Oligomeric structure and the anion transport function of human erythrocyte band 3 protein

Cryo-EM structures of the human band 3 transporter indicate a transport mechanism involving the coupled movement of chloride and bicarbonate ions

The band 3 transporter is a critical integral membrane protein of the red blood cell (RBC), as it is responsible for catalyzing the exchange of bicarbonate and chloride anions across the plasma membrane. To elucidate the structural mechanism of the band 3 transporter, detergent solubilized human ghost membrane reconstituted in nanodiscs was applied to a cryo-EM holey carbon grid to define its composition. With this approach, we identified and determined structural information of the human band 3 transporter. Here, we present 5 different cryo-EM structures of the transmembrane domain of dimeric band 3, either alone or bound with chloride or bicarbonate. Interestingly, we observed that human band 3 can form both symmetric and asymmetric dimers with a different combination of outward-facing (OF) and inward-facing (IF) states. These structures also allow us to obtain the first model of a human band 3 molecule at the IF conformation. Based on the structural data of these dimers, we propose a model of ion transport that is in favor of the elevator-type mechanism.

Heteromeric Solute Carriers: Function, Structure, Pathology and Pharmacology

Solute carriers form one of three major superfamilies of membrane transporters in humans, and include uniporters, exchangers and symporters. Following several decades of molecular characterisation, multiple solute carriers that form obligatory heteromers with unrelated subunits are emerging as a distinctive principle of membrane transporter assembly. Here we comprehensively review experimentally established heteromeric solute carriers: SLC3-SLC7 amino acid exchangers, SLC16 monocarboxylate/H⁺ symporters and basigin/embigin, SLC4A1 (AE1) and glycophorin A exchanger, SLC51 heteromer Ost α-Ost β uniporter, and SLC6 heteromeric symporters. The review covers the history of the heteromer discovery, transporter physiology, structure, disease associations and pharmacology - all with a focus on the heteromeric assembly. The cellular locations, requirements for complex formation, and the functional role of dimerization are extensively detailed, including analysis of the first complete heteromer structures, the SLC7-SLC3 family transporters LAT1-4F2hc, b^0,+AT-rBAT and the SLC6 family heteromer B⁰AT1-ACE2. We present a systematic analysis of the structural and functional aspects of heteromeric solute carriers and conclude with common principles of their functional roles and structural architecture.

Molecular Simulations of Intact Anion Exchanger 1 Reveal Specific Domain and Lipid Interactions

Anion exchanger 1 (AE1) is responsible for the exchange of bicarbonate and chloride across the erythrocyte plasma membrane. Human AE1 consists of a cytoplasmic and a membrane domain joined by a 33-residue flexible linker. Crystal structures of the individual domains have been determined, but the intact AE1 structure remains elusive. In this study, we use molecular dynamics simulations and modeling to build intact AE1 structures in a complex lipid bilayer that resembles the native erythrocyte plasma membrane. AE1 models were evaluated using available experimental data to provide an atomistic view of the interaction and dynamics of the cytoplasmic domain, the membrane domain, and the connecting linker in a complete model of AE1 in a lipid bilayer. Anionic lipids were found to interact strongly with AE1 at specific amino acid residues that are linked to diseases and blood group antigens. Cholesterol was found in the dimeric interface of AE1, suggesting that it may regulate subunit interactions and anion transport.

Interaction of the human erythrocyte Band 3 anion exchanger 1 (AE1, SLC4A1) with lipids and glycophorin A: Molecular organization of the Wright (Wr) blood group antigen

The Band 3 (AE1, SLC4A1) membrane protein is found in red blood cells and in kidney where it functions as an electro-neutral chloride/bicarbonate exchanger. In this study, we have used molecular dynamics simulations to provide the first realistic model of the dimeric membrane domain of human Band 3 in an asymmetric lipid bilayer containing a full complement of phospholipids, including phosphatidylinositol 4,5–bisphosphate (PIP₂) and cholesterol, and its partner membrane protein Glycophorin A (GPA). The simulations show that the annular layer in the inner leaflet surrounding Band 3 was enriched in phosphatidylserine and PIP₂ molecules. Cholesterol was also enriched around Band 3 but also at the dimer interface. The interaction of these lipids with specific sites on Band 3 may play a role in the folding and function of this anion transport membrane protein. GPA associates with Band 3 to form the Wright (Wr) blood group antigen, an interaction that involves an ionic bond between Glu658 in Band 3 and Arg61 in GPA. We were able to recreate this complex by performing simulations to allow the dimeric transmembrane portion of GPA to interact with Band 3 in a model membrane. Large-scale simulations showed that the GPA dimer can bridge Band 3 dimers resulting in the dynamic formation of long strands of alternating Band 3 and GPA dimers.

Human Band 3 (AE1, SLC4A1), an abundant 911 amino acid glycoprotein, catalyzes the exchange of bicarbonate and chloride across the red blood cell membrane, a process necessary for efficient respiration. Malfunction of Band 3 leads to inherited diseases such as Southeast Asian Ovalocytosis, hereditary spherocytosis and distal renal tubular acidosis. Despite much available structural and functional data about Band 3, key questions about the conformational changes associated with transport and the molecular details of its interaction with lipids and other proteins remain unanswered. In this study, we have used computer simulations to investigate the dynamics of Band 3 in lipid bilayers that resemble the red blood cell plasma membrane. Our results suggest that negatively charged phospholipids and cholesterol interact strongly with Band 3 forming an annulus around the protein. Glycophorin A (GPA) interacts with Band 3 to form the Wright (Wr) blood group antigen. We were able to recreate this complex and show that GPA promotes the clustering of Band 3 in red blood cell membranes. Understanding the molecular details of the interaction of Band 3 with GPA has provided new insights into the nature of the Wright blood group antigen.

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.

Asymmetry of inverted-topology repeats in the AE1 anion exchanger suggests an elevator-like mechanism

Anion exchanger 1 catalyzes the transmembrane antiport of chloride and bicarbonate ions through a mechanism that has remained unclear. By modeling its inward-facing state and comparing it with the known outward-facing form, Ficici et al. hypothesize that this transporter features an elevator-like mechanism.

The membrane transporter anion exchanger 1 (AE1), or band 3, is a key component in the processes of carbon-dioxide transport in the blood and urinary acidification in the renal collecting duct. In both erythrocytes and the basolateral membrane of the collecting-duct α-intercalated cells, the role of AE1 is to catalyze a one-for-one exchange of chloride for bicarbonate. After decades of biochemical and functional studies, the structure of the transmembrane region of AE1, which catalyzes the anion-exchange reaction, has finally been determined. Each protomer of the AE1 dimer comprises two repeats with inverted transmembrane topologies, but the structures of these repeats differ. This asymmetry causes the putative substrate-binding site to be exposed only to the extracellular space, consistent with the expectation that anion exchange occurs via an alternating-access mechanism. Here, we hypothesize that the unknown, inward-facing conformation results from inversion of this asymmetry, and we propose a model of this state constructed using repeat-swap homology modeling. By comparing this inward-facing model with the outward-facing experimental structure, we predict that the mechanism of AE1 involves an elevator-like motion of the substrate-binding domain relative to the nearly stationary dimerization domain and to the membrane plane. This hypothesis is in qualitative agreement with a wide range of biochemical and functional data, which we review in detail, and suggests new avenues of experimentation.

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.

