CD4 and the proliferative response is blocked by antibodies to the NZB major histocompatibility complex class II but not by antibodies to an irrelevant H-2 haplotype. NZB splenic T cells also proliferated in response to the internal membrane skeleton protein spectrin. By contrast, T cells from BALB/c and DBA2 mice, which bear the same H-2 haplotype as NZB mice, but which do not develop autoimmune hemolytic anemia (AIHA), fail to respond to Band 3. It is considered that these results support the hypothesis that Band 3-reactive T cells provide help for the production of pathogenic anti-Band 3 autoantibodies in NZB mice. T cells from Coombs'-negative NZB mice as young as 3 weeks old proliferated in response to Band 3; moreover, the RBC from Coombs'-negative mice bore elevated levels of autoantibody as judged by a sensitive direct enzyme-linked anti-globulin test. Thus, the pathology of AIHA develops at a much earlier age than was thought previously.

High level expression, partial purification, and functional reconstitution of the human AE1 anion exchanger in Saccharomyces cerevisiae

Human erythroid anion exchanger AE1 (Band 3) was expressed in the yeast Saccharomyces cerevisiae under the control of the constitutive promoter and transcriptional terminator of the yeast phosphoglycerate kinase gene. AE1 expression in stable yeast transformants was estimated to be approximately 0.7 mg AE1 per liter. Density gradient sedimentation analysis indicated that the AE1 protein was associated with a membrane fraction distinct from plasma membrane, most likely the endoplasmic reticulum. AE1 protein was solubilized from yeast membranes with lysophosphatidyl choline, and the protein, tagged with six histidines at its amino terminus, was purified to 35% homogeneity by metal chelation affinity chromatography. Size-exclusion chromatography in the presence of octaethylene glycol monododecyl ether indicated that the solubilized yeast-expressed AE1, like endogenous erythroid AE1, eluted at a stokes radius of 77 A, consistent with a dimeric oligomeric state. Binding of partially purified yeast-expressed AE1 to 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonate resin was competitive with the transportable substrate chloride but not the nontransported anion citrate, suggesting that the structure of the anion binding site is preserved. The specific activity of sulfate transport by partially purified yeast AE1 was determined in proteoliposomes to be similar to that of authentic AE1 purified from erythrocyte membranes. These data show that this expression system has the capacity to produce functional mammalian plasma membrane anion exchangers at levels sufficient for biochemical and biophysical analysis.

The influence of two anion-transport inhibitors, 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulfonate and 4,4'-dibenzoylstilbene-2,2'-disulfonate, on the self-association of erythrocyte band 3 protein

4,4'-Diisothiocyanatodihydrostilbene-2,2'-disulfonate and 4,4'-dibenzoylstilbene-2,2'-disulfonate potently inhibit the erythrocyte anion transporter. These inhibitors act by binding, with a 1:1 stoichiometry, to the band 3 transport protein. We have studied, by sedimentation equilibrium analysis in an analytical ultracentrifuge, the effect of the two closely related stilbenedisulfonates on the state of association of band 3 in the nonionic detergent nonaethyleneglycol lauryl ether. It was found that covalent binding of 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulfonate to band 3 did not significantly disturb the monomer/dimer/tetramer association equilibrium shown by the unliganded protein. An entirely different result was obtained after addition of 4,4'-dibenzoylstilbene-2,2'-disulfonate to the protein, at both low and high chloride concentrations. The amount of band 3 dimer in the samples increased with increasing inhibitor concentration c1, and for c1 > or = 15 microM virtually all of the protein was present as dimer. After removal of the inhibitor (by gel filtration or dialysis), the original monomer/dimer/tetramer distribution of the band 3 protein was restored. Our data show that the (noncovalent) binding of 4,4'-dibenzoylstilbene-2,2'-disulfonate drastically changes the coupling between band 3 protomers. In addition, a reversible change in the state of association of band 3 induced by ligand binding is demonstrated.

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.

Topological and segmental phylogenetic analyses of the anion exchanger (band 3) family of transporters

Eleven sequenced anion exchanger (AE; band 3) proteins, including five AE1, four AE2 and two AE3 proteins, comprise the anion exchanger family (AEF) of homologous proteins. Eliminating the rat and rabbit proteins that are nearly identical to the corresponding mouse proteins, seven dissimilar members of this family were selected for study, divided into N-terminal, central and C-terminal segments (designated segments 0, 1 and 2, respectively) and analysed separately for sequence similarity and phylogenetic relatedness. Segments 0 are variable in length and sequence, are essentially lacking in some of the members of the AEF, and are not demonstrably homologous in other members of the family. All segments 1 and 2 are homologous, but they exhibit widely differing degrees of sequence divergence. Segments 2 are highly conserved in all AEF proteins. Segments 1 of the AE2 and AE3 proteins are as conserved as are segments 2, but segments 1 of the AE1 proteins have diverged from each other and from the AE2 and AE3 segments 1 much more than have segments 2 of these same proteins. The distributions of various types of amino acid residues in the putative transmembrane helical spanners of the seven dissimilar members of the AEF, based on a modification of the 14-spanner model of Wang et al. (1994) was determined, and this distribution was compared with those of other transmembrane transport proteins of known structure (bacterial rhodopsins, outer membrane porins of Gram-negative bacteria and bacterial photosynthetic reaction centres.(ABSTRACT TRUNCATED AT 250 WORDS)

