The nature of the stable noncovalent dimers of band 3 protein from erythrocyte membranes in solutions of Triton X-100

Does dimeric melittin occur in aqueous solutions?

Melittin, a peptide from bee venom, is known to undergo a monomer / tetramer conversion in aqueous solutions. We have studied the possible participation of dimers in the association equilibrium of melittin by sedimentation equilibrium experiments in the analytical ultracentrifuge and subsequent mathematical analysis of the concentration distributions obtained. It was found that the dimeric state is not significantly populated, the contribution of dimer to the total peptide weight probably being below 0.5%.

Oxidative stress and autologous immunoglobulin G binding to band 3 dimers in newborn erythrocytes

Since birth-induced oxidative stress (OS) results in the removal of erythrocytes from the blood stream, we studied the binding of autologous IgG to erythrocyte band 3 dimers (the 170-kDa band, which marks the erythrocytes for removal) in preterm and term newborns and in adults. The 170-kDa band was present in as much as 74% of preterm, in 21% of term newborns, and in 10% of adults. During erythrocyte ageing "in vitro" (0, 24, and 48 h aerobic incubation), the appearance of the band occurred much faster with erythrocytes from newborns (particularly preterm) than with those from adults. When the blots for the 170-kDa band were quantified by scanning densitometry, it was seen that the 0 time values were significantly higher in preterm compared to term and adult values. After aerobic incubation a progressive increase in the optical density was observed in each group and the densities were higher in preterm than in the other groups. The course of iron release during the various incubations was analogous to that of the 170-kDa band blots, and significant correlations were found at 0 and 48 h. Methemoglobin formation roughly paralleled iron release. Esterified F(2)-isoprostanes (markers of OS) and O(2)(-) production in the nonincubated (0 time) erythrocytes were much higher in newborn (preterm and term) than in adult erythrocytes. Plasma free F(2)-isoprostanes were significantly higher in preterms than in terms and in terms than in adults. Plasma non-protein-bound iron (NPBI) was higher in preterm than in term newborns and not detectable in adults. In conclusion dimers of band 3 with autologous IgG are found under conditions in which OS can be detected in erythrocytes or in plasma: namely in newborns or in aged erythrocytes.

Critical amino acid residues involved in the electrogenic sodium-bicarbonate cotransporter kNBC1-mediated transport

We have previously reported a topological model of the electrogenic Na(+)-HCO(3)(-) cotransporter (NBC1) in which the cotransporter spans the plasma membrane 10 times with N- and C-termini localized intracellularly. An analysis of conserved amino acid residues among members of the SLC4 superfamily in both the transmembrane segments (TMs) and intracellular/extracellular loops (ILs/ELs) provided the basis for the mutagenesis approach taken in the present study to determine amino acids involved in NBC1-mediated ion transport. Using large-scale mutagenesis, acidic and basic amino acids putatively involved in ion transport mediated by the predominant variant of NBC1 expressed in the kidney (kNBC1) were mutated to neutral and/or oppositely charged amino acids. All mutant kNBC1 cotransporters were expressed in HEK-293T cells and the Na(+)-dependent base flux of the mutants was determined using intracellular pH measurements with 2',7'-bis-(carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Critical glutamate, aspartate, lysine, arginine and histidine residues in ILs/ELs and TMs were detected that were essential for kNBC1-mediated Na(+)-dependent base transport. In addition, critical phenylalanine, serine, tyrosine, threonine and alanine residues in TMs and ILs/ELs were detected. Furthermore, several amino acid residues in ILs/ELs and TMs were shown to be essential for membrane targeting. The data demonstrate asymmetry of distribution of kNBC1 charged amino acids involved in ion recognition in putative outward-facing and inward-facing conformations. A model summarizing key amino acid residues involved in kNBC1-mediated ion transport is presented.

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.

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.

