Reconstitution of the erythrocyte anion channel

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

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

Lipid domains in model and biological membranes

Lipid domains that occur within biological of model membranes encompass a variety of structures with very different lifetimes. The separation of membrane lipids into compositional domains can be due to lateral phase separation, immiscibility within a single phase, or interaction of lipids with integral or peripheral proteins. Lipid domains can affect the extent and rate of reactions in the membrane and provide sites for the activity of specialized proteins. Domains are likely to be involved in the process of lipid sorting to various cellular membranes, as well as in other processes which involve membrane budding or invagination.

Reconstitution, identification, and purification of the Torpedo californica electroplax chloride channel complex

A voltage-gated chloride channel was identified in the electric organ of the marine ray Torpedo californica by White and Miller (J. Biol. Chem. 254, 10161-10166 (1979)). The experiments reported here concern the purification and identification of this channel which was accomplished by solubilization of electric organ plasma membranes and reconstitution of the channel into vesicles made of phosphatidylethanolamine, phosphatidylserine, and cholesterol. Channel activity was measured in these vesicles by assaying 36Cl- uptake against an outwardly directed chloride chemical gradient as described by Garty et al. (J. Biol. Chem. 258, 13094-13099 (1983)). Maximal uptake occurred by 15 s. Addition of valinomycin after 10 min released intravesicular 36Cl- suggesting that chloride is moving through a channel. Channel activity was inhibited by DIDS (K0.5 of 56 mM) and NBD chloride (K0.5 of 176 mM). In a 40 lipid/1 protein (w/w) reconstitution, approx. 30% of the vesicles contained a functional chloride channel, based upon uptake done in the presence of chlorotriphenyltin (an anion ionophore), indicating that the Torpedo electric organ is an enriched source as shown by White and Miller (Biophys. J. 35, 455-462 (1981)). The chloride channel was purified approx. 40-fold by sedimentation velocity. In this purified preparation, four polypeptides (210, 95, 55, and 40 kDa) were visible by silver-staining after nonreducing SDS-PAGE. Of the four polypeptides, the largest (210 kDa) is not sufficient for Cl- channel activity by itself, but it is labeled by DIDS, an inhibitor of channel activity. Channel activity was approx. 20-fold greater in material that bound to concanavalin A compared to the concanavalin A flow-through; all four polypeptides were present in the bound materia. It is possible that some of these polypeptides are subunits of the chloride channel.

Enrichment of saturated fatty acid containing phospholipids in sheep brain serotonin receptor preparations: use of microwave irradiation for rapid transesterification of phospholipids

During enrichment of the 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT)-binding serotonin 5-HT1A receptors from sheep brain gray matter (membrane isolation, detergent solubilization and reconstitution into vesicles) a consistent and striking increase in the composition of saturated fatty acids was observed in phospholipids which were coisolated with the receptors. A rapid procedure has been developed for the methylation of free and phospholipid linked fatty acids which were thus analyzed by gas chromatography-mass spectrometry (GC/MS). Esterification of free fatty acids and transesterification of phospholipid linked fatty acids were achieved with 14% boron trifluoride in methanol (BF3-CH3OH) in 20 s and 50 s, respectively, under low power microwave irradiation (60 W) with a post-reaction cooling of less than 5 min. This is in contrast to the conventional method of heating in a boiling water bath for 10-15 min with BF3-CH3OH which is inevitably preceded by time-consuming and inconvenient clamping of vials and followed by cooling for 10 min before the vials can be safely opened. Analysis of fatty acid profiles in phosphatidylethanolamine (PE) and phosphatidylcholine (PC) from egg yolk, phosphatidylinositol (PI) from bovine liver and phosphatidylserine (PS) from bovine brain by both techniques showed comparable results. During detergent solubilization of sheep brain gray matter, the overall proportion of saturated fatty acids in PE (major lipid), PI, PC (major lipid) and PS increased from 50-60% in sheep brain phospholipids to 70-75% in 1.5% CHAPS solubilized, reconstituted and biologically active serotonin 5-HT1A preparations. In sharp contrast, the proportions of saturated fatty acids in 1.5% Triton X-100 solubilized PE (48.1%) (major lipid), PI (63.6%), PC (60.6%) (major lipid) and PS (62.2%) were not significantly different from those in the original sheep brain membranes. Strikingly, this was coupled with the occurrence of very low levels of 5-HT1A receptor activity in the Triton X-100 solubilized preparations. The abundance of 5-HT1A sites in the enriched vesicles obtained only from the CHAPS-solubilized preparations was further confirmed by specific radiolabeling of a 58-kDa polypeptide by the 5-HT1A specific ligand p-aminophenylethyl-m-trifluoromethylphenylpiparazine (PAPP) which was coupled to a 125I-labeled, photoreactive, heterobifunctional cross-linker, sulfosuccinimidyl-2-(p-azidosalicylamido)ethyl-1,3'-dithiopropiona te (SASD). Thus CHAPS-solubilized 5-HT1A receptor preparations are depleted in the more rigid lipids such as sphingolipids and cholesterol, (Banerjee et al. (1990) Biochim. Biophys. Acta 1044, 305-314), but are enriched in vesicle-stabilizing, phospholipid-linked saturated fatty acids which in turn probably stabilize the heptahelical, membrane bound 5-HT1A receptor.

