Reconstitution and partial purification of the sodium and chloride-coupled glycine transporter from rat spinal cord

The (Na+ + Cl-)-coupled glycine transporter has been solubilized from rat spinal cord with 2% cholate and purified 6-7-fold using Wheat Germ Agglutinin-Sepharose 4B. Transport activity - as determined upon reconstitution of the fraction into liposomes - was retained on the column and eluted by N-acetylglucosamine. When the glycoprotein fraction was depleted of the N-acetylglucosamine and applied to a second round of lectin-chromatography, the glycine transport activity was retained and again could be eluted by the sugar. The transporter activity reconstituted from the glycoprotein fraction retains the same features displayed in the synaptic plasma membrane vesicles, namely an absolute dependence on sodium and chloride, electrogenicity and efflux and exchange properties. These observations indicate that the (Na+ + Cl-)-coupled glycine transporter is a glycoprotein.     

Separation and reconstitution of sodium-dependent glucose transport activity from renal brush-border membranes using gel-filtration chromatography

Pig kidney brush-border membrane vesicles were solubilized using a final concentration of 1% Triton X-100, found optimal for quantitative reconstitution of d-glucose transport into liposomes. Using reconstituted proteoliposomes, selective permeability towards d-glucose compared to other sugars tested was shown as well as the main features of d-glucose transport in native membranes, namely sodium dependence and phlorizin inhibition of d-glucose accumulation. After removal of Triton X-100 from the detergent extract, some membrane proteins (about 40%), which are insoluble in the absence of detergent, were isolated. Among these proteins resolubilized by 1% Triton X-100, the component catalyzing the d-glucose transport was located by gel-filtration chromatography separation, using reconstitution of transport as the assay. The active fraction displayed a molecular size of 50 Å; when analyzed on SDS polyacrylamide gel electrophoresis, it contained one major protein subunit with an apparent molecular weight close to 65 000. We conclude that this protein fraction is involved in d-glucose transport by renal brush borders.     

Asymmetric reconstitution of the glucose transporter from Ehrlich ascites cell plasma membrane: role of alkali cations

Gel chromatography of solubilized Ehrlich cell plasma membranes and preformed asolectin vesicles coupled to a freeze-thaw cycle results in the reconstitution of 3-O-methyl-D-glucose transport. The transport activity of the liposomes formed is critically dependent on the cation present during reconstitution. Liposomes formed in K+ show high levels of carrier-mediated 3-O-methyl-D-glucose uptake (495 pmol/min/mg protein) while those formed in Na+ do not (33 pmol/min/mg protein). The inactivity in Na+ is not due to a diminished incorporation of glucose transporter nor is it due to carrier molecules reconstituted with a different orientation from those in K+ liposomes. Instead, the low glucose transport level in Na+ liposomes is related to the small size of vesicles formed with Na+. A second freeze-thaw cycle in K+ causes a two- to threefold increase in the available intravesicular volume of Na+ liposomes and results in an eightfold increase in carrier-mediated 3-O-methyl-D-glucose uptake. K+ liposomes, treated in an identical manner, show only a twofold increase in uptake. The glucose transporter was identified as a protein with a molecular mass range of 44.7 to 66.8 kDa, by the D-glucose-inhibitable photoincorporation of [3H]cytochalasin B. The carrier protein is inserted in reconstituted vesicles in a nonrandom manner with at least 80% of the molecules oriented with the cytoplasmic domain accessible to the external medium. In contrast, the neutral Na+-dependent amino acid transport system appears to be randomly reconstituted.     

Reconstitution into liposomes of the B degrees -like glutamine-neutral amino acid transporter from renal cell plasma membrane

