The human red cell glucose transporter in octyl glucoside. High specific activity of monomers in the presence of membrane lipids

Interactions of drugs and an oligonucleotide with charged membranes analyzed by immobilized liposome chromatography

We studied the effect of charged lipids or detergent on the retention of drugs and an oligonucleotide by immobilized liposome chromatography to characterize solute-membrane interactions. This is a novel approach in analysis of oligonucleotide-liposome interactions. The charged lipids (phosphatidylserine or distearoyltrimethylammoniumpropane) or detergent (sodium dodecylsulfate) interacted electrostatically in a concentration-dependent matter with the solutes. The oligonucleotide ions presumably bound to the liposomes by multipoint interactions, which was saturable. Sodium dodecylsulfate seemed to affect the drug-membrane interactions more strongly than phosphatidylserine did, probably due to different positioning in the bilayer.

Effects of ions and detergents in drug partition chromatography on liposomes

We have determined drug partitioning into phospholipid bilayers by immobilized-liposome chromatography (ILC). Electrostatic effects on the drug partitioning were observed on neutral bilayers at low ionic strength. The size of the counterions affected the partitioning. When liposomes were supplemented with ionic detergents the partitioning of charged drugs was strongly affected, allowing complete separation of drugs of different charges which showed similar retention on neutral bilayers. Partial separation was obtained on bilayers containing fatty acid. Detergent ions or fatty acid inserted into phospholipid bilayers affected the partitioning of drugs much more than did free ions or phospholipid head group charges.

Centrifugal and chromatographic analyses of tryptophan and tyrosine uptake by red blood cells and GLUT1 proteoliposomes with permeability estimates and observations on dihydrocytochalasin B

We analyzed transport into liposomes and proteoliposomes, separated the free and internalized radioactively labeled substrates by size-exclusion chromatography (SEC) and observed a net influx owing to nonfacilitated diffusion across the lipid bilayers during the separation. The permeabilities (10(-9) cm/s) of glucose transporter (GLUT1) proteoliposomes were estimated to be 4.6, 1.0, 1.4 and 2.1 for D-glucose, L-glucose, L-Tyr and L-Trp, respectively; 15, 3.3, 5.1 and 2.1 times higher than the corresponding permeabilities of liposomes. These values indicated that GLUT1 did not transport Tyr or Trp, or transported Tyr, and only Tyr, slowly. This interpretation was supported by further analyses. Dihydrocytochalasin B inhibited the transport of Tyr and, partially, Trp into human red blood cells (centrifugal analyses). It did not inhibit Tyr and Trp influx into GLUT1 proteoliposomes, but partitioned strongly into the bilayers and seemed to make them fragile. The GLUT1 inhibitor cytochalasin B and the GLUT1 substrate 2-deoxy-D-glucose did not inhibit Tyr transport into the cells. Upon immobilized biomembrane affinity chromatography, Trp decreased the cytochalasin B retardation by GLUT1 only at levels far above the physiological Trp concentration. Ethanol (commonly added to aqueous solutions for enhancing a compound's solubility) halved the retardation at 4% (v/v) concentration. Drastic modification of the SEC method is required to allow permeability measurements with nonlabeled and highly permeable substrates.

Immobilized-biomembrane affinity chromatography for binding studies of membrane proteins

Analyses of specific interactions between solutes and a membrane protein can serve to characterize the protein. Frontal affinity chromatography of an interactant on a column containing the membrane protein immobilized in a lipid environment is a simple and robust approach for series of experiments with particular protein molecules. Regression analysis of the retention volumes at a series of interactant concentrations shows the affinity of the protein for the interactant and the amount of active binding sites. The higher the affinity, the fewer sites are required to give sufficient retention. Competition experiments provide the affinities of even weakly binding solutes and the non-specific retention of the primary interactant. Hummel and Dreyer size-exclusion chromatography allows complementary analyses of non-immobilized membrane materials. Analyses of the human facilitative glucose transporter GLUT1 by use of the inhibitor cytochalasin B (radioactively labeled) and the competitive substrate D-glucose (non-labeled) showed that GLUT1 interconverted between two states, exhibiting one or two cytochalasin B-binding sites per two GLUTI monomers, dependent on the membrane composition and environment. Similar analyses of a nucleoside transporter, a photosynthetic reaction center, nicotinic acetylcholine receptors and a P-glycoprotein, alternative techniques, and immobilized-liposome chromatographic approaches are presented briefly.