Effects of ligand-mimetic peptides Arg-Gly-Asp-X (X = Phe, Trp, Ser) on alphaIIbbeta3 integrin conformation and oligomerization

The purpose of this investigation was to determine what structural changes convert "inert" alphaIIbbeta3 integrins into "activated" high-affinity receptors for adhesive proteins. Light scattering, analytical ultracentrifugation, electron microscopy, and molecular modeling were used to probe the conformational states of the alphaIIbbeta3 integrin. Isolated from human blood platelets in octyl glucoside, the alphaIIbbeta3 complex behaved as an asymmetric 230 kDa macromolecule with a z-average translational diffusion coefficient of 2.9 F and a weight-average sedimentation coefficient of 7.7 S. Dynamic light scattering showed that ligand-mimetic peptides (RGDX, X = F, W, S) caused prompt, concentration-dependent increases in the Stokes radius (R(s)) of the alphaIIbbeta3 complex, whereas control peptides of reversed sequence (XDGR, X = F, W, S) had no significant effect. Sedimentation velocity data coupled with time-derivative analyses showed that RGDX peptides shifted the distribution of alphaIIbbeta3 sedimenting species toward smaller s values. Sedimentation equilibrium measurements indicated that a slower increase in the alphaIIbbeta3 molecular weight distribution took place in the presence of RGDX ligand-mimetics. Electron microscopy showed a split of alphaIIbbeta3's globular domain into two distinct nodules in the presence of RGDX peptides; oligomers joined through their stalk regions were seen frequently. These observations suggest that receptor occupancy by ligand-mimetic RGDX peptides is tightly coupled to relatively large changes in the structure of the alphaIIbbeta3 complex. alphaIIbbeta3 bead models were developed to describe quantitatively the ligand-induced transition from a "closed" to an "open" integrin conformation and the limited oligomerization that follows. This provides a new mechanistic framework for understanding integrin activation and the formation of signaling clusters on the surface of stimulated platelets.

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

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

Effect of band 3 subunit equilibrium on the kinetics and affinity of ankyrin binding to erythrocyte membrane vesicles

The membrane-spanning protein, band 3, anchors the spectrin-based membrane skeleton to the lipid bilayer via the bridging protein, ankyrin. To understand how band 3 subunit stoichiometry influences this membrane-skeletal junction, we have induced changes in the band 3 association equilibrium and assayed the kinetics and equilibrium properties of ankyrin binding. We observe that band 3 oligomers convert slowly to dimers and ultimately monomers following removal of ankyrin. Addition of excess ankyrin back to these membranes enriched in dissociated band 3 then shifts band 3 almost entirely to tetramers, confirming that the tetrameric form of band 3 constitutes the preferred oligomeric state of ankyrin binding. 4, 4'-Diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) labeling of band 3, which is shown to shift most of the band 3 population to dimers, eliminates the majority of ankyrin-binding sites on the membrane and greatly reduces retention of band 3 in detergent-extracted membrane skeletons. Furthermore, DIDS- modified membranes lack all low affinity ankyrin-binding sites and roughly half of all high affinity sites. Since labeled membranes lack the rapid kinetic phase of ankyrin binding and exhibit only half of the normal amplitude of the slow kinetic phase, it can be concluded that the rapid phase of ankyrin association involves low affinity sites and the slow phase involves high affinity sites. A model accounting for these data and most previous data on ankyrin-band 3 interactions is provided.

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.

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.