Isolation and characterization of the rainbow trout erythrocyte band-3 protein

Rainbow trout (Salmo gairdneri) band-3 protein was isolated from trout erythrocyte plasma membranes by a combination of preparative SDS/PAGE and electroelution. High purity and recovery of the plasma membranes were achieved by a new method. This was demonstrated using 4,4'diiso-thiocyano[3H2]dihydro-stilbene 2,2'disulfonic acid (3H2DIDS) which specifically labels band-3 protein. On SDS/PAGE, band-3 protein yields a similarly diffuse pattern, as does mammalian band-3 protein, with an apparent Mr of 116,000. In situ chymotryptic cleavage/cross-linking experiments with 3H2DIDS reveal that the fragments cross-link as in human and mouse band-3 proteins but that there are minor differences. Treatment of trout erythrocytes with trypsin results in cleavage of the band-3 protein. Purified polyclonal antibodies raised against trout band-3 protein react with trout band-3 protein and do not crossreact with mouse or human band-3 protein. They react specifically with only one chymotryptic fragment of trout band-3 protein.

Fatty acid composition of lipids which copurify with band 3

In a previous study (L. R. Maneri and P. S. Low (1988) J. Biol. Chem. 263, 16170-16178) we determined that the anion transport protein, band 3, was significantly stabilized by lipids containing saturated and/or long chain fatty acids. To determine whether this thermodynamic preference is reflected in the composition of lipids tightly associating with the anion transporter in vivo, we have analyzed the fatty acid content of phospholipids co-isolating with the purified integral domain of band 3. Our data demonstrate that although stearic acid comprises only 14% of the bulk lipid fatty acids of the red cell membrane, it constitutes -68% of the fatty acids of lipids co-isolating with band 3. Certain other long chain fatty acids were also enriched in the adherent lipids. These results suggest that the fatty acids which most effectively stabilize band 3 also have the highest affinity for the transport protein.

Degradation of the human erythrocyte membrane band 3 studied with monoclonal antibody directed against an epitope on the cytoplasmic fragment of band 3

The mouse hybridoma monoclonal antibody BIII.136 of the IgG2a class is specific for human erythrocyte band-3 protein. It was shown by means of immunoblotting and immunoprecipitation assays that the antibody recognized an epitope located in the cytoplasmic pole of the band-3 molecule within approximately 20 kDa from the N-terminal end. The N-terminal fragments of band-3 protein, migrating in SDS/polyacrylamide gel electrophoresis in the 60-kDa, 40-kDa and 20-kDa regions, were detected with the antibody in untreated red-cell membranes as products of autolysis of band-3 protein. A correlation was found between the amount of these fragments and erythrocyte age, which suggests that partial degradation of band 3 proceeds in vivo during senescence of erythrocytes. The further degradation of band-3 protein in vitro was not observed in intact erythrocytes stored at 4 degrees C, but progressed distinctly after hemolysis of red cells, during washing and storing the membranes.

Dynamic association of band 3 with triton shells in human erythrocyte ghosts

To test a possibility that free band 3 and ankyrin-linked band 3 are exchanged in situ, band 3 was labeled with 125I, using intact red blood cells and lactoperoxidase. The cytoplasmic surface of this labeled band 3 was considered to be intact. When Triton shells were incubated with Triton supernatants prepared from 125I-labeled intact erythrocytes at 37 degrees C in the presence of Mg-ATP under isotonic conditions, the incorporation of free 125I-labeled band 3 to shells was observed. This incorporation was affected by the presence of Triton X-100 in the incubation mixture, and significantly decreased when the content of Triton X-100 was less than 0.04% (v/v). On the other hand, ankyrin-linked 125I-labeled band 3 was released when shells prepared from 125I-labeled intact erythrocytes were incubated with the Triton supernatants at 37 degrees C under the same condition as when free 125I-labeled band 3 incorporation was observed. These results strongly suggest that free and ankyrin-linked band 3 exchanged with each other in the presence of Triton X-100. A water-soluble 43 kDa fragment of band 3 inhibited the incorporation of free 125I-labeled band 3 to the shells and also inhibited the Mg-ATP-dependent shape change of ghosts in the absence of Triton X-100. Both of these inhibitory effects remained, even after 10 min of heat treatment at 100 degrees C, but drastically decreased by treatment with trypsin. Our results strongly suggest that a dynamic exchange of the free band 3 for ankyrin-linked band 3 may occur in intact erythrocytes, and it may even contribute to the shape change of erythrocytes.