Effects of cholesterol and model transmembrane proteins on drug partitioning into lipid bilayers as analysed by immobilized-liposome chromatography

We have analysed how cholesterol and transmembrane proteins in phospholipid bilayers modulate drug partitioning into the bilayers. For this purpose we determined the chromatographic retention of drugs on liposomes or proteoliposomes entrapped in gel beads. The drug retention per phospholipid amount (the capacity factor Ks) reflects the drug partitioning. Cholesterol in the bilayers decreased the Ks value and hence the partitioning into the membrane in proportion to the cholesterol fraction. On average this cholesterol effect decreased with increasing temperature. Model transmembrane proteins, the glucose transporter GLUT1 and bacteriorhodopsin, interacted electrostatically with charged drugs to increase or decrease the drug partitioning into the bilayers. Bacteriorhodopsin proteoliposomes containing cholesterol combined the effects of the protein and the cholesterol and approached the partitioning properties of red blood cell membranes. For positively charged drugs the correlation between calculated intestinal permeability and log Ks was fair for both liposomes and bacteriorhodopsin-cholesterol proteoliposomes. Detailed modeling of solute partitioning into biological membranes may require an extensive knowledge of their structures.

Oligomerization of water and solute channels of the major intrinsic protein (MIP) family

Water and small solute fluxes through cell membranes are ensured in many tissues by selective pores that belong to the major intrinsic protein family (MIP). This family includes the water channels or aquaporins (AQP) and the neutral solute facilitators such as the glycerol facilitator (GlpF). We have compared the characteristics of representatives of each subfamily. Following solubilization in the nondenaturing detergents n-octyl-glucoside (OG) and Triton X-100 (T-X100), AQPs remain in their native homotetrameric state, while GlpF always behaves as a monomer. Solute facilitators are fully solubilized by the detergent N-lauroyl sarcosine (NLS), while AQPs are not. Analyses of mutants and chimeras demonstrate a close correlation between the water transport function and the resistance to NLS solubilization. Thus, AQPs and solute facilitators exhibit different behaviors in mild detergents; this could reflect differences in quaternary organization within the membranes. We propose that the oligomerization state or the strength of self-association is part of the mechanisms used by MIP proteins to ensure solute selectivity.

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

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

Conversion between two cytochalasin B-binding states of the human GLUT1 glucose transporter

Two cytochalasin B-binding states of the human red blood cell facilitative glucose transporter GLUT1 were studied, one exhibiting one cytochalasin B-binding site on every second GLUT1 monomer (state 1) and the other showing one site per monomer (state 2). Quantitative affinity chromatography of cytochalasin B was performed on (a) biotinylated red blood cells, (b) cytoskeleton-depleted red blood cell membrane vesicles, and (c) GLUT1 proteoliposomes. The cells were adsorbed on streptavidin-derivatized gel beads, and the vesicles and proteoliposomes entrapped in dextran-grafted agarose gel beads. Cytochalasin B binding to free vesicles and proteoliposomes was analyzed by Hummel and Dreyer size-exclusion chromatography and ultracentrifugation. Analysis of the biotinylated cells indicated an equilibrium between the two GLUT1 states. GLUT1 in free membrane vesicles attained state 2, but was converted into state 1 on entrapment of the vesicles. Purification of GLUT1 in the presence of non-ionic detergent followed by reconstitution produced GLUT1 in state 1. This state was maintained after entrapment of the proteoliposomes. Finally, GLUT1 showed slightly higher affinity for cytochalasin B in state 1 than in state 2. In summary, the cytochalasin B-binding state of GLUT1 seemed to be affected by (a) biotinylation of the cell surface, (b) removal of the cytoskeleton at high pH and low ionic strength, (c) interaction between the dextran-grafted agarose gel matrix and the membrane vesicles, and (d) reconstitution to form proteoliposomes.