Na+ dependent [3H]glutamine uptake was found in liposomes reconstituted with solubilized rat kidney brush border in the presence of intraliposomal K+. The reconstituted system was optimised with respect to the critical parameters of the cyclic detergent removal procedure, i.e., the detergent used for the solubilization, the protein concentration, the detergent/phospholipid ratio and the number of passages through a single Amberlite column. Time dependent [3H]glutamine accumulation in proteoliposomes occurred only in the presence of external Na+ and internal K+. The transporter showed low if there is any tolerance towards the substitution of Na+ or K+ for other cations. Valinomycin strongly stimulated the transport indicating that it is electrogenic. Intraliposomal glutamine had no effect. From the dependence of the transport rate on the Na+ concentration cooperativity index close to 1 was derived, indicating that 1 Na+ should be involved in the cotransport with glutamine. The electrogenicity of the transport originated from the Na+ transport. Optimal rate of 0.1 mM [3H]glutamine uptake was found in the presence of 50 mM intraliposomal K-gluconate. At higher K-gluconate concentrations the transport rate decreased. The activity of the reconstituted transporter was pH dependent with optimal function in the range pH 6.5-7.0. [3H]glutamine (and [3H]leucine) uptake was inhibited by all the neutral but not by the positively or negatively charged amino acids. The sulfhydryl reagents HgCl2, mersalyl, p-hydroxymercuribenzoate and the substrate analogue 2-aminobicyclo[2,2,1]heptane-2-carboxylate strongly inhibited the transporter, whereas the amino acid analogue alpha-(methylamino)isobutyrate had no effect. The inhibition by mersalyl was protected by the presence of the substrate. On the basis of the Na+ dependence, the electrogenic transport mode and the specificity towards the amino acids, the reconstituted transporter was classified as B degrees-like.     

Functional reconstitution into liposomes and characterization of the carnitine transporter from rat liver microsomes

The carnitine transporter was solubilized from rat liver microsomes with Triton X-100 and reconstituted into liposomes, after addition of Triton X-114, by removing the detergent from mixed micelles by hydrophobic chromatography on Amberlite (Bio-Beads SM 2). The reconstitution was optimized with respect to the detergent/phospholipid ratio, the protein concentration, and the number of passages through a single Amberlite column. The reconstituted carnitine transporter catalyzed a first-order uniport reaction inhibited by HgCl2 and DIDS. The IC50 for HgCl2 was 0.16+/-0.03 mM. The reconstituted transporter also catalyzed carnitine efflux from the proteoliposomes; the efflux was stimulated by externally added long-chain acylcarnitines. Besides carnitine, ornithine, arginine, glutamine and lysine were taken up by the reconstituted liposomes with lower efficiency respect to carnitine. Optimal activity was found at pH 8.0. The Km for carnitine on the external side of the transporter was 10.9+/-0.16 mM. The activation energy of the carnitine transport derived by Arrhenius plot was 16.1 kJ/mol.     

Functional reconstitution of the glycine receptor

The functional reconstitution of the chloride channel coupled glycine receptor is described. Glycine receptors were purified from the cholate extract of rat spinal cord membranes by affinity chromatography and incorporated into phospholipid vesicles by the addition of phosphatidylcholine and removal of detergent by gel filtration. The reconstituted vesicles showed the same polypeptide composition as the purified receptor (proteins of Mr 48,000 and 58,000). The pharmacological characteristics of the glycine receptor were also preserved in the proteoliposomes, as demonstrated by the displacement of [3H]strychnine binding by several glycinergic ligands and by photoaffinity labeling experiments. In order to observe functional responses (i.e., specific agonist-induced anion translocation), we have developed an assay based on the fluorescence quenching of an anion-sensitive entrapped probe, SPQ [6-methoxy-N-(3-sulfopropyl)quinolinium]. Reconstituted vesicles were loaded with the fluorescent probe during a freeze-thaw-sonication cycle in the presence of added liposomes containing cholesterol. In such a reconstituted system, glycine receptor agonists are able to increase the rate of anion influx into the vesicles. The action of agonists is blocked by the simultaneous presence of strychnine or other glycine antagonists. Our results show that the purified 48,000- and 58,000-dalton polypeptides reconstituted into phospholipid vesicles can bind ligands and promote specific ion translocation in a way similar to the glycine receptor in its native environment.     

Solubilization and reconstitution of the catecholamine transporter from bovine chromaffin granules.

The catecholamine transporter from bovine chromaffin granules has been solubilized by using low concentrations of sodium cholate in the presence of phospholipids. The functional solubilized protein has been incorporated into liposomes after removal of the detergent either by gel filtration or by dialysis. Reserpine-sensitive accumulation against a concentration gradient is achieved by artifically imposing a pH gradient across the membrane. In the reconstituted system adenosine 5'-triphosphate (ATP) serves as an energy source only at higher detergent concentrations. The proton-translocating adenosine triphosphatase (ATPase) is solubilized in parallel with the increasing efficiency of ATP as an energy source. Several criteria are proposed to distinguish between carrier-mediated (reserpine sensitive) and unmediated transport in the reconstituted system. The reserpine-sensitive process shows affinity and ss presented in this communication provide further support for the contention that concentrative uptake in biogenic amine storage vesicles is driven by a transmembrane pH gradient, which, in the native system, is generated by a proton-translocating ATPase. Moreover, the assays described provide a tool for the isolation and purification of the transport protein.     