NBCe1A dimer assemble visualized by bimolecular fluorescence complementation

Mutations in the electrogenic Na(+)/HCO3(-) cotransporter (NBCe1) that cause proximal renal tubular acidosis (pRTA), glaucoma, and cataracts in patients are recessive. Parents and siblings of these affected individuals seem asymptomatic although their tissues should make some mutant NBCe1 protein. Biochemical studies with AE1 and NBCe1 indicate that both, and probably all, Slc4 members form dimers. However, the physiologic implications of dimerization have not yet been fully explored. Here, human NBCe1A dimerization is demonstrated by biomolecular fluorescence complementation (BiFC). An enhanced yellow fluorescent protein (EYFP) fragment (1-158, EYFP(N)) or (159-238, EYFP(C)) was fused to the NH2 or COOH terminus of NBCe1A and mix-and-matched expressed in Xenopus oocyte. The EYFP fluorescent signal was observed only when both EYFP fragments are fused to the NH2 terminus of NBCe1A (EYFP(N)-N-NBCe1A w/ EYFP(C)-N-NBCe1A), and the electrophysiology data demonstrated this EYFP-NBCe1A coexpressed pair have wild-type transport function. These data suggest NBCe1A forms dimers and that NH2 termini from the two monomers are in close proximity, likely pair up, to form a functional unit. To explore the physiologic significance of NBCe1 dimerization, we chose two severe NBCe1 mutations (6.6 and 20% wild-type function individually): S427L (naturally occurring) and E91R (for NH2-terminal structure studies). When we coexpressed S427L and E91R, we measured 50% wild-type function, which can only occur if the S427L-E91R heterodimer is the functional unit. We hypothesize that the dominant negative effect of heterozygous NBCe1 carrier should be obvious if the mutated residues are structurally crucial to the dimer formation. The S427L-E91R heterodimer complex allows the monomers to structurally complement each other resulting in a dimer with wild-type like function.

Carbonic anhydrases and their interplay with acid/base-coupled membrane transporters

Carbonic anhydrases (CAs) have not only been identified as ubiquitous enzymes catalyzing the fast reversible hydration of carbon dioxide to generate or consume protons and bicarbonate, but also as intra- and extracellular proteins, which facilitate transport function of many acid/base transporting membrane proteins, coined 'transport metabolon'. Functional interaction between CAs and acid/base transporters, such as chloride/bicarbonate exchanger (AE), sodium-bicarbonate cotransporter (NBC) and sodium/hydrogen exchanger (NHE) has been shown to require both catalytic CA activity as well as direct binding of the enzyme to specific sites on the transporter. In contrast, functional interaction between different CA isoforms and lactate-proton-cotransporting monocarboxylate transporters (MCT) has been found to be isoform-specific and independent of CA catalytic activity, but seems to require an intramolecular proton shuttle within the enzyme. In this chapter, we review the various types of interactions between acid/base-coupled membrane carriers and different CA isoforms, as studied in vitro, in intact Xenopus oocytes, and in various mammalian cell types. Furthermore, we discuss recent findings that indicate the significance of these 'transport metabolons' for normal cell functions.

Compartmentation of GAPDH

The concept of the cytosol as a space that contains discrete zones of metabolites is discussed relative to the contribution of GAPDH. GAPDH is directed to very specific cell compartments. This chapter describes the utilization of GAPDH's enzymatic function for focal demands (i.e. ATP/ADP and NAD(+)/NADH), and offers a speculative role for GAPDH as perhaps moderating local concentrations of inorganic phosphate and hydrogen ions (i.e. co-substrate and co-product of the glycolytic reaction, respectively). Where known, the structural features of the binding between GAPDH and the compartment components are discussed. The nuances, which are associated with the intracellular distribution of GAPDH, appear to be specific to the cell-type, particularly with regards to the various plasma membrane proteins to which GAPDH binds. The chapter includes discussion on the curious observation of GAPDH being localized to the external surface of the plasma membrane in a human cell type. The default perspective has been that GAPDH localization is synonymous with compartmentation of glycolytic energy. The chapter discusses GAPDH translocation to the nucleus and to non-nuclear cellular structures, emphasizing its glycolytic function. Nevertheless, it is becoming clear that alternate functions of GAPDH play a role in compartmentation, particularly in the translocation to the nucleus.

Trafficking defect of mutant kidney anion exchanger 1 (kAE1) proteins associated with distal renal tubular acidosis and Southeast Asian ovalocytosis

Compound heterozygous anion exchanger 1 (AE1) SAO/G701D mutations result in distal renal tubular acidosis with Southeast Asian ovalocytosis. Interaction, trafficking and localization of wild-type and mutant (SAO and G701D) kAE1 proteins fused with hemagglutinin, six-histidine, Myc, or green fluorescence protein (GFP) were examined in human embryonic kidney (HEK) 293 cells. When individually expressed, wild-type kAE1 was localized at cell surface while mutant kAE1 SAO and G701D were intracellularly retained. When co-expressed, wild-type kAE1 could form heterodimer with kAE1 SAO or kAE1 G701D and could rescue mutant kAE1 proteins to express on the cell surface. Co-expression of kAE1 SAO and kAE1 G701D also resulted in heterodimer formation but intracellular retention without cell surface expression, suggesting their trafficking defect and failure to rescue each other to the plasma membrane, most likely the molecular mechanism of the disease in the compound heterozygous condition.

Substrates Induce Conformational Changes in Human Anion Exchanger 1 (hAE1) as Observed by Fluorescence Resonance Energy Transfer

The one-for-one exchange of Cl(-) and HCO(3)(-) ions is catalyzed by human erythrocyte anion exchanger 1 (hAE1) through a ping-pong mechanism whereby the protein exists in two main conformations, with the single anion-binding site exposed at either the cytoplasmic (inner) side (E(i)) or the extracellular side (E(o)), with interconversion between the two states being possible only after anion binding. Steady-state and time-resolved resonance energy transfer (FRET) techniques were used to determine the distance of the binding site for diTBA (bis-(1,3-diethylthiobarbituric acid)trimethine oxonol), a high affinity fluorescent oxonol inhibitor of hAE1, from a benchmark site (probably Lys-430) labeled by external fluorescein maleimide (FM). Using red cell ghost membranes, energy transfer distances were measured in media containing different anions between FM as the donor, covalently attached to one monomer, and diTBA as the acceptor, reversibly bound to the adjacent monomer of a hAE1 dimer. Energy transfer increased significantly in chloride or bicarbonate buffers relative to conditions where no transportable anions were present, that is, in citrate buffer. These differences in transfer efficiencies were interpreted in light of the conformational distributions of hAE1 in various buffers and the possible effects of diTBA itself on the distribution. The analysis indicates that the diTBA binding site comes closer to the FM site by approximately 7 A in chloride buffer as compared to that in citrate (or equivalent changes in diTBA orientation occur) because of the effects of anion binding. This provides the first direct physical evidence for structural changes in hAE1 induced by substrates.