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

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

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

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

Binding of naturally occurring antibodies to oxidatively and nonoxidatively modified erythrocyte band 3

Both oxidative clustering (elicited by diamide treatment) and nonoxidative clustering (elicited by zinc/BS3 (bis[sulfosuccinimidyl]suberate) treatment) of erythrocyte integral membrane proteins induce binding of autologous antibodies with anti-band 3 specificity, followed by complement deposition and phagocytosis. Autologous antibodies eluted from nonoxidatively clustered erythrocytes bind to and stimulate phagocytosis of oxidatively damaged erythrocytes. Those eluted antibodies bind specifically to disulfide-crosslinked band 3 dimers generated by diamide treatment. Band 3 dimerization and antibody binding are abrogated by cleavage of band 3 cytoplasmic domain. Thus, disulfide-crosslinked band 3 dimers are the minimal band 3 aggregate with enhanced affinity for anti-band 3 antibodies. The eluted antibodies do not bind to band 3 dimers generated nonoxidatively by BS3 treatment but bind avidly to larger band 3 clusters generated nonoxidatively by zinc/BS3 treatment. Possibly, disulfide crosslinking of cytoplasmic domain cysteines induces reorientation of intramembrane domains as to expose putative anti-band 3 epitopes and allow bivalent binding of anti-band 3 antibodies. Extensive nonoxidative band 3 clustering appears to disrupt the native band 3 conformation and generate reoriented dimers which expose putative anti-band 3 epitopes in the proper distance and orientation as to allow bivalent antibody binding.

Partial purification of a diacylglycerol lipase from bovine aorta

A diacylglycerol (DG) lipase has been purified from a soluble subcellular fraction of bovine aorta by (NH4)2SO4 precipitation in the presence of 5.0% (w/v) Triton X-100, followed by chromatography on DEAE-Sephacel, heparin-Sepharose and octyl-Sepharose in the presence of either CHAPS or Triton X-100 detergents. Under basal conditions, the hydrolysis of a short-chain [3H]dioctanoylglycerol ([3H]diC8) substrate was much greater than that of a long-chain 1-[1-14C]palmitoyl-2-oleoyl-sn-glycerol (1-[14C]POG) substrate. Lipase activity measured with 1-[14C]POG was markedly enhanced by Triton X-100. In the presence of 0.1% Triton X-100, specific enzyme activities in the octyl-Sepharose fraction determined with 1-[14C]POG or 1-stearoyl-2-[1-14C]-arachidonoyl-sn-glycerol as substrates were the same as that measured with [3H]diC8. MgCl2 (5mM) or CaCl2 (2 mM) also selectively stimulated lipase activity (up to 10-13-fold) measured with the long-chain (1-[14C]POG) substrate only. The increase in relative specific activity in the octyl-Sepharose fraction was 60-fold and 155-fold, based on hydrolysis of [3H]diC8 and 1-[14C]POG (+ Triton X-100), respectively. Unlabelled diC8 was a competitive inhibitor of 1-[14C]POG hydrolysis, suggesting that a single lipase hydrolyses both the short-chain and long-chain DG substrates; selective stimulatory effects of non-ionic detergents and bivalent cations on the hydrolysis of 1-[14C]POG may be due to effects on the physical properties of the substrate preparation. Monoacylglycerol lipase, DG kinase and cholesterol esterase activities could not be detected in the partially purified lipase preparation.

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.

The effect of the presence of integral membrane protein (human band 3) on the membrane lytic properties of melittin in reconstituted systems

Reconstitution of the anion exchange protein from human erythrocytes (band 3) into phospholipid vesicles was shown to have a protective effect on melittin lysis of the vesicles when compared to pure lipid vesicles. Low salt buffer was found to cause an inhibition of lysis in both proteoliposomes and pure lipid vesicles compared to salt buffer. High phosphate concentration did not seem to cause inhibition of lysis in the reconstituted system. However, an inhibition is observed in pure lipid vesicle control, which is contradictory to previous reports.