Temperature-specific inhibition of human red cell Na+/K+ ATPase by 2,450-MHz microwave radiation

The ATPase activity in human red blood cell membranes was investigated in vitro as a function of temperature and exposure to 2,450-MHz continuous wave microwave radiation to confirm and extend a report of Na+ transport inhibition under certain conditions of temperature and exposure. Assays were conducted spectrophotometrically during microwave exposure with a custom-made spectrophotometer-waveguide apparatus. Temperature profiles of total ATPase and Ca+2 ATPase (ouabain-inhibited) activity between 17 and 31 degrees C were graphed as an Arrhenius plot. Each data set was fitted to two straight lines which intersect between 23 and 24 degrees C. The difference between the total and Ca+2 ATPase activities, which represented the Na+/K+ ATPase activity, was also plotted and treated similarly to yield an intersection near 25 degrees C. Exposure of membrane suspensions to electromagnetic radiation, at a dose rate of 6 W/kg and at five temperatures between 23 and 27 degrees C, resulted in an activity change only for the Na+/K+ ATPase at 25 degrees C. The activity decreased by approximately 35% compared to sham-irradiated samples. A possible explanation for the unusual temperature/microwave interaction is proposed.

Are Ca2+-dependent proteases really responsible for Cl(-)-dependent and Ca2+-stimulated binding of [3H]glutamate in rat brain?

The role of Ca2+ ions in [3H]glutamate binding was re-examined using synaptic membranous preparations obtained from the rat brain. In vitro addition (0.1-5 mM) of calcium chloride exhibited a profound enhancement of the binding in a temperature-dependent manner, whereas that of calcium acetate had no significant effect on the binding independently of the incubation temperature. Calcium acetate elicited a significantly additional stimulation of the Cl(-)-induced and temperature-dependent facilitation of the binding. The augmentation by these two ions was invariably eliminated by the addition of an antagonist for the anion channels including picrotoxinin as well as of inhibitors of anion transport such as ethacrynic acid and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. L-Aspartate exerted a more potent inhibitory action on the Cl(-)-dependent binding and Cl(-)-dependent and Ca2+-stimulated binding, than D-aspartate. The latter two bindings were selectively abolished by an agonist (quisqualic acid) and an antagonist (DL-2-amino-4-phosphonobutyric acid) for central glutamate receptors, respectively. It was also found that pretreatment of the membranes with calcium acetate resulted in a complete abolishment of the Ca2+-stimulated binding with a concomitant stimulation of the Cl(-)-dependent binding, which invariably occurred independently of the preincubation temperature (2 or 30 degrees C). No significant alteration was detected in the basal binding following the latter pretreatment. None of various protease inhibitors such as leupeptin, antipain, chymostatin and pepstatin induced a significant alteration in the basal, Cl(-)-dependent, Ca2+-stimulated and Na+-dependent bindings of [3H]glutamate, respectively. These results suggest that Ca2+ ions may elicit their stimulatory action on the Cl(-)-dependent binding of [3H]glutamate even in the absence of Cl- ions added through the temperature-independent and apparently irreversible interaction with the anion transport carriers rather than the direct action on the binding sites of the ligand. The evidence presented here also suggests that the widely held view that Ca2+-dependent proteases are responsible for the exhibition of Cl(-)-dependent and Ca2+-stimulated binding of [3H]glutamate may need to be re-evaluated.

Differentiation of the Ca2+-stimulated binding from the Cl- -dependent binding of [3H]glutamate in synaptic membranes from rat brain

The effect of Ca2+ as well as Cl- ions on [3H]glutamate (Glu) binding was re-examined using rat brain synaptic membranes frozen at -80 degrees C in 0.32 M sucrose. The inclusion of 20 mM ammonium chloride or 20 mM ammonium chloride plus 2.5 mM calcium acetate disclosed the Cl- -dependent binding or Ca2+-stimulated binding even at 2 min after the initiation of incubation at 30 degrees C and each binding reached a plateau within 30 min. In contrast, the binding reached its maximal value within 10 min followed by a progressive decline up to 60 min in the presence of 100 mM sodium acetate. Scatchard analysis revealed that Cl- as well as Cl-/Ca2+ ions invariably caused a significant increment of the number of binding sites without altering their affinity, whereas Na+ ions induced a prominent increment of the density of binding sites with a concomitant lowering of their affinity. DL-2-Amino-4-phosphonobutyric acid selectively abolished the Cl- -dependent and Ca2+-stimulated bindings without significantly affecting the basal or Na+-dependent binding. Quisqualic acid induced a profound inhibition of both Cl- -dependent and Ca2+-stimulated bindings, to a significantly greater extent than that of the basal and Na+-dependent bindings. D-Aspartic acid exhibited a potent inhibition of the Na+-dependent binding with a significantly less potent displacement of the basal, Cl- -dependent and Ca2+-stimulated bindings. An inhibitor of anion transport, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), not only eliminated the Cl- -dependent binding, but also completely abolished the Ca2+-stimulated binding. Scatchard analysis revealed that DIDS (0.1 mM) prevented the Cl- - and Cl-/Ca2+-induced increment of the density of binding sites with no significant change of their affinity. Pretreatment of the membranes with hydrophilic SH-reactive agents such as N-ethylmaleimide and 5,5'-dithiobis-(2-nitrobenzoic acid) invariably resulted in a more sensitive inhibition of the Ca2+-stimulated binding than that of the Cl- -dependent binding, while hydrophobic reagent p-chloromercuribenzoic acid produced a similarly potent elimination of the Cl- -dependent and Ca2+-stimulated bindings. Calcium-stimulated binding was also found to be sensitively diminished by dithiothreitol and dithioerythritol as compared with the Cl- -dependent binding. In vitro addition of L-ascorbic acid (10(-6)-10(-3) M) attenuated the Ca2+-stimulated binding to a significantly greater extent than the inhibition of the Cl- -dependent binding.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Current concepts in membrane protein reconstitution

The reconstitution of integral proteins into artificial lipid vesicles is largely prompted by the complexity of most biological membranes and the inherent difficulty of studying individual components in situ. Ideally, therefore, the reconstituted system should consist of a single protein in a lipid matrix which mimics the native membrane in all but its diversity. While such an approach allows individual components of a complex system to be studied in isolation it should also be sufficiently versatile to permit the generation of increasingly sophisticated multicomponent models. From the considerable number of reconstitution techniques which have been developed I have tried in this review to identify those characteristics of a particular system which maximise both the information it can provide and its versatility.