N-terminal protein sequence analysis of the rabbit erythrocyte lactate transporter suggests identity with the cloned monocarboxylate transport protein MCT1

An improved purification for the rabbit erythrocyte lactate transporter, using aminoethyl-Sepharose chromatography, is described. The process of purification of the 40-50 kDa transporter, labelled with 4,4'-diisothiocyanostilbene-2,2'-disulphonate (DIDS), was followed by Western blotting with anti-DIDS antibodies [Poole, R. C. and Halestrap, A. P. (1992) Biochem. J. 283, 855-862]. Fractions highly-enriched in transporter were further purified by SDS/PAGE and the 40-50 kDa DIDS-labelled polypeptide was subjected to N-terminal protein sequencing. This analysis identified the first 16 amino acids of the protein. With the exception of one conservative substitution, this protein sequence is identical to the N-terminal protein sequence predicted from a cDNA isolated from Chinese hamster ovary cells that encode a monocarboxylate transporter, MCT1 [Kim Garcia, C., Goldstein, J. L., Pathak, R. K., Anderson, R. G. W. and Brown, M. S. (1994) Cell 76, 865-873]. This observation, along with similarities in functional properties, leads us to conclude that lactate transport in rabbit erythrocytes is mediated by the MCT1 monocarboxylate transporter isoform.

Interaction of the interdigitated DPPG or DPPG/DMPC bilayer with human erythrocyte band 3: differential scanning calorimetry and fluorescence studies

Human erythrocyte band 3 reconstituted into phospholipid vesicles has been used for studying the interaction of interdigitated lipid bilayer with an integral membrane protein. The interdigitated gel phase in DPPG/band 3 or DPPG/DMPC/band 3 systems was induced with polymyxin B (PMB) or Tris+. The phase transitions of the vesicles were detected with high-sensitivity differential scanning calorimetry (DSC). The results indicated that band 3 does not cause significant alterations in the interdigitated phase of phospholipids, with only a little decrease of the phase transition enthalpies. Fluorescence measurements showed that the transition of phospholipid/band 3 systems from the non-interdigitated to interdigitated phase is accompanied by marked intrinsic fluorescence changes of band 3. The interdigitated phase of DPPG or DPPG/DMPC vesicles increases the intrinsic fluorescence intensity of band 3 and significantly decreases the accessibility of certain tryptophan residues on the protein to Cs+.

Near-UV circular dichroism of band 3. Evidence for intradomain conformational changes and interdomain interactions

Near-UV circular dichroism (CD) was used to identify differences in the tertiary structure of human erythrocyte band 3, the chloride/bicarbonate exchange protein, consequent to covalent binding of anion transport inhibitors to the intramonomeric stilbenedisulfonate (ISD) site. Isolated intact band 3 and its membrane domain (B3MD) were compared. Spectral differences were observed which involved intradomain effects, in that they were seen both with intact band 3 and with B3MD, or interdomain effects, in that they were observed only for B3MD, but were inhibited when the cytoplasmic domain was attached. The intradomain effect involved a significant loss in optical activity in the Phe/Tyr region of the spectrum below 280 nm. It was seen only when the ISD site had stilbenedisulfonates bound covalently at pH 7.4. Raising the pH to 9.6 after adduct formation "normalized" this spectral change irreversibly. The interdomain effect was identified in the Trp spectral region at 292 nm. There was a significant increase in optical activity at 292 nm when bulky covalent ligands such as DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonate) were bound to B3MD, but not when the same ligands were bound to intact band 3. These latter results offer evidence that certain aspects of the conformational response of the integral domain are inhibited by the presence of an attached cytoplasmic domain. The potential significance of interdomain interactions to band 3 function is discussed briefly.

Differentiation between transmembrane helices and peripheral helices by the deconvolution of circular dichroism spectra of membrane proteins