Structural stability of the erythrocyte anion transporter, band 3, in native membranes and in detergent micelles

The exothermic thermal denaturation transition of band 3, the anion transporter of the human erythrocyte membranes, has been studied by differential scanning calorimetry, in ghost membranes and in nonionic detergent micelles. In detergent micelles the transmembrane domain of band 3 gave an irreversible denaturation transition (C transition). However, no thermal transition was observed for the N-terminal cytoplasmic domain when band 3 was solubilised in detergent micelles. A reduction in enthalpy (190-300 kcal mol-1) with an accompanying decrease in thermal denaturation temperatures (48-60 degrees C) for the C transition was observed in detergent solubilised band 3 when compared with ghost membranes. Unlike ghost membranes, two thermal transitions for band 3 in detergent micelles were observed for the C transition when in the presence of excess covalent inhibitor, 4,4'-diisothiocyanostilbene-2,2'-disulphonate (DIDS), which derive from the thermal unfolding of a single protein with two different thermal stabilities; DIDS-stabilised (75 degrees C) and DIDS-insensitive (62 degrees C). A reduction in the denaturation temperature for the transmembrane domain of band 3 was observed when compared with intact band 3 although no significant differences was observed in the corresponding enthalpy values. This indicates some cooperativity of the two domains of band 3 in maintaining the transmembrane conformation. The results presented in this study show that detergents of intermediate micelle size (e.g. Triton X-100 and C12E8) are required for optimal thermal stability of band 3.

Band 3-hemoglobin associations. The band 3 tetramer is the oxyhemoglobin binding site

The associations between the band 3 protein of the human erythrocyte membrane and oxyhemoglobin, in solutions of a nonionic detergent, were studied by sedimentation equilibrium experiments in the analytical ultracentrifuge. The following results were obtained: (i) hemoglobin is bound virtually exclusively to the band 3 tetramer, but not to the monomer or dimer; (ii) the band 3 tetramer can bind up to four hemoglobin tetramers; (iii) unlike the unstable dimers of unmodified band 3, stable dimers crosslinked via S-S-bridges also represent hemoglobin binding sites.

Rotational diffusion of band 3 in erythrocyte membranes. 1. Comparison of ghosts and intact cells

The rotational diffusion of eosin-labeled 3 in human erythrocyte cells and hemoglobin-free ghosts at 37 degrees C has been studied in detail by polarized delayed luminescence. The time-resolved anisotropy with both cells and freshly prepared ghosts is similar, decaying with well-resolved rotational correlation times of 0.03, 0.2, and greater than or equal to 1 ms. Mild proteolytic removal of the water-soluble 41-kDa cytoplasmic domain of band 3 in ghosts results in a drastic increase in the fractional contributions of the two fastest depolarizing components. Our results, taken together with other data in the literature, imply that several classes of band 3 that differ greatly in mobility exist in ghosts and intact cells. The mobility of one class is hindered due to complexation with other membrane or cytoplasmic proteins mediated via the 41-kDa cytoplasmic domain. However, another class of band 3 molecules exists as homo-or heterooligomeric complexes larger than a dimer that are stabilized by hydrophobic interactions involving the intramembranal domain. Finally, the presence of the (previously undetected) 0.03-ms anisotropy component strongly suggests that a significant fraction of band 3 in both ghosts and intact cells is highly mobile and diffuses at the rate expected for a freely rotating dimer in the erythrocyte membrane.

Isolation and reconstitution of furosemide-binding proteins from Ehrlich ascites tumor cells

Furosemide-binding proteins were isolated from cholate-solubilized membranes of Ehrlich ascites tumor cells by affinity chromatography, using furosemide as ligand. Solubilized proteins retarded by the affinity material were eluted by furosemide. In reducing and denaturing gels, the major proteins eluted by furosemide were 100 and 45 kDa. In nonreducing, non-denaturing gels, homodimers of both polypeptides were found, whereas no oligomeric proteins containing both polypeptides were seen. It is concluded that the furosemide gel binds two distinct dimeric proteins. The isolated proteins were reconstituted into phospholipid vesicles and the K+ transport activity of these vesicles was assayed by measurement of 86Rb+ uptake against a large opposing K+ gradient. The reconstituted system was found to contain a K+ transporting protein, which is sensitive to Ba2+ like the K+ channel previously demonstrated to be activated in intact cells after cell swelling.