Membrane integration and topology of the first transmembrane segment in normal and Southeast Asian ovalocytosis human erythrocyte anion exchanger 1

Anion exchanger 1 (AE1, or Band 3) is an integral membrane glycoprotein found in erythrocytes, responsible for the electroneutral exchange of chloride and bicarbonate ions across the plasma membrane. Southeast Asian ovalocytosis (SAO) results from a nine-amino acid deletion in the first transmembrane segment (TM) of the AE1 protein that abolishes its transport function. The effects of the SAO deletion on: (1) the efficiency of integration of TM1 into the membrane, and (2) the precise positioning of TM1 relative to the membrane were investigated using scanning N-glycosylation mutagenesis in a cell-free transcription/translation system and in transfected HEK293 cells. AE1 or SAO constructs containing either the endogenous N-glycosylation site at Asn642 in extracellular loop 4 (EC4) or single N-glycosylation sites engineered into an expanded extracellular loop 1 (EC1) were used. N-glycosylation efficiency of EC1 in the SAO construct was significantly lower than that of the AE1 construct, indicating that the SAO deletion impairs membrane integration of TM1 and the translocation of EC1 across the membrane. Scanning N-glycosylation mapping of EC1 in the cell-free system and in transfected cells showed that the C-terminus of both AE1 and SAO TM1 were at the same position relative to the membrane. Thus, the SAO deletion is likely to cause a pulling-in of the polar amino acid sequence immediately N-terminal to the deletion into the lipid bilayer, allowing SAO TM1 that was inserted to assume a transmembrane disposition.

Molecular mechanisms of autosomal dominant and recessive distal renal tubular acidosis caused by SLC4A1 (AE1) mutations

Mutations of SLC4A1 (AE1) encoding the kidney anion (Cl⁻/HCO₃⁻) exchanger 1 (kAE1 or band 3) can result in either autosomal dominant (AD) or autosomal recessive (AR) distal renal tubular acidosis (dRTA). The molecular mechanisms associated with SLC4A1 mutations resulting in these different modes of inheritance are now being unveiled using transfected cell systems. The dominant mutants kAE1 R589H, R901X and S613F, which have normal or insignificant changes in anion transport function, exhibit intracellular retention with endoplasmic reticulum (ER) localization in cultured non-polarized and polarized cells, while the dominant mutants kAE1 R901X and G609R are mis-targeted to apical membrane in addition to the basolateral membrane in cultured polarized cells. A dominant-negative effect is likely responsible for the dominant disease because heterodimers of kAE1 mutants and the wild-type protein are intracellularly retained. The recessive mutants kAE1 G701D and S773P however exhibit distinct trafficking defects. The kAE1 G701D mutant is retained in the Golgi apparatus, while the misfolded kAE1 S773P, which is impaired in ER exit and is degraded by proteosome, can only partially be delivered to the basolateral membrane of the polarized cells. In contrast to the dominant mutant kAE1, heterodimers of the recessive mutant kAE1 and wild-type kAE1 are able to traffic to the plasma membrane. The wild-type kAE1 thus exhibits a ‘dominant-positive effect’ relative to the recessive mutant kAE1 because it can rescue the mutant proteins from intracellular retention to be expressed at the cell surface. Consequently, homozygous or compound heterozygous recessive mutations are required for presentation of the disease phenotype. Future work using animal models of dRTA will provide additional insight into the pathophysiology of this disease.

Alterations in band 3 protein and anion exchange in red blood cells of renal failure patients

The precise nature of band 3 protein and its involvement in oxalate exchange in the red blood cells (RBCs) of renal failure patients has not been studied in detail. Therefore, here we studied the oxalate exchange and binding by band 3 protein in RBCs of humans with conditions of acute and chronic renal failure (ARF and CRF). The RBCs of ARF and CRF patients exhibited abnormal red cell morphology and an increased resistance to osmotic hemolysis. Further, an increase in the cholesterol content and decrease in the activities of Na(+)-K(+)-, Ca(2+)-, and Mg(2+)-ATPases of membranes were observed in the RBCs of ARF and CRF patients. A decrease in the oxalate flux was observed in the RBCs of ARF and CRF patients. The oxalate-binding activities of the RBC membranes were significantly lower in ARF (20 pmoles/mg protein) and CRF (5.3 pmoles/mg protein) patients as compared to that in the normal subjects (36 pmoles/mg protein). DEAE-cellulose and Sephadex G-200 column chromatography purification profiles revealed a distinctive shift in oxalate-binding activity of band 3 protein of RBCs of ARF and CRF patients as compared to that of the normal subjects. It was also observed from the binding studies with a fluorescent dye, eosin-5-maleimide, which specifically binds to band 3 protein, that the RBCs of ARF and CRF patients exhibited only 53 and 32% of abundance of band 3 protein, respectively, as compared to that in the RBCs of the normal subjects, thus revealing a decrease in the band 3 protein content in ARF and CRF patients. These results for the first time showed a decrease in the oxalate exchange in RBCs of patients with ARF and CRF, which was also concomitant with the low levels of abundance of band 3 protein.

The disruption of the third extracellular loop of the red cell anion exchanger AE1 does not affect electroneutral Cl−/HCO3− exchange activity

The red cell anion exchanger (band 3; AE1) is a multispanning membrane protein that traverses the bilayer up to 14 times and mediates the stilbene-disulfonate-sensitive, electroneutral exchange of chloride and bicarbonate. Previous studies showed that the integrity of the third extracellular loop (EC3) of the protein was not essential for stilbene-disulfonate-sensitive chloride uptake. Here we demonstrate that the chloride uptake mediated by assemblies separated at EC3 represents the physiological electroneutral Cl(-)/HCO(3)(-) activity associated with intact AE1 protein. This provides further evidence that the 1:5 and 6:14 regions of the protein form discrete folding domains and confirms that the third extracellular loop does not play a pivotal role in AE1 transport function.

Aquaporin-1 and HCO3(-)-Cl- transporter-mediated transport of CO2 across the human erythrocyte membrane

Recent studies have suggested that aquaporin-1 (AQP1) as well as the HCO3(-)-Cl- transporter may be involved in CO2 transport across biological membranes, but the physiological importance of this route of gas transport remained unknown. We studied CO2 transport in human red blood cell ghosts at physiological temperatures (37 degrees C). Replacement of inert with CO2-containing gas above a stirred cell suspension caused an outside-to-inside directed CO2 gradient and generated a rapid biphasic intracellular acidification. The gradient of the acidifying gas was kept small to favour high affinity entry of CO2 passing the membrane. All rates of acidification except that of the approach to physicochemical equilibrium of the uncatalysed reaction were restricted to the intracellular environment. Inhibition of carbonic anhydrase (CA) demonstrated that CO2-induced acidification required the catalytic activity of CA. Blockade of the function of either AQP1 (by HgCl2 at 65 microM) or the HCO3(-)-Cl- transporter (by DIDS at 15 microM) completely prevented fast acidification. These data indicate that, at low chemical gradients for CO2, nearly the entire CO2 transport across the red cell membrane is mediated by AQP1 and the HCO3--Cl- transporter. Therefore, these proteins may function as high affinity sites for CO2 transport across the erythrocyte membrane.

ThermoKinetic modelling. Membrane potential as a dependent variable in ion transport processes

We show how to incorporate the membrane potential and its effects on the kinetics of ion transport processes into kinetic models.

