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

Hemoglobin Binding to the Red Blood Cell (RBC) Membrane Is Associated with Decreased Cell Deformability

The deformability of red blood cells (RBCs), expressing their ability to change their shape as a function of flow-induced shear stress, allows them to optimize oxygen delivery to the tissues and minimize their resistance to flow, especially in microcirculation. During physiological aging and blood storage, or under external stimulations, RBCs undergo metabolic and structural alterations, one of which is hemoglobin (Hb) redistribution between the cytosol and the membrane. Consequently, part of the Hb may attach to the cell membrane, and although this process is reversible, the increase in membrane-bound Hb (MBHb) can affect the cell's mechanical properties and deformability in particular. In the present study, we examined the correlation between the MBHb levels, determined by mass spectroscopy, and the cell deformability, determined by image analysis. Six hemoglobin subunits were found attached to the RBC membranes. The cell deformability was negatively correlated with the level of four subunits, with a highly significant inter-correlation between them. These data suggest that the decrease in RBC deformability results from Hb redistribution between the cytosol and the cell membrane and the respective Hb interaction with the cell membrane.

Architecture of the human erythrocyte ankyrin-1 complex

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

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.

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.

pH-sensitive self-associations of the N-terminal domain of NBCe1-A suggest a compact conformation under acidic intracellular conditions

NBCe1-A is an integral membrane protein that cotransports Na⁺ and HCO₃^- ions across the basolateral membrane of the proximal tubule. It is essential for maintaining a homeostatic balance of cellular and blood pH. In X-ray diffraction studies, we reported that the cytoplasmic, N-terminal domain of NBCe1-A (NtNBCe1-A) is a dimer. Here, biophysical measurements show that the dimer is in a concentration-dependent dynamic equilibrium among three additional states in solution that are characterized by its hydrodynamic properties, molar masses, emission spectra, binding properties, and stabilities as a function of pH. Under physiological conditions, dimers are in equilibrium with monomers that are pronounced at low concentration and clusters of molecular masses up to 3-5 times that of a dimer that are pronounced at high concentration. The equilibrium can be influenced so that individual dimers predominate in a taut conformation by lowering the pH. Conversely, dimers begin to relax and disassociate into an increasing population of monomers by elevating the pH. A mechanistic diagram for the inter-conversion of these states is given. The self-associations are further supported by surface plasmon resonance (SPR-Biacore) techniques that illustrate NtNBCe1-A molecules transiently bind with one another. Bicarbonate and bicarbonate-analog bisulfite appear to enhance dimerization and induce a small amount of tetramers. A model is proposed, where the Nt responds to pH or bicarbonate fluctuations inside the cell and plays a role in self-association of entire NBCe1-A molecules in the membrane.

Band 3 (AE1, SLC4A1)-mediated transport of stilbenedisulfonates. III: Role of solute and protein structure in proton-activated stilbenedisulfonate influx

DBDS (4,4'-dibenzamido-2,2'-stilbenedisulfonate) influx into magnesium resealed ghosts (MRSG) occurs over the anion/proton co-transport pH range (pK approximately 5.0). Here, factors are studied which may influence the pH dependence of DBDS transport. Accumulation of various stilbenedisulfonate (SD) molecules was studied and found to be correlated with the hydrophobicity of the R-groups (Hansch factor), not protonation of the sulfonates. The role of proton binding to glutamate 681 was found not to be part of the rate-limiting step in DBDS uptake by MRSG. Finally, the pH dependence of changes in quaternary structure/conformational state was investigated using an assay involving photo-crosslinking of band 3 subunits in the presence of DASD (4,4'-diazido-2,2'-stilbenedisulfonate). Lowering the pH promoted intersubunit crosslinking by DASD, with a pK value of 4.75+/-1.0. This value is comparable to the pK for DBDS binding to the "second" class of sites on control band 3 (pK = 5.01+/-0.16), and to DBDS influx into control MRSG (pK values between 4.57+/-0.15 and 4.7+/-0.1). Susceptibility to photo-crosslinking was reversed by raising the pH prior to initiation of the reaction. Significantly, no photo-crosslinking was observed between pH 6.0 and 8.0, where band 3 subunits are known to exist as stable dimers and tetramers. We conclude that intersubunit photo-crosslinking does not simply involve random collision between photo-activated DASD and band 3. Rather, proton binding to band 3 either alters the conformation at the interface between subunits of pre-existing tetramers, or it promotes self-association of stable dimers to a "novel" tetrameric conformational state.