Model membranes bearing glycolipids and glycoproteins

The forces that hold cell membrane components together are non-covalent and thermodynamically favoured in aqueous media. Hence virtually any glycolipid or membrane glycoprotein might be expected to be incorporable into lipid bilayer membranes and this expectation has been borne out. In addition methods have been developed for linking lipid fragments to species that would not otherwise be expected to associate with bilayers. Techniques that have been successfully used to generate bilayer structures bearing glycolipids and glycoproteins include hydration of films dried down from non-aqueous solutions of the components, detergent removal from aqueous component solutions, exogenous addition to preformed membranes, and various organic solvent injection or reverse phase approaches. Bilayer association of glycolipids and membrane glycoproteins, with preservation of specific receptor function, seem easy to achieve--in fact difficult not to achieve. Optimization of receptor function to accurately mimic that of cell membranes and efficient preservation of functions such as transport or second messenger activation, are typically more demanding, although still feasible. A systematic approach can give considerable insight into the processes involved via identification of minimal necessary factors. Unfortunately, the actual relative arrangement of components, so critical to subtleties of glycolipid and glycoprotein function, remains almost totally unknown for lack of morphological information in the size range of individual macromolecules. The latter problem has come to be the most critical limitation to many studies.

Effect of detergent on protein structure. Action of detergents on secondary and oligomeric structures of band 3 from bovine erythrocyte membranes

With special interest in the mode of action of zwitterionic detergents on proteins, a variety of detergents were examined for their ability to disrupt the secondary and quaternary structures of an anion transport protein, band 3, and its cytoplasmic 38 kDa fragment from bovine erythrocyte membranes and for their effect on the binding of an anion transport inhibitor to band 3. Nonionic detergents and Chaps also acted as a nondenaturant in these instances, as well accepted for other proteins. Though deoxycholate and cholate inhibited the binding of an anion transport inhibitor to band 3, these detergents did not show any effect on the native structure of band 3. Zwitterionic detergents (Zwittergent 3-10, Zwittergent 3-12 and N, N-dimethyl-N-dodecyl glycine) were suggested to denature the water-soluble 38 kDa fragment at concentrations above the critical micelle concentration, but to be weak in disrupting interacting forces between hydrophobic membrane-bound domains of band 3. The results indicated that these zwitterionic detergents are similar in the mode of denaturing action to dodecyltrimethylammonium bromide rather than sodium dodecyl sulfate.

The use of 2,2,6,6-tetramethyl-4-maleimido-piperidin-1-oxyl in electron paramagnetic resonance spin label studies of drug interactions with erythrocyte membranes

The electron spin resonance (EPR) spectrum of erythrocyte membranes labeled with 2,2,6,6-tetra-methyl-4-maleimido-piperidin-1-oxyl (MAL-6) indicates both weakly and strongly immobilized labeling sites. Changes in the ratio of the membrane's low field spectral peaks (W/S) have often been used to monitor drug-erythrocyte interactions. We have investigated a number of experimental factors that may influence this ratio even in the absence of drug. Instrumental settings on the EPR spectrometer had no obvious effect. As the weight ratio of label/protein decreased, the W/S ratio increased. Similarly an increase in labeling time and temperature lead to an increase in the ratio. The ratio also increased with time after labeling; this change was most marked in samples kept at 37 degrees C, but was insignificant in samples kept at 4 degrees C. Increasing the viscosity of the sample with low-molecular-weight substances such as sucrose or glycerol, but not with those with high molecular weight such as dextran and PVP, caused a reduction in the ratio. Increasing the pH and changing the buffer concentrations also lead to a small increase in W/S. These results suggest that it is very important that all of these factors be kept constant and at some optimal value if reliable and consistent results are to be obtained using this method to monitor drug-erythrocyte interactions.