Macrophages and HIV infection: therapeutical approaches toward this strategic virus reservoir

Cells of macrophage lineage represent a key target of human immunodeficiency virus (HIV) in addition to CD4-lymphocytes. The absolute number of infected macrophages in the body is relatively low compared to CD4-lymphocytes. Nevertheless, the peculiar dynamics of HIV replication in macrophages, their long-term survival after HIV infection, and their ability to spread virus particles to bystander CD4-lymphocytes, make evident their substantial contribution to the pathogenesis of HIV infection. In addition, infected macrophages are able to recruit and activate CD4-lymphocytes through the production of both chemokines and virus proteins (such as nef). In addition, the activation of the oxidative pathway in HIV-infected macrophages may lead to apoptotic death of bystander, not-infected cells. Finally, macrophages are the most important target of HIV in the central nervous system. The alteration of neuronal metabolism induced by infected macrophages plays a crucial role in the pathogenesis of HIV-related encephalopathy. Taken together, these results strongly support the clinical relevance of therapeutic strategies able to interfere with HIV replication in macrophages. In vitro data show the potent efficacy of all nucleoside analogues inhibitors of HIV-reverse transcriptase in macrophages. Nevertheless, the limited penetration of some of these compounds in sequestered districts, coupled with the scarce phosphorylation ability of macrophages, suggests that nucleoside analogues carrying preformed phosphate groups may have a potential role against HIV replication in macrophages. This hypothesis is supported by the great anti-HIV activity of tenofovir and other acyclic nucleoside phosphonates in macrophages that may provide a rationale for the remarkable efficacy of tenofovir in HIV-infected patients. Non-nucleoside reverse transcriptase inhibitors (NNRTI) do not affect HIV-DNA chain termination, and for this reason their antiviral activity in macrophages is similar to that found in CD4-lymphocytes. Interestingly, protease inhibitors (PIs), acting at post-integrational stages of virus replication, are the only drugs able to interfere with virus production and release from macrophages with established and persistent HIV infection (chronically-infected cells). Since this effect is achieved at concentrations and doses higher than those effective in de-novo infected CD4-lymphocytes, it is possible that lack of adherence to therapy, and/or suboptimal dosage leading to insufficient concentrations of PIs may cause a resumption of virus replication from chronically-infected macrophages, ultimately resulting in therapeutic failure. For all these reasons, therapeutic strategies aimed to achieve the greatest and longest control of HIV replication should inhibit HIV not only in CD4-lymphocytes, but also in macrophages. Testing new and promising antiviral compounds in such cells may provide crucial hints about their efficacy in patients infected by HIV.

The complex of band 3 protein of the human erythrocyte membrane and glyceraldehyde-3-phosphate dehydrogenase: stoichiometry and competition by aldolase

The cytoplasmic domain of band 3, the main intrinsic protein of the erythrocyte membrane, possesses binding sites for a variety of other proteins of the membrane and the cytoplasm, including the glycolytic enzymes glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and aldolase. We have studied the stoichiometry of the complexes of human band 3 protein and GAPDH and the competition by aldolase for the binding sites. In addition, we have tried to verify the existence of mixed band 3/GAPDH/aldolase complexes, which could represent the nucleus of a putative glycolytic multienzyme complex on the erythrocyte membrane. The technique applied was analytical ultracentrifugation, in particular sedimentation equilibrium analysis, on mixtures of detergent-solubilized band 3 and dye-labelled GAPDH, in part of the experiments supplemented by aldolase. The results obtained were analogous to those reported for the binding of hemoglobin, aldolase and band 4.1 to band 3: (1) the predominant or even sole band 3 oligomer forming the binding site is the tetramer. (2) The band 3 tetramer can bind up to four tetramers of GAPDH. (3) The band 3/GAPDH complexes are unstable. (4) Artificially stabilized band 3 dimers also represent GAPDH binding sites. In addition it was found that aldolase competes with GAPDH for binding to the band 3 tetramer, and that ternary complexes of band 3 tetramers, GAPDH and aldolase do exist.

Processing of N-linked oligosaccharide depends on its location in the anion exchanger, AE1, membrane glycoprotein

The human erythrocyte anion exchanger (AE)1 (Band 3) contains a single complex N-linked oligosaccharide that is attached to Asn(642) in the fourth extracellular loop of this polytopic membrane protein, while other isoforms (AE2, AE3 and trout AE1) are N-glycosylated on the preceding extracellular loop. Human AE1 expressed in transfected human embryonic kidney (HEK)-293 or COS-7 cells contained a high-mannose oligosaccharide. The lack of oligosaccharide processing was not due to retention of AE1 in the endoplasmic reticulum since biotinylation assays showed that approx. 30% of the protein was expressed at the cell surface. Moving the N-glycosylation site to the preceding extracellular loop in an AE1 glycosylation mutant (N555) resulted in processing of the oligosaccharide and production of a complex form of AE1. A double N-glycosylation mutant (N555/N642) contained both a high-mannose and a complex oligosaccharide chain. The complex form of the N555 mutant could be biotinylated showing that this form of the glycoprotein was at the cell surface. Pulse-chase experiments showed that the N555 mutant was efficiently converted from a high-mannose to a complex oligosaccharide with a half-time of approx. 4 h, which reflected the time course of trafficking of AE1 from the endoplasmic reticulum to the plasma membrane. The turnover of the complex form of the N555 mutant occurred with a half-life of approx. 15 h. The results show that the oligosaccharide attached to the endogenous site in extracellular loop 4 in human AE1 is not processed in HEK-293 or COS-7 cells, while the oligosaccharide attached to the preceding loop is converted into the complex form.

Volume expansion stimulates p72(syk) and p56(lyn) in skate erythrocytes

Hypotonic volume expansion of skate erythrocytes rapidly stimulates the tyrosine phosphorylation of band 3, the membrane protein thought to mediate the osmotically sensitive taurine efflux. Skate erythrocytes possess numerous tyrosine kinases including p59fyn, p56lyn, pp60(src), and p72(syk), demonstrated by immune complex assays measuring autocatalytic kinase activity. Inclusion of the cytoplasmic domain of band 3 in this assay showed that only Syk and Lyn can directly phosphorylate the cytoplasmic domain of band 3. Upon cell volume expansion, Syk activity was increased as assessed by three different assays (immune complex assay measuring autophosphorylation, assay of the level of phosphotyrosine of the immunoprecipitated kinase, and assay of level of 32P in the kinase immunoprecipitated from cells prelabeled with 32PO4 and then volume-expanded). The tyrosine kinase Lyn was also stimulated by volume expansion, most notably when analyzed by the latter two methods. Volume expansion stimulated a large increase in the ability of Syk to phosphorylate band 3 at times that coincide with the stimulation of taurine flux. The stilbene piceatannol inhibited Syk preferentially over Lyn and other tyrosine kinases and inhibited volume-stimulated taurine efflux in a concentration-dependent manner similar to that for the inhibition of Syk. Two major phosphorylation peaks were detected in tryptic digests of cdb3 separated by reverse phase HPLC. Edman degradation demonstrated a phosphotyrosine in a YXXL motif. In conclusion, p72(syk) appears to be a strong candidate as a pivotal signal-transducing step in the volume-activated taurine efflux in skate red cells. The level of band-3 phosphorylation may be regulated, in addition, by a protein-tyrosine phosphatase of the 1B variety.

Surfactants in membrane solubilisation

An understanding of the action of many drugs requires a knowledge of how the drug reaches the site of action in a cell. A detailed knowledge of the structure and function of cell membranes is often required to understand the transport of drugs across the plasma membrane. To obtain this information proteins must be isolated. The isolation and characterisation of cell membrane proteins usually requires the solubilisation of the membrane and a method of separation of the various membrane proteins and glycoproteins. The starting point for such an investigation is the choice of a suitable surfactant (detergent) to solubilise the membrane. This review considers the range of surfactants that are available for membrane solubilisation, how surfactants interact with membranes, the part they play in the separation of integral membrane proteins and in the reconstitution of membrane proteins for functional studies. The solubilisation of specific membrane proteins and glycoproteins including the human erythrocyte anion transporter, mitochondrial porin, sarcoplasmic reticulum Ca(2+)-ATPase, the ATPase-active multidrug transporter P-glycoprotein, bacteriorhodopsin and rhodopsin are also discussed.