The interpretation of the circular dichroism (CD) spectra of proteins to date requires additional secondary structural information of the proteins to be analyzed, such as X-ray or NMR data. Therefore, these methods are inappropriate for a CD database whose secondary structures are unknown, as in the case of the membrane proteins. The convex constraint analysis algorithm (Perczel, A., Hollósi, M., Tusnády, G., & Fasman, G. D., 1991, Protein Eng. 4, 669-679), on the other hand, operates only on a collection of spectral data to extract the common spectral components with their spectral weights. The linear combinations of these derived "pure" CD curves can reconstruct the original data set with great accuracy. For a membrane protein data set, the five-component spectra so obtained from the deconvolution consisted of two different types of alpha helices (the alpha helix in the soluble domain and the alpha T helix, for the transmembrane alpha helix), a beta-pleated sheet, a class C-like spectrum related to beta turns, and a spectrum correlated with the unordered conformation. The deconvoluted CD spectrum for the alpha T helix was characterized by a positive red-shifted band in the range 195-200 nm (+95,000 deg cm2 dmol-1), with the intensity of the negative band at 208 nm being slightly less negative than that of the 222-nm band (-50,000 and -60,000 deg cm2 dmol-1, respectively) in comparison with the regular alpha helix, with a positive band at 190 nm and two negative bands at 208 and 222 nm with magnitudes of +70,000, -30,000, and -30,000 deg cm2 dmol-1, respectively.

Anti-Cdc25 antibodies inhibit guanyl nucleotide-dependent adenylyl cyclase of Saccharomyces cerevisiae and cross-react with a 150-kilodalton mammalian protein

The CDC25 gene product of the yeast Saccharomyces cerevisiae has been shown to be a positive regulator of the Ras protein. The high degree of homology between yeast RAS and the mammalian proto-oncogene ras suggests a possible resemblance between the mammalian regulator of Ras and the regulator of the yeast Ras (Cdc25). On the basis of this assumption, we have raised antibodies against the conserved C-terminal domain of the Cdc25 protein in order to identify its mammalian homologs. Anti-Cdc25 antibodies raised against a beta-galactosidase-Cdc25 fusion protein were purified by immunoaffinity chromatography and were shown by immunoblotting to specifically recognize the Cdc25 portion of the antigen and a truncated Cdc25 protein, also expressed in bacteria. These antibodies were shown both by immunoblotting and by immunoprecipitation to recognize the CDC25 gene product in wild-type strains and in strains overexpressing Cdc25. The anti-Cdc25 antibodies potently inhibited the guanyl nucleotide-dependent and, approximately 3-fold less potently, the Mn(2+)-dependent adenylyl cyclase activity in S. cerevisiae. The anti-Cdc25 antibodies do not inhibit cyclase activity in a strain harboring RAS2Val-19 and lacking the CDC25 gene product. These results support the view that Cdc25, Ras2, and Cdc35/Cyr1 proteins are associated in a complex. Using these antibodies, we were able to define the conditions to completely solubilize the Cdc25 protein. The results suggest that the Cdc25 protein is tightly associated with the membrane but is not an intrinsic membrane protein, since only EDTA at pH 12 can solubilize the protein. The anti-Cdc25 antibodies strongly cross-reacted with the C-terminal domain of the Cdc25 yeast homolog, Sdc25. Most interestingly, these antibodies also cross-reacted with mammalian proteins of approximately 150 kDa from various tissues of several species of animals. These interactions were specifically blocked by the beta-galactosidase-Cdc25 fusion protein.

Anti-Cdc25 antibodies inhibit guanyl nucleotide-dependent adenylyl cyclase of Saccharomyces cerevisiae and cross-react with a 150-kilodalton mammalian protein

The CDC25 gene product of the yeast Saccharomyces cerevisiae has been shown to be a positive regulator of the Ras protein. The high degree of homology between yeast RAS and the mammalian proto-oncogene ras suggests a possible resemblance between the mammalian regulator of Ras and the regulator of the yeast Ras (Cdc25). On the basis of this assumption, we have raised antibodies against the conserved C-terminal domain of the Cdc25 protein in order to identify its mammalian homologs. Anti-Cdc25 antibodies raised against a beta-galactosidase-Cdc25 fusion protein were purified by immunoaffinity chromatography and were shown by immunoblotting to specifically recognize the Cdc25 portion of the antigen and a truncated Cdc25 protein, also expressed in bacteria. These antibodies were shown both by immunoblotting and by immunoprecipitation to recognize the CDC25 gene product in wild-type strains and in strains overexpressing Cdc25. The anti-Cdc25 antibodies potently inhibited the guanyl nucleotide-dependent and, approximately 3-fold less potently, the Mn(2+)-dependent adenylyl cyclase activity in S. cerevisiae. The anti-Cdc25 antibodies do not inhibit cyclase activity in a strain harboring RAS2Val-19 and lacking the CDC25 gene product. These results support the view that Cdc25, Ras2, and Cdc35/Cyr1 proteins are associated in a complex. Using these antibodies, we were able to define the conditions to completely solubilize the Cdc25 protein. The results suggest that the Cdc25 protein is tightly associated with the membrane but is not an intrinsic membrane protein, since only EDTA at pH 12 can solubilize the protein. The anti-Cdc25 antibodies strongly cross-reacted with the C-terminal domain of the Cdc25 yeast homolog, Sdc25. Most interestingly, these antibodies also cross-reacted with mammalian proteins of approximately 150 kDa from various tissues of several species of animals. These interactions were specifically blocked by the beta-galactosidase-Cdc25 fusion protein.