Catabolism of the anion transport protein in human erythrocytes

We identified the catabolic products of protein 3 in human erythrocytes. Protein 3, the major protein of the erythrocyte membrane, functions in anion transport and reacts covalently with tritiated 4,4'-diisothiocyano-1,2-diphenylethane-2,2'-disulfonic acid ([3H]DIDS), a very selective inhibitor of anion transport. In this study, [3H]DIDS was used to label protein 3 in the membranes of normal cells and those from a donor heterozygous for a variant of protein 3, defined by its elongated amino-terminal end. Both types of cells contained [3H]DIDS-labeled peptides other than protein 3. A protein fragment of 60K molecular weight was found in normal cells, whereas both 60K and 63K fragments were identified in cells from the heterozygote. These peptides are identical with those generated by treatment of intact erythrocytes with Pronase or chymotrypsin. A polyclonal rabbit antibody specific for the purified 60K fragment of protein 3 was used to detect this protein and its products in the erythrocyte membrane. Autoradiographs of membrane peptides that were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, transferred to nitrocellulose, and allowed to react with the monospecific antibody showed, in addition to protein 3, a 60K fragment and fragments in the 40K region and in the 20-30K region. Cells containing the protein 3 variant yielded two fragments showing a 3K difference in molecular weight in all three regions, demonstrating that degradation of protein 3 is identical in normal erythrocytes and those heterozygous for the variant. This observation also confirms the common derivation of the fragments from protein 3.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of lipid composition on the barrier properties of band 3-containing lipid vesicles

Band 3 protein has been incorporated into lipid vesicles consisting of 94:6 (molar ratio) egg phosphatidylcholine-bovine heart phosphatidylserine or total erythrocyte lipids by means of a Triton X-100 Bio-Beads method, with an additional sonication step prior to the removal of the detergent. This methods results, for both types of band 3 lipid vesicles, in rather homogeneous vesicles with comparable protein content and vesicle trap. Freeze-fracture electron microscopy revealed that band 3-egg phosphatidylcholine-bovine heart phosphatidylserine vesicles have considerably more intramembrane particles as compared to the band 3-erythrocyte lipid vesicles. The dimensions of the nonspecific permeation pathways present in the band 3-lipid vesicles were measured using an influx assay procedure for nonelectrolytes of different size, in which the vesicles were sampled and subsequently freed from nonenclosed labeled permeant by means of gel-filtration. The band 3-egg phosphatidylcholine-bovine heart phosphatidylserine vesicles have nonspecific permeation pathways (pores), with diameters of up to 60 A. In contrast, the band 3-total erythrocyte lipid vesicles are more homogeneous and show much smaller nonspecific permeation pathways, having a diameter of about 12 A. These results suggest that the nonspecific permeability of the band 3-lipid vesicles is strongly lipid-dependent. Increase in specific anion permeability expected as a consequence of the presence of band 3 in the erythrocyte lipid vesicles was found to be very limited. However, stereospecific, phloretin-inhibitable D-glucose permeability could clearly be demonstrated in these vesicles. The difference of the nonspecific permeability of the band 3-egg phosphatidylcholine-bovine heart phosphatidylserine vesicles and band 3-erythrocyte lipid vesicles, is discussed in the light of the presence of defects at the lipid/protein interface and protein aggregation, which may induce formation of pores.

Concentration dependence of the unidirectional sulfate and phosphate flux in human red cell ghosts under selfexchange and under homoexchange conditions

The concentration dependence of the sulfate and the phosphate selfexchange and homoexchange fluxes was studied in resealed red cell ghosts (25 degrees C, pH 7.3). The selfexchange fluxes were calculated from the rate constant of the tracer back-exchange and from the intracellular substrate anion content. The homoexchange fluxes were determined from the initial cis-to-trans tracer fluxes and the initial specific substrate anion activities at the cis-membrane side. Sulfate and phosphate concentrations ranging from approx. 2-100 mM were employed. The selfexchange fluxes of sulfate and of phosphate exhibit sigmoidal flux/concentration curves. The apparent Hill coefficients were in the range of 1.2-1.4 indicating a type of positive cooperativity. Under homoexchange conditions the positive cooperativity of the flux/concentration curves disappears. The outward homoexchange fluxes of sulfate and phosphate display a saturation kinetics with Hill coefficients close to 1.0. The inward homoexchange fluxes exhibit a negative type of cooperativity with Hill coefficients smaller than 1.0. The sulfate and the phosphate half-saturation concentrations for the outer and the inner membrane surface are equal in size and amount to approx. 35 mM for sulfate and to approx. 110 mM for phosphate, respectively. The positive cooperativity of the unidirectional sulfate and phosphate fluxes under selfexchange conditions and the disappearance of the positive cooperativity under homoexchange conditions indicate a cooperativity of the translocation process. The saturation of the outward homoexchange flux and the negative cooperativity of the inward homoexchange flux suggest a substrate anion binding according to the law of mass action at the inner and a negative cooperativity of substrate anion binding at the outer membrane surface.

The anion permeability of vesicles reconstituted with intrinsic proteins from the human erythrocyte membrane