Incorporation of bacterial membrane proteins into liposomes: factors influencing protein reconstitution

Meningococcal and gonococcal outer membrane proteins were reconstituted into liposomes using detergent-mediated dialysis. The detergents octyl glucopyranoside (OGP), sodium cholate and Empigen BB were compared with respect to efficiency of detergent removal and protein incorporation. The rate of OGP removal was greater than for cholate during dialysis. Isopycnic density gradient centrifugation studies showed that liposomes were not formed and hence no protein incorporation occurred during dialysis from an Empigen BB containing reconstitution mixture. Cholate-mediated reconstitution yielded proteoliposomes with only 75% of the protein associated with the vesicles whereas all of the protein was reconstituted into the lipid bilayer during OGP-mediated reconstitution. Essentially complete protein incorporation was achieved with an initial protein-to-lipid ratio of 0.01:1 (w/w) in the reconstitution mixture; however, at higher initial protein-to-lipid ratios (0.02:1) only 75% protein incorporation was achieved. Reconstituted proteoliposomes were observed as large (>300 nm), multilamellar structures using cryo-electron microscopy. Size reduction of these proteoliposomes by extrusion did not result in significant loss of protein or lipid. Extruded proteoliposomes were unilamellar vesicles with mean diameter of about 100 nm.     

Reconstitution into liposomes and functional characterization of the carnitine transporter from renal cell plasma membrane

The carnitine transporter was solubilized from rat renal apical plasma membrane (brush-border membrane) with C12E8 and reconstituted into liposomes by removing the detergent from mixed micelles by hydrophobic chromatography on Amberlite XAD-4. The reconstitution was optimised with respect to the protein concentration, the detergent/phospholipid ratio and the number of passages through a single Amberlite column. The reconstituted carnitine transporter catalysed a first-order antiport reaction (carnitine/carnitine or carnitine/substrate) stimulated by external, not internal, Na+, with a positive cooperativity. Na+ was co-transported with carnitine. Optimal activity was found between pH 5.5 and pH 6.0. The sulfhydryl reagents MTSES, MTSET and mercurials strongly inhibited the transport. Substrate analogues inhibited the transport; the most effective were acylcarnitines and betaine, followed by dimethylglicine, tetraethylammonium and arginine. Besides carnitine, only acylcarnitines and betaine were efficiently translocated. The Km for carnitine on the external and internal side of the transporter was 0.08 and 1.2 mM, respectively. The transporter is asymmetrical and it is unidirectionally inserted into the proteoliposomal membrane with an orientation corresponding to that of the native membrane. The reconstituted carnitine transporter corresponds, very probably, to the OCTN2 protein.     

Purification of the sodium- and chloride-coupled glycine transporter from central nervous system.

We have recently developed a reconstitution assay which allows the rapid determination of sodium- and chloride-dependent glycine transport activity of many fractions (López-Corcuera, B., and Aragón, C. (1989) Eur. J. Biochem. 181, 519-524). In this paper we report the purification of the sodium- and chloride-coupled glycine transporter from pig brain stem. Transporter is solubilized from plasma membrane vesicles with 2% cholate and purified by sequential chromatography on phenyl-Sepharose, wheat germ agglutinin-Sepharose, and hydroxylapatite columns, followed by a 5-20% sucrose density gradient fractionation. Taking into account the inactivation suffered by the transporter, a final increase in specific activity of about 450-fold is achieved. Although two polypeptides with apparent molecular masses of 100 and 37 kDa are progressively enriched during the chromatographic steps, only the 100-kDa band comigrates with transport activity along the density gradient. This band is finally isolated to apparent homogeneity in the active fractions. We conclude that the 100-kDa band represents the glycine transporter. Finally, the pure transporter can be reconstituted into liposomes, retaining the absolute dependence on sodium and chloride gradients, the electrogenicity, the glycine affinity, the substrate specificity, and the sensitivity to group-selective modifiers characteristic of the native transporter.     

