Fractionation of human red cell membrane proteins by ion-exchange chromatography in detergent on Mono Q, with special reference to the glucose transporter

Tailored nanodisc immobilization for one-step purification and reconstitution of cytochrome P450: A tool for membrane proteins' hard cases

A novel nanodisc-based immobilization method was developed for high-efficient purification and reconstitution of cytochrome P450 in one step. Using membrane scaffold protein containing a histidine tag, charged-nanodiscs were prepared in the form of self-assembly of lipid-protein nanoparticles. Their properties including the particle diameter and its distribution and Zeta potential were controlled well by adjusting molar ratios of phospholipids to membrane scaffold protein. At an optimum lipid-to-membrane scaffold protein molar ratio of 60:1, uniformly regular-shaped and discoidal nanodiscs with an average particle diameter of 10 nm and Zeta potential of -19 mV were obtained. They can be well fractionated by size exclusion chromatography. Charged-nanodiscs were successfully immobilized onto Ni-chelating microspheres via histidine tags with a density of 6.6 mg membrane scaffold protein/mL gel. After being packed in a column, chromatography studies demonstrated that this nanodisc-immobilized chromatographic medium had a specific binding to cytochrome P450 in rat liver microsome. Nanodiscs containing cytochrome P450 can be furthermore eluted from the column with a diameter of about 87.0 nm and height of about 8.0 nm, respectively. The purity of cytochrome P450 after purification increased 25 folds strikingly. This nanodisc-immobilized chromatography method is promising for the one-step purification and reconstitution of membrane protein.

Enhanced detergent extraction for analysis of membrane proteomes by two-dimensional gel electrophoresis

Background

The analysis of hydrophobic membrane proteins by two-dimensional gel electrophoresis has long been hampered by the concept of inherent difficulty due to solubility issues. We have optimized extraction protocols by varying the detergent composition of the solubilization buffer with a variety of commercially available non-ionic and zwitterionic detergents and detergent-like phospholipids.

Results

After initial analyses by one-dimensional SDS-PAGE, quantitative two-dimensional analyses of human erythrocyte membranes, mouse liver membranes, and mouse brain membranes, extracted with buffers that included the zwitterionic detergent MEGA 10 (decanoyl-N-methylglucamide) and the zwitterionic lipid LPC (1-lauroyl lysophosphatidylcholine), showed selective improvement over extraction with the common 2-DE detergent CHAPS (3 [(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate). Mixtures of the three detergents showed additive improvements in spot number, density, and resolution. Substantial improvements in the analysis of a brain membrane proteome were observed.

Conclusion

This study demonstrates that an optimized detergent mix, coupled with rigorous sample handling and electrophoretic protocols, enables simple and effective analysis of membrane proteomes using two-dimensional electrophoresis.

Differential solubilization of osteoblastic alkaline phosphatase from human primary bone cell cultures

Mineralization of cartilage and bone requires alkaline phosphatase activity. In order to study the enzymatic properties of bone alkaline phosphatase in bone disease and more particularly in patients with osteoporosis and osteoarthritis, we investigated the solubilization of alkaline phosphatase from primary bone cell cultures derived from human bone explants. To study the release of alkaline phosphatase from membranes, several detergents at a concentration above the critical micellar concentration and cholesterol were used. Solubilized alkaline phosphatase was characterized by enzymatic activity and electrophoresis analysis. Almost all the alkaline phosphatase was solubilized using non-ionic detergent as n-octylglucoside and hecameg. In comparison with initial membranous activity, the solubilized activity was increased by a factor, i.e. 2 +/- 0.05 (SEM, n = 3) (with n-octylglucoside), i.e. 2.1 +/- 0.05 (SEM, n = 3) (with Hecameg). With an ionic detergent (sodium dodecylsulfate), zwitterionic detergent ((cholamido propyl) dimethylammonio 1 propane sulfonate) and cholesterol, a fraction of alkaline phosphatase was resistant to solubilization. Electrophoresis studies showed that released alkaline phosphatase was a glycosylphosphatidylinositol protein (amphipatic form) with 140 kDa as apparent molecular weight. A hydrophilic form was obtained by treatment with a specific lipase. This study showed differential solubilization of osteoblastic alkaline phosphatase from human primary bone cell cultures. Better extractibility and higher activation of this membrane anchored enzyme were obtained with non-ionic detergents.

Influence of the size of the polar head of non-ionic detergents on membrane proteins immunoaffinity purification

Nonionic polyoxyethylene type detergents (CxEy) are widely used to solubilize and purify membrane proteins. The detergent hydrophobic moiety (Cx) replaces phospholipids at exposed hydrophobic regions of the membrane proteins. During chromatography on an immobilized anti-Kell antibody to purify Kell protein (an integral erythrocyte protein), it was observed that the size of the polar head of an non ionic detergent added to the mobile phase appeared to influence the interaction of the detergent-protein complex with the immobilized antibody. Further studies were performed using another erythrocyte membrane protein, Glycophorin C and three anti-GPC monoclonal antibodies directed against three epitopes of the extracytoplasmic domain of the protein. The interaction of GPC with the three Protein A-coupled monoclonal antibodies was studied in the presence of three detergents C12E<9>, C13E<15> and C12E<23>. It was observed in batch mode and in column chromatography experiments that the adsorption of GPC to the immunoaffinity supports decreased as the size of the detergent polar head increased. Thus, the polyoxyethylene chain of a detergent might prevent the interaction of the detergent-protein complex with the immobilized antibody.