Band 3 protein was reconstituted with lipid vesicles consisting of 94:6 (molar ratio) egg phosphatidylcholine-bovine heart phosphatidylserine in a 2500:1 phospholipid:protein molar ratio by means of a Triton X-100/beads method. The SO2-4 permeability of the resulting vesicles was measured using an influx assay procedure in which the vesicles were sampled and subsequently eluted over Sephadex columns at appropriate time intervals. The accuracy of the assay was greatly increased by using an internal standard in order to correct for vesicle recovery. In agreement with previous work, it could be demonstrated that incorporation of band 3 in the vesicles caused an increase in SO2-4 permeability, which could be (partially) inhibited by high concentrations of DIDS or a competitive anion such as thiocyanate. However, the magnitude of the increased SO2-4 permeability was highly variable, even when vesicles were reconstituted using band 3 isolated from one batch of ghosts. In addition, the SO2-4 influx curves showed complex kinetics. These results are related to the existence of vesicle heterogeneity with respect to protein content and vesicle size as revealed by stractan density gradient centrifugation and freeze-fracture electron microscopy. Band 3 incorporation also increased the L-glucose permeability of the vesicles which could also be inhibited by DIDS. Glycophorin, which has no known transport function, reconstituted with lipid vesicles consisting of 94:6 (molar ratio) egg phosphatidylcholine-bovine heart phosphatidylserine in a 400:1 phospholipid:protein molar ration increased the bilayer permeability towards SO2-4 as well as towards L-glucose. Surprisingly, the SO2-4 permeability in the vesicles could also be inhibited by DIDS and thiocyanate. It is concluded that the use of DIDS and a competitive anion, thiocyanate, in order to prove that band 3 is functionally reconstituted, is highly questionable. The increased SO2-4 and L-glucose permeability of band 3-lipid as well as glycophorin-lipid vesicles and the inhibitory action of DIDS are discussed in the light of the presence of defects at the lipid/protein interface and protein aggregation, which may induce the formation of pores. Since the band 3-lipid vesicles are more permeable for SO2-4 than for L-glucose, in contrast to the glycophorin-containing vesicles, it is suggested that some anion specificity of the increased bilayer permeability in the band 3-lipid vesicles is still preserved.

Characterization of phosphoenolpyruvate transport across the erythrocyte membrane. Evidence for involvement of band 3 in the transport system

Phosphoenolpyruvate was found to be transported across the erythrocyte membrane by a carrier-mediated transport system. The transport of phosphoenolpyruvate was competitively inhibited by inorganic phosphate (Ki = 24 mM) and pyridoxal 5-phosphate (Ki = 0.2 mM), whereas the transport was noncompetitively inhibited by L(+)-lactate (Ki = 37 mM). Specific inhibitors for the inorganic anion transport system such as 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and 4,4'-dinitrostilbene-2,2'-disulfonic acid strongly inhibited the phosphoenolpyruvate transport. The transport was irreversibly inhibited by treating erythrocytes with pyridoxal 5-phosphate and NaBH4. Transport activities of phosphoenolpyruvate and inorganic phosphate in the treated cells were similarly inhibited by pyridoxal 5-phosphate depending on its concentrations. 4,4'-Dinitrostilbene-2,2'-disulfonic acid protected both transport activities against the pyridoxal 5-phosphate/NaBH4 treatment. The major integral membrane protein, band 3, was preferentially labelled by treating erythrocytes with pyridoxal 5-phosphate and NaB[3H]H4. The radioactive incorporation into band 3 was confirmed by two-dimensional gel electrophoresis combining isoelectric focusing in the first dimension and sodium dodecyl sulfate/polyacrylamide gel electrophoresis in the second dimension. Taken together, these results suggest that band 3 mediates the transport of phosphoenolpyruvate as well as inorganic phosphate.

Phospholipid dependence of the anion transport system of the human erythrocyte membrane. Studies on reconstituted band 3/lipid vesicles

Band 3 protein extracted from human erythrocyte membranes by Triton X-100 was recombined with the major classes of phospholipid occurring in the erythrocyte membrane. The resulting vesicle systems were characterized with respect to recoveries, phospholipid composition, protein content and vesicle size as well as capacity and activation energy of sulfate transport. Transport was classified into band-3-specific fluxes and unspecific permeability by inhibitors. Transport number (sulfate ions per band 3 per minute) served as a measure of functional therapy after reconstitution. The transport properties of band 3 proved to be insensitive to replacement of phosphatidylcholine by phosphatidylethanolamine, while sphingomyelin and phosphatidylserine gradually inactivated band-3-specific anion transport when present at mole fractions exceeding 30 mol%. The activation energy of transport remained unaltered in spite of the decrease in transport numbers. The results, which are discussed in terms of requirements of band 3 protein function with respect to the fluidity and surface charge of its lipid environment, provide a new piece of evidence that the transport function of band 3 protein depends on the properties of its lipid environment just as the catalytic properties of some other membrane enzymes. The well-established species differences in anion transport (Gruber, W. and Deuticke, B. (1973) J. Membrane Biol. 13, 19-36) may to some extent reflect this lipid dependence.

Functional reassembly of lymphocyte lentil lectin receptor glycoproteins into lipid bilayer vesicles

Plasma membrane vesicles purified from pig mesenteric lymph nodes were solubilized using the mild, readily-dialyzable detergent dodecyltrimethylammonium bromide, and lentil lectin receptor glycoproteins were isolated by affinity chromatography. The receptor fraction showed 12 major bands on SDS-polyacrylamide gel electrophoresis representing 8-9% of the membrane protein. 5'-Nucleotidase (EC 3.1.3.5) was very effectively solubilized by the detergent and was recovered in high yield in the receptor fraction. Receptor glycoproteins were reassembled into large unilamellar vesicles of phosphatidylcholine/phosphatidylserine (mean diameter 0.235 microm) using a detergent dialysis technique. Sixty to seventy percent of the glycoprotein and most of the 5'-nucleotidase activity is associated with the phospholipid vesicles. 5'-Nucleotidase is reassembled in a symmetrical fashion and is inhibited by binding of concanavalin A, lentil lectin and pea lectin but not by succinyl-concanavalin A. Measured values for Ki and maximal inhibition are similar to those observed with intact plasma membrane vesicles. Hemagglutination inhibition studies showed that the reassembled receptors effectively bind lentil lectin. Thus lymphocyte membrane glycoproteins reassembled into phospholipid vesicles seem to retain at least part of their function in that enzyme activities such as 5'-nucleotidase remain intact and the receptors effectively bind lentil lectin.