Kinetics of the reconstituted dicarboxylate carrier from rat liver mitochondria

The dicarboxylate carrier from rat liver mitochondria was purified by the Amberlite/hydroxyapatite procedure and reconstituted in egg yolk phospholipid vesicles by removing the detergent with Amberlite. The efficiency of reconstitution was optimized with respect to the ratio of detergent/phospholipid, the concentration of phospholipid and the number of Amberlite column passages. In the reconstituted system the incorporated dicarboxylate carrier catalyzed a first-order reaction of malate/phosphate exchange. V of the reconstituted malate/phosphate exchange was determined to be 6000 mumol/min per g protein at 25 degrees C. This value was independent of the type of substrate present at the external or internal space of the liposomes (malate, phosphate or malonate). The half-saturation constant was 0.49 mM for malate, 0.54 mM for malonate and 1.41 mM for phosphate. The activation energy of the exchange reaction was determined to be 95.8 kJ/mol. The transport was independent of the external pH in the range between pH 6 and 8.     

Solubilization and reconstitution of the renal phosphate transporter

Proteins from brush-border membrane vesicles of rabbit kidney cortex were solubilized with 1% octylglucoside (protein to detergent ratio, 1:4 (w/w). The solubilized proteins (80.2 +/- 2.3% of the original brush-border proteins, n = 10, mean +/- S.E.) were reconstituted into artificial lipid vesicles or liposomes prepared from purified egg yolk phosphatidylcholine (80%) and cholesterol (20%). Transport of Pi into the proteoliposomes was measured by rapid filtration in the presence of a Na+ or a K+ gradient (out greater than in). In the presence of a Na+ gradient, the uptake of Pi was significantly faster than in the presence of a K+ gradient. Na+ dependency of Pi uptake was not observed when the liposomes were reconstituted with proteins extracted from brush-border membrane vesicles which had been previously treated with papain, a procedure that destroys Pi transport activity. Measurement of Pi uptake in media containing increasing amounts of sucrose indicated that Pi was transported into an intravesicular (osmotically sensitive) space, although about 70% of the Pi uptake appeared to be the result of adsorption or binding of Pi. However, this binding of Pi was not dependent upon the presence of Na+. Both Na+-dependent transport and the Na+-independent binding of Pi were inhibited by arsenate. The initial Na+-dependent Pi transport rate in control liposomes of 0.354 nmol Pi/mg protein per min was reduced to 0.108 and 0 nmol Pi/mg protein per min in the presence of 1 and 10 mM arsenate, respectively. Future studies on reconstitution of Pi transport systems must analyze and correct for the binding of Pi by the lipids used in the formation of the proteoliposomes.     

Reconstitution of the glucose transporter from rat skeletal muscle

As a step in the purification and characterization of the glucose transporter from rat skeletal muscle, we have reconstituted glucose transport activity in liposomes. Plasma membranes were prepared from skeletal muscle which display D-glucose reversible binding of cytochalasin B (10 pmol sites/mg protein; KD = 0.3 microM). Older rats gave a slightly lower specific activity and much lower yield of sites per g muscle than young rats. Glucose transport activity was reconstituted into liposomes by the freeze-thaw procedure using either plasma membranes directly or cholate-extracted membrane proteins; the latter gave a 50% higher specific activity. The reconstituted transport activity was stereospecific, saturable, and inhibited by cytochalasin B, phloretin, and mercuric chloride. The optimum cholate concentration for extraction and reconstitution of transport activity was about 1.5%, and the highest specific activity of reconstituted transport was seen only at low ratios of protein to lipid in the reconstitution. Chromatography on agarose lentil lectin and agarose ethanethiol doubled both the specific activity of reconstituted transport and the fraction of glucose uptake which was stereospecific. In all of these respects the results were similar to our results with the bovine heart transporter (T. J. Wheeler and M. A. Hauck, Biochim. Biophys. Acta 818, 171-182 (1985)). Our findings suggest that further purification procedures developed for the heart transporter may be applicable to the skeletal muscle transporter as well.     