Freeze-thaw immobilization of liposomes in chromatographic gel beads: evaluation by confocal microscopy and effects of freezing rate

Biological membranes immobilized in chromatographic gel beads constitute a multifunctional affinity matrix. Membrane protein-solute interactions and drug partitioning into the lipid bilayers can conveniently be studied. By the use of confocal laser-scanning microscopy (CLSM) the distribution of immobilized model membranes in the beads has been visualized for the first time. Freeze-thaw-immobilized liposomes in Superdex 200 gel beads were situated in a thick shell surrounding a liposome-free core. The amount of phospholipids immobilized by freeze-thawing was dependent on the temperature in the cooling bath and the type of test tube used. A bath temperature of -25 degrees C gave higher immobilization yield than freezing at -75 or -8 degrees C did. Freeze-thawing in the presence of liposomes did not affect the gel bead shape or the refractive index homogeneity of the agarose network of the beads, as shown by confocal microscopy.

Chromatography on cells: analyses of solute interactions with the glucose transporter Glut1 in human red cells adsorbed on lectin-gel beads

The affinities of the human red cell glucose transporter Glut1 for D-glucose and cytochalasin B (CB) and the stoichiometry of CB binding vary with the Glut1 environment. In order to study the native state of Glut1 we adsorbed human red cells to wheat germ lectin agarose gel beads for frontal affinity chromatographic analyses. Glut1 showed relatively high affinities for D-glucose (Kd 12+/-1 mM) and CB (Kd 59+/-17 nM). The number of CB-binding sites per Glut1 monomer, 0.46+/-0.16, was approximately doubled upon coating the cells with polylysine, which induced cell association.

Biomembrane-affinity centrifugal analyses of solute interactions with membrane proteins

We have developed a rapid centrifugal method for analyzing solute interactions with membrane proteins in cytoskeleton-depleted membrane vesicles or proteoliposomes sterically immobilized in Superdex 200 gel beads. The size and density of the gel beads allow fast sedimentation in a bench-top centrifuge. Biospecific interactions of cytochalasin B and D-glucose with the human red cell glucose transporter, Glut1, were analyzed. The binding constants and the molar ratio of inhibitor sites per protein monomer agreed well with recent results obtained by frontal affinity chromatography.

Chromatographic approaches to liposomes, proteoliposomes and biomembrane vesicles

Size-exclusion chromatography has been used for fractionation of liposomes, proteoliposomes and biomembrane vesicles of up to approximately 500 nm in size and for separation of these entities from smaller components. Liposome sizes, encapsulation stability, and solute affinities for membrane proteins have been determined. Counter-current distribution in aqueous two-phase systems has widened the range of applications to larger structures. Immobilized biomembrane vesicles and (proteo)liposomes provide stationary phases for chromatographic analysis of specific or nonspecific membrane-solute interactions.

Biomembrane affinity chromatographic analysis of inhibitor binding to the human red cell nucleoside transporter in immobilized cells, vesicles and proteoliposomes

The affinity of the human red cell nucleoside transporter for the transport inhibitor nitrobenzylthioinosine decreases upon protein purification. The affinity was highest for the whole cells (Kd, 0.04 nM), lowered upon cytoskeleton depletion (Kd, 0.2 nM) and lowest after partial purification and reconstitution (Kd, 0.3 nM), as determined by frontal affinity chromatography.

Glucose affinity for the glucose transporter Glut1 in native or reconstituted lipid bilayers. Temperature-dependence study by biomembrane affinity chromatography

The affinity of D-glucose and the transport inhibitor cytochalasin B (CB) for the glucose transporter Glut1 was studied at 5-42 degrees C by quantitative frontal affinity chromatography on sterically immobilized human red cell membrane vesicles, and on proteoliposomes containing reconstituted red cell membrane proteins. Glut1 in the vesicles showed the highest glucose affinity; the dissociation constant Kd(glc) was nearly constant (16 +/- 3 mM) from 15 degrees C to 37 degrees C. For Glut1 in proteoliposomes Kd(glc) decreased from 56 mM at 5 degrees C to 26 mM at 42 degrees C. The CB-Glut1 affinity was strongest around 20 degrees C and was mostly higher with the vesicles, Kd (CB) being 49 nM at 19 degrees C. The entropy and entropy and enthalpy changes for the interactions were calculated.