Automated purification of human protein band 3, the major integral protein of the erythrocyte membrane

Human erythrocyte protein band 3 was purified from a Triton X-100 extract of white ghosts. This purification, including an ion-exchange chromatography and a group-affinity chromatography, was automated. The apparatus was assembled from commercially available elements and allowed for the recovery of 2 to 3 mg pure band 3 in 2 hr. The purification could be repeated several times a day. The advantages of automation are discussed.

Cation dependence of the phosphorylation of specific residues in red cell membrane protein band 3

Phosphorylation of anion channel protein (ACP), the major component of erythrocyte protein band 3, was achieved in red cell ghosts in buffers containing vanadate (an inhibitor of phosphatases) and Mg2+ or Mn2+, known specific activators of the various kinases present in the red cell membrane. The anion channel protein was isolated to purity and the phosphorylated aminoacids were determined. The present results show that the phosphorylation of anion channel protein in its membraneous environment leads to an equal phosphorylation of tyrosine and serine plus threonine in the presence of Mg2+. In contrast, phosphotyrosine represents 80% of the total when Mn2+ is the activator.

A new method for the reconstitution of the anion transport system of the human erythrocyte membrane

The anion transport protein of the human erythrocyte membrane, band 3, was solubilized and purified in solutions of the non-ionic detergent Triton X-100. It was incorporated into spherical lipid bilayers by the following procedure: Dry phosphatidylcholine was suspended in the protein solution. Octylglucopyranoside was added until the milky suspension became clear. The sample was dialyzed overnight against detergent-free buffer. Residual Triton X-100 was removed from the opalescent vesicle suspension by sucrose density gradient centrifugation and subsequent dialysis. Sulfate efflux from the vesicles was studied, under exchange conditions, using a filtration method. Three vesicle subpopulations could be distinguished by analyzing the time course of the efflux. One was nearly impermeable to sulfate, and efflux from another was due to leaks. The largest subpopulation, however, showed transport characteristics very similar to those of the anion transport system of the intact erythrocyte membrane: transport numbers (at 30 degrees C) close to 20 sulfate molecules per band 3 and min, an activation energy of approx. 140 kJ/mol, a pH maximum at pH 6.2, saturation of the sulfate flux at sulfate concentrations around 100 mM, inhibition of the flux by H2DIDS and flufenamate (approx. KI-values at 30 degrees C: 0.1 and 0.7 microM, respectively), and "right-side-out" orientation of the transport protein (as judged from the inhibition of sulfate efflux by up to 98% by externally added H2DIDS). Thus, the system represents, for the first time, a reconstitution of all the major properties of the sulfate transport across the erythrocyte membrane.

Formation and properties of tetramers of band 3 protein from human erythrocyte membranes in planar lipid bilayers

Lipid bilayer experiments were performed in the presence of solubilized band 3 protein from human red cell membranes. Band 3 protein increased the conductance of the lipid membranes by several orders of magnitude. Membrane conductance was found to be dependent on the fourth power of protein concentration. This shows that four band 3 subunits form an ion permeable pathway in the lipid bilayer membranes. It also shows that, in the membranes, the protein molecules undergo an association equilibrium which involves at least the monomer and the tetramer of the protein, relaxation towards equilibrium being rapid on the time scale of the experiment. The increase in bilayer conductance induced by the band 3 tetramer could be inhibited by pretreatment of the protein with several SH-reagents (pCMB, pCMBS, DTNB) which also inhibit water transport across the human red cell membrane. Other SH-reagents which do not influence water transport (iodoacetamide, N-ethylmaleimide) did not show any influence on the band 3 induced conductance increase. A band 3-mediated exchange of anions comparable to that in the erythrocyte membrane did not occur in the system studied by us. Our results suggest that, in the human erythrocyte membrane, a pore formed by the band 3 tetramer could be the pathway responsible for the protein-mediated part of water transport.