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.

The phosphate carrier from yeast mitochondria. Dimerization is a prerequisite for function

Wild type phosphate carrier (PIC) from Saccharomyces cerevisiae and recombinant PIC proteins with different C-terminal extensions were expressed in Escherichia coli as inclusion bodies. From these, PIC was isolated with the detergent sodium lauroyl sarcosinate in a form, partially monomeric and unfolded. This PIC associates to stable dimers after exchanging the detergent to the polyoxyethylene detergent C12E8 and dialysis. Combining two differently tagged monomers of PIC and following this with affinity chromatography yields defined homo- and heterodimeric forms of PIC, which are all fully active after reconstitution. As a member of the mitochondrial carrier family PIC is supposed to function as a homodimer. We investigated its dimeric nature in the functionally active state after reconstitution. When reconstituting PIC monomers a sigmoidal dependence of transport activity on the amount of inserted protein is observed, whereas insertion of PIC dimers leads to a linear dependence. Heterodimeric PIC constructs consisting of both an active and an inactivated subunit do not catalyze phosphate transport. In contrast, reconstitution of a mixture of active and inactive monomeric subunits led to partially active carrier. These experiments prove (i) that PIC does not function in monomeric form, (ii) that PIC dimers are stable both in the solubilized state and after membrane insertion, and (iii) that transport catalyzed by PIC dimers involves functional cross-talk between the two monomers.

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.

Cytoskeleton-membrane connections in the human erythrocyte membrane: band 4.1 binds to tetrameric band 3 protein

Band 4.1 provides, besides ankyrin, the main linkage between the erythrocyte membrane and its cytoskeleton. Its predominant binding sites in the membrane are located on the glycophorins. However, the cytoplasmic domain of band 3 can also bind band 4.1. We have studied which of the different band 3 oligomers observed (monomers, dimers, tetramers) can act as band 4.1 binding sites, by equilibrium sedimentation experiments on mixtures of purified band 3 and dye-labelled band 4.1 in solutions of a nonionic detergent. At low molar ratios of band 4.1 and band 3, the sedimentation equilibrium distributions obtained could all be perfectly fitted assuming that only two dye-labelled particles were present: uncomplexed band 4.1 and a complex formed between one band 4.1 molecule and one band 3 tetramer. The presence of small amounts of complexes containing band 3 monomers or dimers could not be completely ruled out but is unlikely. On the other hand, stabilized band 3 dimers effectively bound band 4.1. At higher molar band 4.1/band 3 ratio, the band 3 tetramer apparently could bind up to at least four band 4.1 molecules. The band 4.1/band 3 tetramer complex was found to be unstable. The results described, together with those reported previously, point at a prominent role of tetrameric band 3 in ligand binding.

Gel filtration chromatographic studies of the isolated membrane domain of band 3

We have investigated the oligomeric state of the membrane domain of band 3 (MDB3) in non-ionic detergent solution using Sepharose CL-4B gel filtration chromatography to study the hydrodynamic properties of the protein as a function of its concentration. The studies were performed in a C12E9 (polyoxyethylene-9-lauryl ether) buffer containing phosphatidylcholine and sodium chloride, which significantly slow a dilution-induced band 3 conformational change, and an associated aggregation process. Under these conditions native MDB3 eluted predominantly as a single Gaussian peak with a Stokes radius of 76 +/- 14 A, at all protein concentrations studies between 0.2 and 12 microM. This value agrees with the calculated Stokes radius (74 A) determined from the crystal structure of the MDB3 dimer. The Stokes radius of the MDB3 monomer was obtained experimentally by treating native MDB3 with 0.5% SDS, and exchanging the SDS for C12E9 on the Sepharose column. SDS-treated MDB3 showed two peaks whose ratio was strongly dependent on applied protein concentration. The peak representing the largest material had a Stokes radius of 69.7 +/- 14 A, which is essentially the same as the native MDB3 dimer. The peak representing the smaller material had a Stokes radius of 36 +/- 9 A, and was assigned as the MDB3 monomer in C12E9. Evidence is discussed which indicates that the C12E9 monomer specifically self-associates to form a functional MDB3 dimer. We conclude that native MDB3 exists as a stable dimer in mixed micellar solutions composed of C12E9 and phosphatidylcholine, and that the dimer can be dissociated to monomers only by denaturation.

Attempts to use liposomes and RBC ghosts as vectors in drug and antisense therapy of virus infection

Selective targeting of drugs or oligonucleotide for the treatment of viral diseases or cancer is the objective of new strategies that pursue therapy optimization and reduction of toxicity. In this work we report two protocols based on encapsulation of anti-human immunodeficiency virus drugs within targeted liposomes or erythrocytes. Both have been shown to be effective for the specific delivery of drugs or oligonucleotide in the treatment of viral infection.

Red blood cells mediated delivery of 9-(2-phosphonylmethoxyethyl)adenine to primary macrophages: efficiency metabolism and activity against human immunodeficiency virus or herpes simplex virus

Red blood cells (RBC) may act as selective carriers of drugs to macrophages, an important reservoir of viruses such as human immunodeficiency virus (HIV) and herpes simplex virus type 1 (HSV-1). We therefore assessed the incorporation of 9-(2-phosphonylmethoxyethyl)adenine (PMEA), a potent inhibitor of HIV and HSV-1) into RBC, its delivery to macrophages and its activity against HIV or HSV-1. Loading of PMEA in artificially aged opsonized RBC affords significant levels of intracellular PMEA. RBC metabolize PMEA to its active congener PMEA-diphosphate, although with low efficiency. Exposure of macrophages to RBC-encapsulated PMEA inhibits the replication of both HIV and HSV-1 (about 90% inhibition at the highest RBC:macrophages ratios) even if RBC were removed before virus challenge. By contrast, the antiviral activity of free PMEA removed before virus challenge was irrelevant at concentrations up to 150-fold higher than the 50% effective concentration (EC50). Finally, the antiviral effect of RBC-encapsulated PMEA correlates with PMEA levels in macrophages about 500-fold higher than those achieved by free PMEA (at concentrations 10-fold higher than the EC50). The efficacy of RBC-mediated delivery to macrophages of PMEA (and perhaps of compounds with shorter intracellular half-lives) warrants further studies in infectious diseases involving phagocytizing cells as main targets of the pathogen.

Associations between erythrocyte band 3 protein and aldolase in detergent solution. Determining their stoichiometry by analytical ultracentrifugation

The cytoplasmic domain of band 3, the predominant polypeptide of the erythrocyte membrane, represents a binding site for certain glycolytic enzymes. We have studied the association between human band 3 protein and aldolase, in order to clarify the role of the different band 3 oligomers as ligand binding sites. The experiments were performed on mixtures of solubilized band 3 and aldolase in solutions of a nonionic detergent, nonaethyleneglycol lauryl ether. The main technique applied was sedimentation equilibrium analysis in an analytical ultracentrifuge. In addition, nonequilibrium centrifugation techniques were used. To facilitate the evaluations, the aldolase was labelled with a dye. The following results were obtained. (1) With unmodified band 3, aldolase is bound exclusively or at least predominantly to the band 3 tetramer (but not to monomers or dimers). (2) The band 3 tetramer can bind up to four aldolase tetramers. (3) The band 3 tetramer/aldolase complex is unstable on the time scale of the techniques used. (4) Stable band 3 dimers (stabilized either covalently or noncovalently) can also associate with aldolase and can bind up to two aldolase tetramers. The results described, together with those reported previously, point at a prominent role of the band 3 tetramer in ligand binding.