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

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

Immobilization of human red cells in gel particles for chromatographic activity studies of the glucose transporter Glut1

Chromatography on a novel stationary phase, human red cells immobilized in a gel bed, was introduced for analysis of activities of the glucose transporter Glut1 in the cell membrane. A gel containing positively charged ligands was synthesized from derivatized acrylamide monomers. Red cells were immobilized in gel particles which were packed into a column tube for chromatographic analyses over periods of 10-15 days. D-Glucose was separated from L-glucose on a 1.1-ml bed with a retention volume difference of 0.23 ml, approximately equal to the total inner volume of immobilized intact cells and of ghosts probably formed from lysed cells during the immobilization. The separation was suppressed by the glucose-transport inhibitor cytochalasin B. The interactions between D-glucose, the transport inhibitor forskolin and Glut1 were analyzed by quantitative frontal affinity chromatography. The dissociation constants at room temperature were 6.8 mM for D-glucose binding and 1.8 microM for glucose-displaceable binding of forskolin, in good agreement with published values. The results suggest that chromatography on immobilized cells is a potentially useful tool for studies on cellular membrane functions.

Immobilized membrane vesicle or proteoliposome affinity chromatography. Frontal analysis of interactions of cytochalasin B and D-glucose with the human red cell glucose transporter

Human red cell membrane vesicles stripped of peripheral proteins and proteoliposomes with reconstituted red cell glucose transporter (Glut1) were sterically immobilized in gel beads by freezethawing. The specific interactions between the transport inhibitor cytochalasin B (CB), D-glucose, and Glut1 were analyzed by quantitative frontal affinity chromatography. The dissociation constants, Kd(CB), for the interaction between CB and Glut1 in vesicles and proteoliposomes were similar, the average value being 92 +/- 5 nM at an ionic strength I of 0.05. Kd(CB) for Glut1 in vesicles decreased with increasing ionic strength to become 46 nM at I = 0.5. The affinity of glucose was significantly higher for Glut1 in vesicles (Kd = 24 +/- 2 mM) than for reconstituted Glut1 (Kd = 37 +/- 2 mM). The frontal analysis allowed determination of the amount of CB binding sites, which was found to be 0.33 +/- 0.06 mol per mole of Glut1 monomer (Mr = 54 000). The CB binding capacity of Glut1 in the vesicles and the proteoliposomes was stable in the presence of dithioerythritol over periods of several weeks at room temperature.

Monomeric human red cell glucose transporter (Glut1) in non-ionic detergent solution and a semi-elliptical torus model for detergent binding to membrane proteins

The self-association state of the human red cell glucose transporter (Glut1) in octaethylene glycol n-dodecyl ether (C12E8) and n-octyl beta-D-glucopyranoside (OG) solution was analyzed in the presence of reductant by gel filtration with light-scattering, refractivity and absorbance detection, and by ultracentrifugation. The C12E8-Glut1 complex was essentially monomeric, whereas OG-Glut1 also formed dimers and larger oligomers. C12E8-Glut1 retained substantial glucose transport activity even after depletion of endogenous lipids by gel filtration, as shown by reconstitution and transport measurements. Removal of endogenous lipids from OG-Glut1 abolished the activity unless phosphatidylcholine was included in the eluent. The binding of C12E8 and OG to Glut1 was determined by gel filtration with refractivity and absorbance detection or with radioactive tracer to be 1.86 +/- 0.07 and 1.84 +/- 0.09 g/g polypeptide, respectively. A structural model was proposed in which non-ionic detergent forms a semi-elliptical torus (SET) surrounding the transmembrane protein. The torus thickness was assumed to be equal to the radius (short half-axis) of a spherical (oblate ellipsoidal) free detergent micelle and the polar head groups of the detergent molecules were predicted to be situated just outside the hydrophobic surface of the protein. The experimental detergent binding values and those obtained from the SET model together confirmed that Glut1 was monomeric in C12E8 solution and provided constraints on the shape and size of the hydrophobic transmembrane region of Glut1 in alpha-helical and beta-barrel topology models.