A screening procedure for the solubilization of chloroplast membrane proteins from the marine green macroalga Ulva lactuca using RP–HPLC–MALDI-MS

A protocol for purification and analysis of chloroplast membrane proteins in the green macroalga Ulva lactuca has been developed, including reversed phase high performance liquid chromatography (RP-HPLC) and matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Five different solvents were evaluated for extraction of membrane proteins by three methods. The highest protein yield was achieved when proteins were extracted directly from the chloroplasts using the solvent hexafluoroisopropanol. A range of proteins of increasing hydrophobicity was separated by HPLC. Analysis of both HPLC fractions and non-separated samples by MALDI-TOF-MS revealed proteins with molecular weights spanning between 1 and 376 kDa.

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.

Properties of the detergent solubilised cytochrome c oxidase (cytochrome cbb(3)) purified from Pseudomonas stutzeri

Cytochrome cbb(3) is a cytochrome c-oxidising isoenzyme that belongs to the superfamily of respiratory haem/copper oxidases. We have developed a purification method yielding large amounts of pure cbb(3) complex from the soil bacterium Pseudomonas stutzeri. This cytochrome cbb(3) complex consists of three subunits (ccoNOP) in a 1:1:1 stoichiometry and contains two b-type and three c-type haems. The protein complex behaves as a monomer with an overall molecular weight of 114.0+/-8.9 kDa and a s(0)(20,w) value of 8.9+/-0.3 S as determined by analytical ultracentrifugation. Crystals diffracting to 5.0 A resolution have been grown by the vapour diffusion sitting drop method to an average size of 0.1 x 0.1 x 0.3 mm. This is the first crystallisation report of a (cbb(3))-type oxidase.

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.

High-yield expression and functional analysis of Escherichia coli glycerol-3-phosphate transporter

The glycerol-3-phosphate (G3P) transporter, GlpT, from Escherichia coli mediates G3P and inorganic phosphate exchange across the bacterial inner membrane. It possesses 12 transmembrane alpha-helices and is a member of the Major Facilitator Superfamily. Here we report overexpression, purification, and characterization of GlpT. Extensive optimization applied to the DNA construct and cell culture has led to a protocol yielding approximately 1.8 mg of the transporter protein per liter of E. coli culture. After purification, this protein binds substrates in detergent solution, as measured by tryptophan fluorescence quenching, and its dissociation constants for G3P, glycerol-2-phosphate, and inorganic phosphate at neutral pH are 3.64, 0.34, and 9.18 microM, respectively. It also shows transport activity upon reconstitution into proteoliposomes. The phosphate efflux rate of the transporter in the presence of G3P is measured to be 29 micromol min(-1) mg(-1) at pH 7.0 and 37 degrees C, corresponding to 24 mol of phosphate s(-1) (mol of protein)(-1). In addition, the glycerol-3-phosphate transporter is monomeric and stable over a wide pH range and in the presence of a variety of detergents. This preparation of GlpT provides ideal material for biochemical, biophysical, and structural studies of the glycerol-3-phosphate transporter.