Transmembrane Ca2+ gradient is essential for high anion transport activity of human erythrocytes

The role of a transmembrane Ca2+ gradient in anion transport by Band 3 of human resealed erythrocyte ghosts has been studied. The results show that a transmembrane Ca2+ gradient is essential for the conformation of erythrocyte Band 3 with higher anion transport activity. The dissipation of the transmembrane Ca2+ gradient by the ionophore A23187 inhibits the anion transport activity. The extent of this inhibition approaches 90% as the Ca2+ concentration on both sides of the ghost membrane is increased to 1.0 mM and half-maximum inhibitions is observed at 0.25 mM Ca2+. Addition of ATP (0.4 mM) to the resealing medium can partly reestablish the transmembrane Ca2+ gradient by activation of Ca(2+)-ATPase and alleviate the inhibition to some extent. N-ethylmaleimide, an inhibitor of erythrocyte Ca(2+)-ATPase, prevents such restoration. Electron micrographs reveal that numerous larger intramembranous particle can be observed on the P-faces of freeze-fractured resealed ghosts in the absence of a transmembrane Ca2+ gradient.

The orientation of eosin-5-maleimide on human erythrocyte band 3 measured by fluorescence polarization microscopy

The dominant motional mode for membrane proteins is uniaxial rotational diffusion about the membrane normal axis, and investigations of their rotational dynamics can yield insight into both the oligomeric state of the protein and its interactions with other proteins such as the cytoskeleton. However, results from the spectroscopic methods used to study these dynamics are dependent on the orientation of the probe relative to the axis of motion. We have employed polarized fluorescence confocal microscopy to measure the orientation of eosin-5-maleimide covalently reacted with Lys-430 of human erythrocyte band 3. Steady-state polarized fluorescence images showed distinct intensity patterns, which were fit to an orientation distribution of the eosin absorption and emission dipoles relative to the membrane normal axis. This orientation was found to be unchanged by trypsin treatment, which cleaves band 3 between the integral membrane domain and the cytoskeleton-attached domain. this result suggests that phosphorescence anisotropy changes observed after trypsin treatment are due to a rotational constraint change rather than a reorientation of eosin. By coupling time-resolved prompt fluorescence anisotropy with confocal microscopy, we calculated the expected amplitudes of the e-Dt and e-4Dt terms from the uniaxial rotational diffusion model and found that the e-4Dt term should dominate the anisotropy decay. Delayed fluorescence and phosphorescence anisotropy decays of control and trypsin-treated band 3 in ghosts, analyzed as multiple uniaxially rotating populations using the amplitudes predicted by confocal microscopy, were consistent with three motional species with uniaxial correlation times ranging from 7 microseconds to 1.4 ms.

Organic osmolyte channels: transport characteristics and regulation

Erythrocytes of the skate (Raja erinacea) exposed to hypotonic stress swell and then undergo a volume regulatory decrease by releasing taurine and other osmolytes. Previous studies showed that taurine release occurs via a volume-activated, Na(+)-independent, bi-directional transporter that has the properties of a size-limited channel. We now report on the transport characteristics of this channel and its regulation. Kinetic, competition and inhibitor studies indicate that polyols (myo-inositol) and trimethylamines (betaine) are transported by the same channel as taurine. Although the identity of the channel is still unknown a variety of evidence suggests that band 3 is involved in either channel formation or regulation. Hypotonicity causes phosphorylation and structural changes in band 3. Under isotonic conditions band 3 is predominantly in the dimeric form. Hypotonicity causes a shift to tetrameric band 3. We hypothesize that the band 3 tetramer either forms or regulates an osmolyte channel. The finding that expression of band 3 protein increases osmolyte channel activity in Xenopus oocytes supports this hypothesis.

Determination of the orientation of a band 3 affinity spin-label relative to the membrane normal axis of the human erythrocyte

The orientation of the nitroxide moiety of an isotopically substituted spin-labeled derivative of dihydrostilbenedisulfonate ([15N,2H13]-SL-H2DADS-maleimide) covalently coupled at the extracellular stilbenedisulfonate binding site of the human erythrocyte anion exchange protein, band 3, has been determined relative to the membrane normal axis of intact cells. The X-band linear electron paramagnetic resonance (EPR) spectra of [15N,2H13]-SL-H2DADS-maleimide-labeled band 3 in intact erythrocytes oriented by flow through an EPR flat cell have been obtained for two orthogonal orientations of the sample in the DC magnetic field. Two different methods of analysis have provided very similar values for the angles alpha 1 and beta 1 which uniquely define the orientation of the nitroxide axis frame relative to the membrane normal axis. In the first approach, a variable fraction of the cells, f, were taken to be biconcave disks perfectly oriented relative to the flat cell surface with the remainder, 1-f, isotropically oriented. Simultaneous nonlinear least squares analysis of the spectra obtained at the two sample orientations yielded best fit values of f = 0.60, alpha 1 = 58 degrees, and beta 1 = 36 degrees. In the second approach, the EPR spectra of flow-oriented intact erythrocytes labeled with the fatty acid spin-label, [15N,2H12]-5-nitroxyl stearate, have been obtained at the two sample orientations. These two spectra have been used to determine a model-independent distribution of membrane normal orientations in the sample. Using this experimentally determined membrane normal orientation distribution, the EPR spectra of [15N,2H13]-SL-H2DADS-maleimide-labeled erythrocytes were then reanalyzed to obtain a second determination of the nitroxide orientation, alpha 1 = 61 degrees and beta 1 = 37 degrees. The orientation of the nitroxide with respect to the membrane normal axis determined in the present study is nearly identical to the orientation of the nitroxide with respect to the uniaxial rotational diffusion axis, alpha = 66 degrees and beta = 34 degrees, as determined from saturation transfer EPR (ST-EPR) studies [Hustedt, E. H., & Beth, A. H. (1995) Biophys. J. 69, 1409-1423]. This result supports the conclusion that the motion observed using ST-EPR spectroscopy is, in fact, the uniaxial rotational diffusion of band 3 about the membrane normal.

Synthesis and characterization of a novel spin-labeled affinity probe of human erythrocyte band 3: characteristics of the stilbenedisulfonate binding site

A new spin-labeled maleimide derivative of the anion exchange inhibitor 4-4'-diaminodihydrostilbene-2,2'-disulfonate (H2DADS) has been synthesized as a site-specific molecular probe of the stilbenedisulfonate binding site of the anion exchange protein 1 (AE-1; band 3) in human erythrocytes. This probe, SL-H2DADS-maleimide, specifically and covalently labels the Mr 17 kDa integral membrane segment of band 3 with a 1:1 stoichiometry and inhibits essentially 100% of the band 3-mediated anion exchange. The linear V1 EPR spectrum of spin-labeled intact erythrocytes is indicative of a spatially isolated probe which is effectively immobilized on the submicrosecond time scale. Several independent lines of experimental evidence have shown that the nitroxide moiety of SL-H2DADS-maleimide-labeled band 3 is sequestered in a highly protected protein environment. These results are consistent with the observation that the spin-label is rigidly linked to band 3 in a fixed orientation with respect to the membrane normal axis [Hustedt, E. J., & Beth, A. H., (1996) Biochemistry 35, 6944-6954]. The nitroxide moieties of the SL-H2DADS-maleimide-labeled band 3 dimer are greater than 20 A from each other and are also more than 20 A from a monomer-monomer contact surface defined by cross-linking with the spin-labeled reagent BSSDA [bis(sulfo-N-succinimidyl)doxyl-2-spiro-5'-azelate]. These properties make SL-H2DADS-maleimide an extremely useful molecular probe for characterization of the physical properties of the band 3 stilbenedisulfonate binding site, determination of distances between the stilbenedisulfonate site and other segments of band 3, and investigation of the global rotational dynamics of human erythrocyte band 3.