Functional reconstitution into liposomes and characterization of the carnitine transporter from rat liver microsomes

The carnitine transporter was solubilized from rat liver microsomes with Triton X-100 and reconstituted into liposomes, after addition of Triton X-114, by removing the detergent from mixed micelles by hydrophobic chromatography on Amberlite (Bio-Beads SM 2). The reconstitution was optimized with respect to the detergent/phospholipid ratio, the protein concentration, and the number of passages through a single Amberlite column. The reconstituted carnitine transporter catalyzed a first-order uniport reaction inhibited by HgCl₂ and DIDS. The IC₅₀ for HgCl₂ was 0.16 ± 0.03 mM. The reconstituted transporter also catalyzed carnitine efflux from the proteoliposomes; the efflux was stimulated by externally added long-chain acylcarnitines. Besides carnitine, ornithine, arginine, glutamine and lysine were taken up by the reconstituted liposomes with lower efficiency respect to carnitine. Optimal activity was found at pH 8.0. The Km for carnitine on the external side of the transporter was 10.9 ± 0.16 mM. The activation energy of the carnitine transport derived by Arrhenius plot was 16.1 kJ/mol.     

Reconstitution and purification of the sodium- and chloride-coupled gamma-aminobutyric acid transporter from rat brain

The sodium- and chloride-coupled gamma-aminobutyric acid transporter from rat brain has been highly purified. Synaptic plasma membranes from rat brain were extracted with cholate in the presence of 10% ammonium sulfate. The soluble extract was incorporated into liposomes consisting of asolectin and crude brain lipids. Brain lipids markedly enhanced the transport activity. The resulting proteoliposomes catalyzed sodium- and chloride-coupled gamma-aminobutyric acid transport which, in the presence of internal potassium, was greatly (up to 20-fold) stimulated by valinomycin. Using this transport of the reconstituted system as an assay, the transporter was purified by the following steps. The cholate extract was fractionated by ammonium sulfate. The activity was not precipitated by 50% but could be precipitated by 70% ammonium sulfate. The cholate and ammonium sulfate were removed on a Sephadex G-50 column. Subsequently, the transporter was partially purified on DEAE-cellulose in a mixture of Triton X-100 and octyl glucoside. The active fractions were chromatographed on a hydroxylapatite column in the presence of Triton X-100. Although the increase in specific activity was only up to 100-fold, this was due to partial inactivation. The actual purification was at least 1000-fold. The purified transporter exhibited the same features of the synaptic plasma membrane vesicles, namely dependence on sodium and chloride, electrogenicity, and a similar affinity. The sodium dodecyl sulfate gel pattern indicated that a major protein ran as a 24-kDa band. This band may represent the gamma-aminobutyric acid transporter.     

Radiation inactivation studies on the rabbit kidney sodium-dependent glucose transporter

Rabbit kidney cortical brush-border membrane vesicles were irradiated in the frozen state with increasing doses of high energy electrons from a Van de Graaff generator. Sodium-dependent D-glucose and L-alanine transport showed a simple exponential loss of activity with increasing radiation dosage. Target size calculation based on these data gives estimates of 1.0 X 10(6) daltons for the glucose transporter and 1.2 X 10(6) daltons for the alanine transporter. A highly purified glucose transport protein extracted from rabbit kidney cortex was similarly irradiated both before and after reconstitution into liposomes. The target size of this purified glucose transporter was 343,000 daltons, based on inactivation of transport. The intensity of the major 165,000-dalton sodium dodecyl sulfate-gel electrophoresis band of this preparation was decreased by radiation. The decrease in staining intensity was dose-dependent, yielding a target size of 298,000 daltons, in situ. We propose that the purified glucose transporter reconstituted into liposomes is a tetramer comprised of 85,000-dalton subunits.     

Reconstitution into liposomes and functional characterization of the carnitine transporter from renal cell plasma membrane

The carnitine transporter was solubilized from rat renal apical plasma membrane (brush-border membrane) with C₁₂E₈ and reconstituted into liposomes by removing the detergent from mixed micelles by hydrophobic chromatography on Amberlite XAD-4. The reconstitution was optimised with respect to the protein concentration, the detergent/phospholipid ratio and the number of passages through a single Amberlite column. The reconstituted carnitine transporter catalysed a first-order antiport reaction (carnitine/carnitine or carnitine/substrate) stimulated by external, not internal, Na⁺, with a positive cooperativity. Na⁺ was co-transported with carnitine. Optimal activity was found between pH 5.5 and pH 6.0. The sulfhydryl reagents MTSES, MTSET and mercurials strongly inhibited the transport. Substrate analogues inhibited the transport; the most effective were acylcarnitines and betaine, followed by dimethylglicine, tetraethylammonium and arginine. Besides carnitine, only acylcarnitines and betaine were efficiently translocated. The Km for carnitine on the external and internal side of the transporter was 0.08 and 1.2 mM, respectively. The transporter is asymmetrical and it is unidirectionally inserted into the proteoliposomal membrane with an orientation corresponding to that of the native membrane. The reconstituted carnitine transporter corresponds, very probably, to the OCTN2 protein.     