Stilbenedisulfonate Binding Kinetics to Band 3 (AE 1): Relationship between Transport and Stilbenedisulfonate Binding Sites and Role of Subunit Interactions in Transport

Stilbenedisulfonates are competitive inhibitors of band 3 (AE1) anion exchange. It has been assumed that competitive binding implies that the stilbenedisulfonates bind to the transport site. In this paper, I summarize briefly an extensive series of stopped-flow fluorescence kinetic studies which indicate that stilbenedisulfonates do not compete with substrate anions directly, but rather that they behave as allosteric competitive inhibitors of monovalent anion binding to band 3. Monovalent anions lower stilbenedisulfonate affinity by accelerating the rate of their release from band 3, without changing the value of the initial second-order "on" kinetic constant. In addition, partial covalent labeling of the band 3 population with stilbenedisulfonates revealed subunit interaction effects: (a) in steady-state and pre-steady-state transport kinetic studies, (b) in studies on the kinetics of reversible stilbenedisulfonate binding to the unlabeled portion of the band 3 population, and (c) in microcalorimetric studies of the thermal unfolding of the membrane domain of band 3. Studies on the kinetics of reversible stilbenedisulfonate binding to erythrocyte membranes from an individual with Southeast Asian ovalocytosis also revealed subunit interaction effects. The demonstration of allosteric competition between stilbenedisulfonates and substrate anions and the observation of numerous examples of subunit interaction effects suggest that allosteric effects may play a significant role in band 3 function.

Interaction of membrane proteins and lipids with solubilizing detergents

Detergents are indispensable in the isolation of integral membrane proteins from biological membranes to study their intrinsic structural and functional properties. Solubilization involves a number of intermediary states that can be studied by a variety of physicochemical and kinetic methods; it usually starts by destabilization of the lipid component of the membranes, a process that is accompanied by a transition of detergent binding by the membrane from a noncooperative to a cooperative interaction already below the critical micellar concentration (CMC). This leads to the formation of membrane fragments of proteins and lipids with detergent-shielded edges. In the final stage of solubilization membrane proteins are present as protomers, with the membrane inserted sectors covered by detergent. We consider in detail the nature of this interaction and conclude that in general binding as a monolayer ring, rather than as a micelle, is the most probable mechanism. This mode of interaction is supported by neutron diffraction investigations on the disposition of detergent in 3-D crystals of membrane proteins. Finally, we briefly discuss the use of techniques such as analytical ultracentrifugation, size exclusion chromatography, and mass spectrometry relevant for the structural investigation of detergent solubilized membrane proteins.

Mechanism of band 3 dimer dissociation during incubation of erythrocyte membranes at 37 degrees C

The mechanism of dissociation of the stable dimer of band 3 was investigated during the incubation of isolated erythrocyte membranes or resealed ghosts at 37 degrees C. The kinetics of changes in the structural and functional integrity of the membrane domain of band 3 (MDB3) were measured and correlated with the change in the Stokes radius of band 3. MDB3 integrity was determined as follows: (1) by measuring the fluorescence emission spectrum of 4, 4'-di-isothiocyanostilbene-2,2'-disulphonate (DIDS) bound covalently to MDB3; (2) by measuring the number of DIDS covalent binding sites present after incubation of unlabelled resealed ghosts; and (3) by measuring the anion transport V(max) by using the same resealed ghosts. Incubation of membranes at 37 degrees C caused the dissociation of band 3 dimers to monomers but only after a lag period lasting approx. 50 h. The observation of such a lag implies that dissociation involves a sequence of molecular events beginning with some type of initial process. We have discovered that this initial process involves a conformation change in MDB3. There was a shift in the fluorescence spectrum for DIDS-labelled band 3 and a decrease in the DIDS binding capacity and transport activity of the unlabelled protein. Incubation of membranes at 4 degrees C inhibited the conformational change in MDB3 and the dissociation of dimers. Furthermore, no conformational change in MDB3 was observed when erythrocytes were incubated at 37 degrees C. We suggest that MDB3 unfolding is the molecular event responsible for the subsequent dissociation of stable dimers of band 3 to monomers during the incubation of erythrocyte membranes at 37 degrees C. The monomers so generated are either not functional in anion exchange or they have an attenuated functionality. The absence of a conformational change for band 3 in erythrocytes might imply that haemolysis perturbs the membrane structure and somehow predisposes band 3 to the conformational change that occurs during incubation at 37 degrees C.