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

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

Local structural difference between human and bovine band 3 in the anion transport inhibitor-binding region

We have examined molecular properties of inhibitor-complexed human and bovine band 3, an anion transport protein of erythrocyte membrane, in order to demonstrate the structural characteristics of the inhibitor binding region. Band 3 modified with DIDS (4,4'-diisothiocyano-2,2'-stilbenedisulfonate), a potent anion transport inhibitor, generated a positive circular dichroic band at a wavelength of 345 nm, corresponding to a DIDS chromophore. The dichroic spectra of human band 3-DIDS complex and its bovine counterpart differed markedly in their ellipticity. Under the conditions that H2DIDS (the dihydro-derivative of DIDS) cross-linked two chymotryptic fragments of human band 3, the reagent failed to cross-link the equivalent bovine fragments. The inhibitory effect of PLP (pyridoxal 5'-phosphate), a substrate and affinity label, on phosphate influx into red blood cells was more pronounced for human band 3 than for bovine band 3. The residue Lys-562 of human band 3 was found to be modified with PLP, while the corresponding residue of bovine band 3 was devoid of reactivity with PLP.

Synthesis and characterization of N-acylated, pH-sensitive 'caged' aminophospholipids

A series of acid-labile 'caged' phosphatidylserine (PS) and phosphatidylethanolamine (PE) molecules have been synthesized by N-acylation of the aminophospholipid with maleic, citraconic, dimethylmaleic, phthalic, or 3,4,5,6-tetrahydrophthalic anhydride. N-citraconyl-dioleoylphosphatidylethanolamine (C-DOPE) and N-citraconyl-dioleoyl-phosphatidylserine (C-DOPS) exhibited the highest degree of sensitivity to acidic pH; incubation at pH 5.5 and 6.5, respectively, resulted in 50% cleavage to the native aminophospholipid within 60 min. Significant cleavage of the phthalyl- and maleyl-PE derivatives did not occur at physiologically relevant pH values (pH 5.5-8), while tetrahydrophthalyl-PE and dimethylmaleyl-PE could not be isolated, reflecting their inherent instability. At pH 5.5, the half time for cleavage of C-DOPE and C-DOPS was 110 min and 85 min, respectively. The utility of these 'caged' lipids was demonstrated in human erythrocytes. When mixed with cells, C-DOPS, or the short chain analog, N-citraconyl-dilauroylphosphatidylserine (C-DLPS), transferred from liposome membranes into erythrocytes and remained in the cell outer monolayer. Low pH treatment released the citraconyl group and the free PS was transported to the inner monolayer by the aminophospholipid transporter. These novel 'caged' phospholipids may be useful for the study of transmembrane aminophospholipid transport, protein-lipid interactions and membrane fusion.

Sulfate restriction induces hyposecretion of the adhesion proteoglycan and cell hypomotility associated with increased 35SO4(2-) uptake and expression of a band 3 like protein in the marine sponge, Microciona prolifera

Sulfate is an important component relating to normal proteoglycan secretion and normal motility in the marine sponge, Microciona prolifera. The following alterations were observed in sponge cells when sulfate free artificial sea water was used as the suspension medium: 1) impairment of aggregation, 2) loss of cell movements, 3) a marked reduction in the secretion of the adhesion proteoglycan (AP). Reversal of this effect occurred if sulfate depleted cells were again rotated in sulfate containing artificial sea water. Motility and reaggregation of sulfate deprived cells could be completely restored by purified AP, but only if cells were first pre-conditioned in normal sea water. Comparisons of 35SO4(2-) uptake between normal and sulfate deprived cells which had been treated to reduce preformed secretions showed a marked increase in 35SO4(2-) uptake and incorporation which could be greatly augmented in the presence of Ca2+/Mg2+. Excessive retention of AP in sulfate starved cells demonstrated by immunostaining suggested that AP secretion and cellular motility may be controlled by a sulfate dependent secretogogue or that undersulfated AP itself had developed a secretory defect. SDS-PAGE of Triton treated cellular extracts demonstrated a 116 kDa 35SO4(2-) sulfated band which co-migrated with AP, but only in extracts derived from sulfate starved cells. Western blots prepared from such extracts incubated in the presence of a monoclonal anti-band 3 antibody demonstrated labelling of a single 97 kDa band only in material from sulfate deprived cells. The absence of this component in normal cell extracts indicated that this protein may be involved in facilitated sulfate transport. This study lends support to a heretofore unrecognized role for sulfate in cell motility and secretion.

Red cell membrane protein band 4.2: phenotypic, genetic and electron microscopic aspects

The present status of band 4.2 has been reviewed from the standpoint of protein chemistry, gene analysis and clinical hematology. Band 4.2 plays an important role in various cellular functions. In 142 GCT-->ACT band 4.2 deficiency, abnormalities of the cytoskeletal network were clearly observed by electron microscopy and by ektacytometry, although the cytoskeletal proteins themselves were essentially normal in these red cells. The physiological states of band 3 in situ in the membranes were also affected in band 4.2 deficiency, as detected by electron microscopy, although again the biochemical properties of band 3 itself were essentially normal in these red cells. Other disorders of band 4.2 deficiency in the absence of the 142 GCT-->ACT mutation appear to be most interesting in the pathogenesis of hemolysis. In some of the band 4.2 anomalies, other membrane proteins including band 3 would appear to be most pathognomonic for the disease states. These conditions require elucidation by protein chemistry and gene analysis. The control mechanism of the gene expression should also be clarified to understand the important role of band 4.2 in health and disease.

Amino acid sequences around exofacial proteolytic cleavage sites of band 3 from bovine and porcine erythrocytes

1. Amino acid sequences of bovine and porcine band 3, an erythrocyte anion transporter, were determined. 2. The sequence of bovine band 3 was positioned to residues 519-599 (the numbering is based on human band 3), in which probably 6 residues were unidentified. 3. Binding site of DIDS (4,4'-diisothiocyanostilbene-2,2'-disulfonate), a potent anion transport inhibitor, was identified as Lys-539 in the bovine case. 4. A loop (residues 551-567), which provides exofacial proteolytic cleavage sites, contains only 53% homology between human and bovine, whereas the residues flanking it on either side are > 84% homologous. 5. Furthermore, the loop of porcine band 3 was indicated to consist of a 6 or 7-residues short peptide as compared with those of other species.