Glucose transport in vesicles reconstituted from Saccharomyces cerevisiae membranes and liposomes.

Glucose transport activity was reconstituted into liposomes by the freeze-thaw-sonication procedure from unextracted Saccharomyces cerevisiae membranes and preformed phospholipid liposomes. Fluorescence-dequenching measurements with octadecylrhodamine B chloride (R18)-labeled membranes showed that the yeast membrane lipids are diluted by the liposome lipids after the freeze-thaw-sonication procedure. At lipid-to-protein ratios greater than 75:1, vesicles with single transporters were formed. Reconstituted specific activity was increased at least twofold if the liposomes contained 50 mol% cholesterol. A further increase in specific activity, from 3- to 10-fold, was achieved by fractionation of the membranes on a Renografin gradient before reconstitution. Examination of the fractions from the Renografin gradient by sodium dodecyl sulfate-gel electrophoresis showed a parallel enrichment of glucose transport activity and a number of proteins including one with an apparent Mr of ca. 60,000, which might be the glucose transporter. Finally, preliminary kinetic analysis of glucose transport activity in vesicles reconstituted at a high lipid-to-protein ratio gave a Vmax of ca. 2.8 mumol/mg of protein per min at 23 degrees C and a Km of ca. 8 mM. The latter value corresponds to the kinase-independent, low-affinity component of glucose transport observed in wild-type cells.     

Cholesterol is required for the reconstruction of the sodium- and chloride-coupled, gamma-aminobutyric acid transporter from rat brain

The reconstruction of the purified sodium- and chloride-coupled gamma-aminobutyric acid transporter from rat brain into asolectin liposomes requires the addition of brain lipids (Radian, R., and Kanner, B. I. (1985) J. Biol. Chem. 260, 11859-11865). The reconstitution assay was used to identify the component(s) from brain lipids responsible for the stimulation during the fractionation of brain lipids. The distribution of the active component was found to be similar to that of cholesterol. Furthermore, cholesterol was found to mimic the effect of brain lipids and it stimulated the transport activity up to 20-fold. Optimal reconstituted transport activity was achieved with mixtures of cholesterol and any one of several phospholipids, such as phosphatidylcholine, phosphatidylserine or phosphatidylglycerol. gamma-Aminobutyric acid transport in these liposomes of defined composition exhibited all the properties of the native transporter, such as the absolute dependence on sodium and chloride and electrogenicity. Cholesterol could not be replaced by cholest-4-en-3one and other steroids, and thus its effect is probably not due to effects on membrane fluidity. The requirement was also not due to effects on intactness of the liposomes or incorporation of proteins into them. Furthermore it was found that the reconstitution of the sodium and potassium coupled L-glutamic acid transporter from rat brain also required cholesterol. However, in this case the optimal activity was reached by 4-5-fold lower levels of cholesterol than those necessary for gamma-aminobutyric acid transport. When cholesterol depletion from the transporters was incomplete, addition of exogenous brain lipids was not required. Thus, if the cholesterol was still associated with the transporter proteins, its final concentration, as a fraction of the total lipids present in the reconstitution mixture, was only about 0.01 mol%. Thus, it is likely that the effects of cholesterol are due to direct interactions with the cotransporters and not to an average effect on membrane properties.     

Purification and reconstitution of the high affinity choline transporter

The high-affinity choline transporter has been solubilized from synaptosomal membranes by various detergents. The solubilized carrier protein has been incorporated into liposomes after removal of the detergent by dialysis. Using the reconstitution of choline transport activity as an assay, the components catalyzing choline translocation were purified from the detergent extract by ion-exchange chromatography on a Mono-Q column followed by immunoaffinity chromatography. Monitoring the active fractions by sodium dodecylsulfate polyacrylamide gel electrophoresis and isoelectrofocussing gave one major protein with an apparent molecular weight of about 90,000 and an isoelectric point of pH 4.7. The isolated protein appeared to be heavily glycosylated as shown by lectin binding; upon treatment with endoglycosidase F the polypeptide was degraded to an apparent molecular weight of about 65,000. Accumulation of choline into liposomes reconstituted with the purified protein was driven by artificially imposed sodium gradients and inhibited by hemicholinium-3.