Isoelectric focusing in immobilized pH gradients of the glucose transporter from human red cell membranes

Isoelectric focusing of the human red cell glucose transporter (a transmembrane protein) was performed in immobilized pH gradients. Isoelectric focusing of integral membrane proteins presents problems that are related to the amphiphilic nature of these proteins. Solubilizing additives must be used to counteract hydrophobic effects. In our case, urea and the nonionic detergent, Triton X-100, were used. Focusing was done at 15 degrees C. The isoelectric point (pI) of the glucose transporter (freshly purified by anion-exchange chromatography in the presence of octyl glucoside) was determined at 8.4 +/- 0.05 (n = 9), in good agreement with our earlier determinations by two-dimensional electrophoresis with isoelectric focusing in the presence of carrier ampholytes in the first dimension. The width of the focused zone was approximately 0.1 pH unit, more narrow than after focusing with carrier ampholytes. In an immobilized pH gradient from pH 7 to 10, the transporter region at pH 8.4 comprised one major and one or two minor zones. The pH interval 4-10 was also used and showed a single transporter zone. The glucose transporter tends to self-associate in detergent solution. Octyl glucoside-purified glucose transporter formed oligomers during incubation at 37 degrees C. Upon focusing, these oligomers appeared in a wide pH interval far below pH 8.4.

Effects of pH on the activity of the human red cell glucose transporter Glut 1: transport retention chromatography of D-glucose and L-glucose on immobilized Glut 1 liposomes

The facilitative glucose transporter Glut 1 from human red cells was reconstituted into liposomes that were size-fractionated and immobilized in an octyl sulfide-Sephacryl S-1000 column. D-[14C]Glucose was eluted later than L-[3H]glucose from the Glut 1 liposome column (by delta V microliters), apparently because the D-glucose was transported through the liposomes. The corresponding difference with protein-free liposomes was delta V0. The Glut 1 transport retention chromatographic effect, delta VG = delta V - delta V0, 40-50 microliters at pH 7, was nearly constant at pH 6-10 (400 mM NaCl, 23 degrees C, internal liposome volume approximately 240 microliters) but decreased steeply below pH 5 to become zero at pH 3.6. The decrease corresponded to a pKa of approximately 4.4 and was partly reversible above pH 4.7. Similarly, glucose exchange by non-immobilized freeze-thawed proteoliposomes with Glut 1 slowed down drastically as the pH was lowered from pH 5.5 to 4; and octyl glucoside-solubilized Glut 1 lost half its activity in 15 min at pH 4.5 (low ionic strength, 2 degrees C) as shown by glucose exchange determinations at pH 7.2 The results suggest that Glut 1 is inactivated at low pH upon protonation of carboxylate groups of pKa approximately 4.4-4.8. It seems likely that carboxylate groups form hydrogen bonds to transported D-glucose.

Active and monomeric human red cell glucose transporter after high performance molecular-sieve chromatography in the presence of octyl glucoside and phosphatidylserine or phosphatidylcholine

The human red cell glucose transporter (Glut 1) was purified by ion-exchange chromatography in the presence of octyl glucoside. The state of association of the protein was studied, and the transport activity was determined after exchange of copurified membrane lipids for phosphatidylserine (PS) or phosphatidylcholine (PC). The purpose was to analyze the Glut 1 preparation for homogeneity and activity prior to attempts at crystallization. Analyses by high performance molecular-sieve chromatography showed that the Glut 1 was monomeric immediately after the ion-exchange purification: the Mr of the Glut 1 polypeptide was estimated to be 49,000 +/- 6000 by TSKgel G3000SW chromatography monitored by low-angle laser light-scattering photometry, differential refractometry and UV photometry. This required determination of the absorption coefficient of the Glut 1, which was measured to be 1.13 +/- 0.03 ml mg-1 cm-1 at 280 nm, referring to the polypeptide concentration. The Mr value is consistent with the cDNA-deduced Mr 54,117 of the very similar HepG2 glucose transporter polypeptide. At 2 degrees C, pH 7 and an ionic strength of 0.06 M, the Glut 1 associated gradually during three days to form oligomers. These formed much more rapidly at room temperature or at high ionic strength. Freshly prepared Glut 1 retained high activity after separation from membrane lipids on a TSKgel G3000SW column in the presence of 40 mM octyl glucoside and 1 mM PS or PC. In contrast, most of the activity was lost when the membrane lipids were separated from the protein in the absence of eluent lipids. The presence of a phospholipid was thus essential for retention of high activity of the Glut 1 in octyl glucoside and PC was nearly as effective as PS.