Structural and binding properties of the stilbenedisulfonate sites on erythrocyte band 3: an electron paramagnetic resonance study using spin-labeled stilbenedisulfonates

Two new spin-label derivatives of 4,4'-diaminodihydrostilbene-2,2'-disulfonate (H2-DADS) have been chemically synthesized and employed in electron paramagnetic resonance (EPR) studies of binding to the anion exchange protein (band 3) in intact human erythrocytes. Equilibrium binding studies with the 4-monoacyl-spin-label derivative (mono-SL-H2-DADS) indicated an effective dissociation constant of 11 microM and substantial negative cooperativity in isotonic citrate buffer, pH 7.4, at 20 degrees C. The 4,4'-diacyl-spin-label derivative (di-SL-H2-DADS) bound with an effective dissociation constant of 54 microM and no detectable cooperativity under the same binding conditions. The findings of substantial negative cooperativity in binding of the less bulky mono-SL-H2-DADS and no cooperativity for di-SL-H2-DADS suggest the presence of an allosteric coupling between the stilbenedisulfonate sites on adjacent band 3 monomers rather than steric interactions. There were approximately 1 x 10(6) binding sites per erythrocyte for both the mono- and di-SL-H2-DADS derivatives, and the binding of each was blocked by pretreatment of intact cells with 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS), a highly specific covalent inhibitor of anion exchange. EPR spectra collected over a wide range of concentrations of mono-SL-H2-DADS indicated that binding resulted in immobilization of the probe and that, even upon near saturation of available binding sites, there were no detectable dipole-dipole interactions between bound probes. EPR spectra collected using di-SL-H2-DADS revealed the presence of intramolecular dipole-dipole interactions between spin-label moieties on opposite ends of this biradical probe, but no intermolecular dipole-dipole interactions between separate bound probes. These data indicate that di-SL-H2-DADS binds to the stilbenedisulfonate binding site on band 3 in a bent conformation and further suggest that the termini of these binding sites on adjacent monomers are greater than 20 A apart.

The state of association of Band 3 of the human erythrocyte membrane: evidence of a hexamer

Band 3 of the human erythrocyte membrane was isolated from 32P-labelled erythrocytes in non-ethyleneglycol n-dodecyl ether (C12E9), Triton X-100, and Brij 58 solutions, and its states of association were studied by sucrose gradient sedimentation and by Sepharose 4B column chromatography. The sedimentation and elution profiles indicated that Band 3 exists under two stable forms in each detergent solution, a slow form and a fast form. The fraction of the fast form in a Brij 58 solution was 2-3-times higher than those of the fast forms in C12E9 and Triton X-100 solutions. Moreover, depending of the conditions of isolation, only the slow form or the fast form was present in a Brij 58 solution. The apparent values of sedimentation coefficients, Stokes radii, and effective masses of the slow and fast forms in C12E9, Triton X-100, and Brij 58 solutions were determined. On the basis of these values, we have concluded that the slow and fast forms of Band 3 were dimer and tetramer in C12E9 and Triton X-100 solutions but were dimer and hexamer in a Brij 58 solution.

Kinetics of conformational changes associated with inhibitor binding to the purified band 3 transporter. Direct observation of allosteric subunit interactions

Subunit interaction effects were identified for isolated human erythrocyte band 3, the anion exchanger, by observing both static and stopped-flow kinetic protein fluorescence changes associated with inhibitor binding to the intramonomeric stilbenedisulfonate site. We measured the rate of conformational changes associated with reversible binding of H2DIDS (4,4'-diisothiocyanodihydrostilbene-2,2'-disulfonate). The rate of H2DIDS release was also measured. As a test for subunit interactions, we studied the effect of partial labeling of the band 3 monomer population with H2DIDS on the equilibrium and kinetics of H2DIDS reversible binding to the remaining monomers. The results showed biphasic kinetics for control band 3, with a pseudo-first-order ligand dependence for the fast phase followed by a slow ligand-independent relaxation. A second-order "on" rate constant for the fast phase was determined to be (1.2 +/- 0.1) x 10(7) M-1 s-1, while the associated "off" rate constant was found to be 1.1 +/- 0.5 s-1. From these kinetic constants, we calculated a Kd value of 95 +/- 50 nM, which is in excellent agreement with the Kd value determined at thermodynamic equilibrium (110 +/- 9 nM). Covalent labeling of 75% of the band 3 monomer population with H2DIDS changed the kinetics of the fast phase, slowing the apparent rate by changing the order of the reaction from pseudo-first-order to zero-order. Partial labeling did not affect the ligand-independent relaxation. Separate measurements of the H2DIDS "off" rate also showed a biphasic time course, with a 20-fold difference in apparent rate constants.(ABSTRACT TRUNCATED AT 250 WORDS)

The major integral proteins of the human red cell

The structures and functions of the major human red cell integral membrane proteins are summarized in this review. The proteins that are discussed are the anion transporter (band 3), the sialic acid-rich glycophorins and the glucose transporter. Band 3 (AE1) is a member of a family of anion transporters which carry out Cl-/HCO3- exchange. AE1 is largely restricted to red cells and functions in CO2 transport between the tissues and lungs. In addition to its transport function band 3 acts as an anchor site to the membrane of the red cell skeleton, and also binds a number of cytoplasmic red cell proteins. Variant forms of band 3 are known and some of these have an effect on red cell function and viability. The glycophorins comprise three major proteins, glycophorin A (GPA), glycophorin B (GPB) and glycophorin C (GPC). GPA and GPB (together with another putative gene product, GPE) are closely related products of highly homologous genes located in tandem on the human chromosome. The similarity between the genes gives rise to a number of genetic variants as a result of unequal crossover events. The gene products are erythroid specific. The function of the proteins is not clearly established, but GPA appears to have a role in facilitating the movement of band 3 to the cell surface during the biosynthesis of the latter. The GPC gene is not related to the GPA, GPB and GPE gene family. This gene gives rise to GPC and a form of GPC which is truncated at the N-terminus and is designated GPD. GPC functions in anchoring the red cell skeleton to the membrane, and absence of the protein is associated with red cell abnormalities. GPC transcripts are found in many other tissues, where they probably also have a role in cytoskeletal interactions. The red cell glucose transporter (GLUT1) is a member of the gene family of passive glucose transporters. GLUT1 is not erythroid specific but is also present in several other tissues.

Lys-430, site of irreversible inhibition of band 3 Cl- flux by eosin-5-maleimide, is not at the transport site

Although eosin-5-maleimide (EM) covalently labels band 3 and has been thought to react at the external-facing anion transport site, EM reversibly inhibits Cl- exchange at 0 degrees C in a noncompetitive fashion, indicating that under these conditions it does not bind to the transport site [Knauf, P.A., N.M. Strong, J. Penikas, R.B. Wheeler, Jr., and S.J. Liu. Am. J. Physiol. 264 (Cell Physiol. 33): C1144-C1154 1993]. To see whether or not the covalent labeling by EM takes place at the same noncompetitive site as the reversible binding, we examined the dependence of reaction rate on EM concentration. The reaction rate saturates with increasing EM concentration, indicating that reversible binding precedes covalent reaction and that EM therefore acts as an affinity label. A more complex model in which reversible binding prevents a bimolecular reaction at a different site cannot, however, be ruled out. Cl- gradients across the membrane affect EM reversible binding in a manner suggesting that EM binds preferentially to the Eo form of band 3, with the transport site unloaded and facing outward. Thus EM binds to and probably reacts covalently with a site that is different from the transport site, but whose conformation is affected by the orientation of the transport site. Lysine-430, the amino acid residue which is covalently labeled by EM (4), may be near the transport site but does not seem to be directly involved in the binding of transported substrates such as chloride. EM binding to one band 3 monomer decreases the reactivity of the adjacent monomer but does not decrease the affinity constant of the reversible binding step that precedes covalent reaction. Although a small fraction (approximately 1%) of band 3 monomers fail to react with EM, EM nearly completely inhibits transport in those monomers with which it reacts.