New approaches for the reconstitution and functional assay of membrane transport proteins. Application to the anion transporter of human erythrocytes

The human red blood cell anion transport protein, band 3, was isolated and reconstituted into lipid vesicles. The main feature of the new reconstitution is the replacement of native lipids and of solubilizing detergent by externally added lipids, while band 3 protein is immobilized on a gel matrix. The vesicles formed upon detergent removal and sonication are unilamellar and sealed, and band 3 protein is the major polypeptide detectable in them. The method consists of: (a) solubilization of alkali-treated red blood cell membranes by Triton X-100; (b) binding of glycophorin and band 3 protein to diethylaminoethyl (DEAE)-cellulose in Triton X-100 solution, followed by high ionic strength elution; (c) band 3 protein complexation to organomercurial Sepharose; (d) exchange of the Triton X-100 with the dialyzable detergent octylglucopyranoside, while band 3 protein is complexed to the column; (e) elution of band 3 by cysteine (5 mM) in the presence of octylglucopyranoside; (f) addition of lipids (asolectin or egg phosphatidylcholine) to the protein-detergent suspension; and (g) dialysis of the mixture against 1% bovine serum albumin to remove the detergent completely. The vesicles were assayed for anion transport capacity by a novel procedure which is based on the fluorescent substrate N-(2-aminoethylsulfonate)7-nitrobenz-2-oxa-1,3-diazole (NBD-taurine) and on anti-NBD-antibodies as quenchers of extravesicular NBD-taurine fluorescence. Efflux of NBD-taurine from vesicles was monitored in a continuous mode as a decrease in intravesicular fluorescence. The band 3-mediated flux was approx. 50% inhibitable by externally added disulfonic stilbenes, indicating the random distribution of band 3 protein in reconstituted vesicles. Both the specific transfer rate (i.e., nmol substrate/mg protein per min) of band 3 and its energy of activation (Ea) in the artificial lipid milieu were similar to those obtained with the native system. Glycophorin incorporation into this milieu had no significant effect on the associated anion transport properties.

Co-operative binding of concanavalin A to a glycoprotein in lipid bilayers

Lectin-binding curves are reported for a concanavalin A receptor glycoprotein in lipid bilayers and intact cells. The results are consistent with previous studies of the structurally dissimilar transmembrane glycoprotein, glycophorin. High-affinity lectin binding to model membranes was influenced by the presence of apparently unrelated macromolecules, which we suggest is an example of receptor modulation by local interactions. Furthermore, high-affinity binding to the model membranes displayed characteristics, including positive cooperativity, similar to those seen with intact cells.

Heterogeneity of human erythrocyte band 3 analyzed by two-dimensional gel electrophoresis

Human erythrocyte membrane and purified band 3 were separated initially by isoelectric focusing and then examined in a second dimension by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. Band 3 was segregated into three major bands whether the protein was contained within the membranes or was present in the isolated state. The isoelectric points of these major bands were 5.25, 5.35 and 5.70. Of chymotryptic fragments of band 3, the 60-kDa fragment was also separated into three major bands whose pI values were 4.75, 5.10 and 5.30. The multiplicity of band 3 appears to be due to different charges carried by the peptide(s) and is not ascribed to oxidation of band 3 during its preparation. Isoelectric points of the purified 60-kDa fragment were different from the pI values of the fragment coexisting with the complementary 35-kDa fragment, in which case the pI values were exactly the same as those of intact band 3. This suggests that these fragments interact tightly in situ even after being cleaved by chymotrypsin, and the tight interaction must still be present during electrophoresis in the first dimension.

Phosphate transport in human red blood cells: concentration dependence and pH dependence of the unidirectional phosphate flux at equilibrium conditions