The isoelectric point of the human red cell glucose transporter

The isoelectric point (pI) of the human red cell glucose transporter (Glut 1) was determined. Inconsistent values of 6.0, 6.4-6.5 and 8 have been reported earlier. Integral membrane proteins from human red cells were analyzed by two-dimensional electrophoresis with isoelectric focusing and sodium dodecyl sulfate gel electrophoresis (2D-PAGE). A zone of monomeric Glut 1 was found at pH 8.7, but most of the Glut 1 focused at pH 6-7 together with the anion transporter and other components. Purified Glut 1 focused only at pH 8.5 +/- 0.2 (S.D., n = 12) and deglycosylated purified Glut 1 only at pH 8.4 +/- 0.1 (n = 5), as shown by 2D-PAGE. The absence of Glut 1 below pH 8 in the latter cases was confirmed by immunoblotting with a monoclonal antibody. Furthermore, Glut 1 was photoaffinity-labelled with [3H]cytochalasin B and subjected to isoelectric focusing in one dimension. The pI of the labelled Glut 1 was 8.6 +/- 0.3 (n = 11). A pI of 9.1 was calculated for the Glut 1 polypeptide on the basis of amino acid composition and pKa values for amino acid side groups. The sialic acid content of the glycosylated transporter from fresh red cells was determined at approximately 2.1 sialic acid residues per transporter, which corresponds to a calculated pI of 8.8. The pI values of other human glucose transporter polypeptides of the facilitative diffusion type (Glut 2, 3, 4 and 5) were calculated at 8.4, 7.4, 7.1 and 6.2, respectively.

Liposome chromatography: liposomes immobilized in gel beads as a stationary phase for aqueous column chromatography

Liposomes have been used as a stationary phase for column chromatography with an aqueous mobile phase. They were immobilized in the pores of carrier gel beads by two methods: (A) hydrophobic ligands were coupled to the matrix of gel beads, which then were packed into a column and liposomes were applied and became associated with the ligands by hydrophobic interaction; and (B) phospholipids and detergent were dialysed in the presence of gel beads; many of the liposomes that formed in the pores of the beads were sterically immobilized by the gel matrix. Proteoliposomes containing red cell glucose transport protein in the lipid bilayers were immobilized in a column by method A. This column retained D-glucose longer than L-glucose. In contrast to L-glucose, D-glucose was transported into and out of the immobilized liposomes, causing an increased retention. Liposomes with (stearylamine)+ or (phosphatidylserine)- in their lipid bilayers were immobilized by method B and the gel beads were packed into a column. A protein of opposite charge was applied in excess. Under suitable conditions, the protein molecules became close-packed on the liposome surfaces. Ion-exchange chromatographic experiments with proteins showed that these sterically immobilized liposomes were also stable enough to be used as a stationary phase. The loss of lipids was 5-23% in the first run at high protein load and with sodium chloride gradient elution but was lower in subsequent runs. It is proposed that water-soluble molecules can be separated and their interactions with liposome surfaces studied by chromatography on immobilized liposomes in detergent-free aqueous solution. Membrane proteins can be inserted and ligands can be anchored in the lipid bilayers for chromatographic purposes.

Substantial glucose leakage from liposomes on filters and upon molecular-sieve chromatography in determinations of reconstituted glucose-transport activity and liposome volumes

Transport-protein activities are often determined by procedures that involve isolation of liposomes containing the transported radioactive solute. We determined the activity of the human red cell glucose transporter in liposomes and, by similar procedures, internal volumes of liposomes. For these purposes, we isolated freeze-thawed liposomes loaded with [14C]glucose, either by filtration on cellulose-nitrate and cellulose-acetate filters, or by chromatography on Sephadex. The interaction of liposomes with filters caused substantial leakage of [14C]glucose. About half of the internal [14C]glucose was released on the filters from glucose-transporter liposomes with inhibited transport. Chromatography at high flow rate provided higher and more accurate values than did the filtration procedure. Leakage corrections could be made by use of flow-cell scintillation elution profiles. The ratios between the corrected chromatographic volume values and the filtration values were 1.4-3.0 for liposomes without protein, 2.4-4.0 for glucose-transporter liposomes and 3.6-7.9 for liposomes with several human red cell integral membrane proteins. The D-glucose equilibrium exchange with glucose-transporter liposomes at 50 mM D-glucose was 2.0 nmol D-glucose per microgram transporter per second as determined by use of chromatography at high flow rate. The filtration procedure gave only 0.6 nmol.microgram-1.s-1 due to the [14C]glucose leakage. In our experiments, the chromatographic procedure thus proved superior.

Water-soluble proteins do not bind octyl glucoside as judged by molecular sieve chromatographic techniques

It is well known that the non-ionic detergent octyl glucoside (1-O-n-octyl-beta-D-glucopyranoside) solubilizes biological membrane components. It forms complexes with membrane-spanning proteins by hydrophobic interactions and it forms mixed micelles with membrane lipids. In contrast, non-ionic detergents usually do not bind to water-soluble proteins. According to a recent report, substantial and cooperative binding of octyl glucoside to several water-soluble proteins does occur near the critical micelle concentration. However, data have been obtained that contradict this report. No decrease was found in the elution volumes of five water-soluble proteins on molecular sieve chromatography on two Superose columns in tandem when 35 mM octyl glucoside was included in the eluent. No binding of the detergent to these proteins was observed at 20 or 22.5 mM octyl glucoside on molecular sieve chromatography on a TSK SW guard column as determined by differential refractometry and UV spectrophotometry of the proteins in the absence or presence of octyl glucoside. The experiments were done with the same buffer system and with six of the proteins used in the reported study. It is concluded that, as expected, there is no binding of octyl glucoside to water-soluble proteins above the detection limit (0.1 g detergent/g protein) of the refractometric method. The binding of, on average, 1.3 +/- 0.2 g of detergent per gram of water-soluble protein that was observed at 20 mM octyl glucoside in the reported study is not consistent with the present results.

Binding of sodium dodecyl sulphate to an integral membrane protein and to a water-soluble enzyme. Determination by molecular-sieve chromatography with flow scintillation detection

We have determined the binding of sodium dodecyl sulphate (SDS) to the human red cell glucose transporter (polypeptide, Mr 54,117) and to a water-soluble enzyme, N-5'-phosphoribosylanthranilate isomerase-indole-3-glycerol-phosphate synthase (PRAI-IGPS) from Escherichia coli (Mr 49,484). [35S]SDS was equilibrated with each protein on molecular-sieve chromatography at a series of SDS concentrations. The binding ratios of SDS to protein were determined by flow scintillation detection and automated amino acid analyses. Unexpectedly the glucose transporter, which is a transmembrane protein, bound about the same amount of SDS per gram of protein as did the enzyme. At 1.6 mM SDS, slightly below the critical micelle concentration (CMC) (1.8 mM) in the eluent, the binding ratio was 1.6 g SDS/g protein for both the glucose transporter and PRAI-IGPS. At 2.0 mM SDS (above the CMC) the glucose transporter showed a binding ratio of 1.7 g SDS/g protein. The corresponding value for the enzyme was about 1.5 g/g. The SDS-glucose transporter complex seems to be more compact than the SDS-enzyme complex as judged by molecular-sieve chromatography and by SDS-polyacrylamide gel electrophoresis. Recent neutron scattering results have shown a protein-decorated triple-micelle structure for the SDS-PRAI-IGPS complex. Hypothetically, the more compact SDS-glucose transporter complex may therefore consist of a dual-micelle structure. The molecular-sieve gel beads bound considerable amounts of SDS. The SDS binding to the gel matrix and to the proteins increased with increasing SDS concentration up to at least 1.6-2.0 mM SDS. In the case of the water-soluble enzyme a shoulder was observed in the binding curve at 1 mM SDS, probably reflecting a change in the conformation of the complex.