The concentration dependence and the pH dependence of the phosphate transport across the red cell membrane were investigated. The unidirectional phosphate fluxes were determined by measuring the 32P-phosphate self-exchange in amphotericin B (5 mumol/liter) treated erythrocytes at 25 degrees C. The flux/concentration curves display an S-shaped increase at low phosphate concentrations, a concentration optimum in the range of 150 to 200 mM phosphate and a self-inhibition at high phosphate concentrations. The apparent half-saturation concentrations, P(0.5), range from 50 to 70 mM and are little affected by pH. The self-inhibition constants, as far as they can be estimated, range from 400 to 600 mM. The observed maximal phosphate fluxes exhibit a strong pH dependence. At pH 7.2, the actual maximal flux is 2.1 X 10(-6) moles . min-1 . g cells-1. The ascending branches of the flux/concentration curves were fitted to the Hill equation. The apparent Hill coefficients were always in the range of 1.5-2.0. The descending branches of the flux/concentration curves appear to follow the same pattern of concentration response. The flux/pH curves were bell-shaped and symmetric with regard to their pH dependence. The pH optimum is at approximately pH 6.5-6.7. The apparent pK of the activator site is in the range of 7.0 to 7.2, while the apparent pK for the inactivating site is in the range of 6.2 to 6.5. The pK-values were not appreciably affected by the phosphate concentration. According to our studies, the transport system possesses two transport sites and probably two modifier sites as indicated by the apparent Hill coefficients. In addition, the transport system has two proton binding sites, one with a higher pK that activates and one with a lower pK that inactivates the transport system. Since our experiments were executed under self-exchange conditions, they do not provide any information concerning the location of these sites at the membrane surfaces.

Mediated transport of anions in band 3-phospholipid vesicles

Band 3 protein, extracted from human erythrocyte membranes by Triton X-100, was recombined with egg lecithin/cholesterol mixtures to form small unilamellar vesicles at a yield of 15-20%. These systems exhibited sulfate fluxes which were inhibitable by stilbene disulfonates and other inhibitors. Maximal inhibition could only be obtained when inhibitors were present at both membrane surfaces. Inhibitor constants I50 were higher than in the native membrane. Quantitatively, transport function was retained at least 60%, as related to the amount of protein involved. Sulfate transport in the recombinates resembled transport in the native membrane with respect to temperature dependence (Ea = 29-32 kcal/mol), pH dependence between pH 6.5 and 7.8, and the relationship between new and exchanges fluxes. In contrast to the native cell, concentration dependence was linear up to 80 mM sulfate, which may be indicative of a lowered affinity for the substrate. Lactate transport in these systems, although substantial, was insensitive to stilbene disulfonates as well as to mercurials, indicating that band 3 is not involved in the specific monocarboxylate transfer in the erythrocyte. Anion transport in band 3-lipid recombinates was insensitive to cholesterol between 0 and 27 mol%. Treatment with proteases, while not affecting transport per se, abolished sensitivity to stilbene disulfonate inhibitors. These observations indicate a number of disturbances of band 3 after recombination, in spite of a preservation of the major transport properties.

Detection of the associated state of membrane proteins by polyacrylamide gradient gel electrophoresis with non-denaturing detergents. Application to band 3 protein from erythrocyte membranes

Polyacrylamide gradient gel electrophoresis was carried out in micellar solutions of various detergents which differ in degree of potency to denature proteins. From the application of this method to band 3 protein from erythrocyte membranes, it was suggested that the procedure was useful in studying the molecular state of membrane proteins. The electrophoretic behaviors of human and bovine band 3 protein did not show any species specificity in either a denature state and a state resembling the native state. As well as in nonionic detergent solutions, the dimeric and tetrameric structures of bovine band 3 protein were preserved in sodium deoxycholate solution, in which protein complexes maintained in nonionic detergent solutions are frequently dissociated. Even in dodecyltrimethylammonium bromide solution, which is a denaturant for water-soluble proteins, part of the band 3 protein was still present as the oligomer. The results suggest that the oligomeric form of band 3 protein is the stable structure and that the dimer and tetramer possibly coexist in membranes.

A relationship between anion transport and a structural transition of the human erythrocyte membrane

Scanning microcalorimetry was employed as an aid in examining some structural features of the anion transport system in red blood cell vesicles. Two structural transitions were previously shown to be sensitive to several covalent and non-covalent inhibitors of anion transport in red cells. In this study, these transitions were selectively removed, either thermally or enzymatically, and the subsequent effect on 35SO2- 4 efflux in red cell vesicles was determined. It is shown that removal of one of these transitions (B2) has a negligible inhibitory effect on anion transport. Cytoplasmic, intermolecular disulfide linkages between band 3 dimers are known to form during the B2 transition. The integrity of the 4,4-diisothiocyanostilbene-2,2-disulfonate-sensitive C transition, on the other hand, is shown to be a requirement for anion transport. The localized region of the membrane giving rise to this transition contains the transmembrane segment of band 3, as well as membrane phospholipids. The calorimetric results suggest a structure of band 3 which involves independent structural domains, and are consistent with the transmembrane segment playing a direct role in the transport process.

The transbilayer movement of phosphatidylcholine in vesicles reconstituted with intrinsic proteins from the human erythrocyte membrane

Vesicles have been prepared from 18 : 1c/18 : 1c-phosphatidylcholine with or without purified glycophorin or partially purified band 3 (obtained by organomercurial gel chromatography). The vesicles have been characterized by freeze-fracture electron microscopy, binding studies to DEAE-cellulose, 31P-NMR and K+ trap measurements. Pools of phosphatidylcholine available for exchange have been investigated using phosphatidylcholine exchange protein from bovine liver. The protein-containing vesicles both exhibit exchangeable pools larger than the fraction of phosphatidylcholine in the outer monolayer, whereas in the protein-free vesicles the exchangeable pool is consistent with the outer monolayer. The results indicate that both glycophorin and the partially purified band 3 preparation enhance the transbilayer movement of phosphatidylcholine.