Sodium dodecyl sulphate-protein complexes. Changes in size or shape below the critical micelle concentration, as monitored by high-performance agarose gel chromatography

We have determined the sodium dodecyl sulphate (SDS) concentration needed to complete the formation of SDS-protein complexes. A Superose-6 column was equilibrated with SDS for 7 h. A sample of a native protein or an SDS-protein complex was applied, and the elution volume, Ve, was determined. Then the SDS concentration, CSDS, was changed, etc., i.e., Ve was determined as a function of CSDS. The critical micelle concentration of SDS (cmcSDS) was 1.8 mM in the eluent (ionic strength 0.10 M). Native bovine carbonic anhydrase (BCA) formed an SDS complex above 0.2 mM SDS. As CSDS was increased, Ve decreased gradually in two main transitions, (TI) at 0.2-1.0 mM and (TII) at 1.2-2.0 mM SDS. These concentrations are corrected for a lag in the column equilibration with SDS. SDS-BCA, pre-equilibrated at 1.6 mM SDS, showed transitions similar to those observed with native BCA, except that transition TII included a minor transition at 2.0-2.2 mM SDS. The SDS complexes of reduced and carboxamidomethylated bovine serum albumin, of N-5'-phosphoribosylanthranilate isomerase-indole-3-glycerol-phosphate synthase from Escherichia coli (PRAI-IGPS) and of two tryptic fragments of this enzyme behaved similarly. For SDS-PRAI-IGPS the major part of transition TII was completed at 1.6-1.7 mM SDS, as shown by analyses after 20-h column equilibrations with increasing as well as decreasing CSDS. The SDS complex of an integral membrane protein, the glucose transporter from human red cells, was smaller or less elongated than the SDS complexes of water-soluble proteins of the same polypeptide length. The formation of all five SDS-protein complexes investigated was practically completed at cmcSDS.

Entrapment of lipid vesicles and membrane protein-lipid vesicles in gel bead pores

Phospholipid vesicles were entrapped in gel beads of Sepharose 6B and Sephacryl S-1000 during vesicle preparation by dialysis. Egg-yolk phospholipids solubilized with cholate or octyl glucoside were dialysed together with gel beads for 2.5 days in a flat dialysis bag. Some vesicles were formed in gel bead pores and vesicles of sufficient size became trapped. Red cell membrane protein-phospholipid vesicles could be immobilized in the same way. Non-trapped vesicles were carefully removed by chromatographic procedures and by centrifugation. The amount of entrapped vesicles increased with the initial lipid concentration and was dependent on the relative sizes of vesicles and gel pores. The largest amount of trapped vesicles, corresponding to 9.5 mumol of phospholipids per ml gel, was achieved when Sepharose 6B gel beads were dialysed with cholate-solubilized lipids at a concentration of 50 mM. In this case the vesicles had an average diameter of 60 nm and an internal volume of 15 microliters/ml gel. The amount of vesicles trapped in Sephacryl S-1000 gel beads upon dialysis under the same conditions was smaller: 2.2 mumol of phospholipids per ml gel. Probably most of the gel pores were too large to trap such vesicles. Larger vesicles, with an average diameter of 230 nm, were entrapped in the Sephacryl S-1000 matrix in an amount corresponding to 3.0 mumol phospholipids per ml gel upon dialysis of the gel beads and octyl glucoside-solubilized lipids at a concentration of 20 mM. The internal volume of these vesicles was 22 microliters/ml gel. The yield of immobilized phospholipids was up to 19%. The entrapped vesicles were somewhat unstable: 9% of the phospholipids were released during 9 days of storage at 4 degrees C. By the dialysis entrapment method vesicles can be immobilized in the gel beads without using hydrophobic ligands or covalent coupling.