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

Human red cell membranes were stripped of peripheral proteins and partially solubilized with 50-260 mM octyl glucoside at 2-14 mg protein/ml, to find conditions that afford a high concentration of active glucose transporter after purification on DEAE-cellulose. Transporter-egg yolk phospholipid vesicles were prepared by gel filtration. The specific D-glucose equilibrium exchange activities increased with increasing dilution of the glucose transporter. At 260 mM octyl glucoside the glucose transporter became partially denaturated. At 225 mM detergent the DEAE-cellulose chromatography showed one main and one minor fraction of active glucose transporter. Nucleoside transport activity was enriched in the minor fraction. Solubilization with 75 mM octyl glucoside at 8 mg protein/ml gave a maximal concentration of purified transporter, 0.8 mg/ml, probably corresponding to complete solubilization. The phospholipids were partially retarded on the DEAE-cellulose. The specific D-glucose equilibrium exchange was high, up to 200 nmol glucose/micrograms transporter in two min at 50 mM glucose. High performance gel filtration in octyl glucoside indicated that the transporter formed dimers during the fractionation. These eluted at Mr 125,000, partially separated from the phospholipids, which appeared at Mr 55,000 (cf. Mascher, E. and Lundahl, P. (1987) J. Chromatogr. 397, 175-186). The D-glucose transport activity was low in the main fraction and high in the transporter-phospholipid fraction. Mixing of these fractions did not increase the activity. The glucose transporter is probably dependent on one or more specific membrane lipid(s). Presumably the transporter dimerizes and loses activity upon removal of these lipids.

Chromatography of complex protein mixtures

This review has shown that a variety of chromatographic techniques are available for fractionating proteins. Fortunately, high-quality columns of every type described in this review are commercially available. Most water-soluble proteins may be eluted from size-exclusion, hydrophobic-interaction, ion-exchange, metal chelate, and bioaffinity columns with ease. When this is the case, high recovery and retention of biological activity are the norm. The exception is reversed-phase chromatography where the organic solvents and acids used in polypeptide elution denature many proteins. When problems do occur, they are generally the result of unique structural features of the protein. Very hydrophobic proteins have presented the biggest problem in that they are difficult to solubilize, particularly with retention of biological activity. It has been found that zwitterionic and non-ionic detergents are the most suitable solubilizing agents, but urea has also been used in cases where hydrophobic interacts are not as strong. Unfortunately, there is still an element of trial-and-error in selecting the most suitable solubilizing agent. Heterogeneous glycosylation of proteins also presents a problem. Both neutral and charged monosaccharides can be incorporated into proteins through multiple steps at several sites. Thus, there is the potential in a sample for a large number of glycoprotein species which have the same polypeptide backbone and differing amounts of oligosaccharide. A problem arises when size-exclusion, ion-exchange, hydrophobic-interaction, reversed-phase and bioaffinity systems begin to discriminate between these very similar glycoprotein species. Chromatographic peaks can become very broad, due to incomplete fractionation, and the polypeptide chain of interest can be associated with multiple peaks. The separation of glycoproteins requires much more study before logical procedures can be suggested for column selection and operation. Aggregated species are another class of proteins which present occasional problems. Multimeric proteins are adsorbed to sorbents by a series of forces, among which are hydrogen bonding, hydrophobic interactions, and electrostatic forces. These forces are also responsible for the maintenance of quaternary structure in proteins. When the same forces dominate both retention of protein structure and adsorption at the sorbent surface, the quaternary structure of the protein can be disrupted during elution. Very basic proteins also present a problem in some cases. Columns with residual negative charges, such as a silica-based reversed-phase column, adsorb anionic species so strongly that they are difficult to elute.(ABSTRACT TRUNCATED AT 400 WORDS)

High-performance agarose gel chromatography in octyl glucoside of integral membrane proteins from human red cells, with special reference to the glucose transporter

Integral membrane proteins and lipids from human red cells were fractionated in the presence of octyl glucoside by high-performance gel chromatography on a 22-ml column of the small-bead cross-linked agarose gel Superose 6, at 5 degrees C, pH 7.6 and 30-50 mM detergent. To avoid aggregation a relatively high flow-rate, 9 ml/h, was chosen. At low ionic strength four main fractions were resolved, which contained anion transporter multimers(I), glycophorin oligomers(II), glucose transporter dimers(III) and phospholipids(IV). In 0.5 M sodium chloride the resolution was lower but the yield of the glucose transporter was markedly higher, and chromatography of partially purified glucose transporter gave a protein recovery of about 90%. The apparent Mr values for the octyl glucoside complexes of the main components were: anion transporter, 900,000; glycophorin A, 210,000-360,000, dependent on ionic strength; glucose transporter, 110,000-160,000; lipids, 70,000. Some components aggregated with time: at a flow-rate of 1 ml/h mainly glycophorins and the glucose transporter were eluted, but no anion transporter, and fractionation performed 20 h after solubilization showed extensive aggregation of proteins. Superose-6 chromatography of glucose transporter and membrane lipids that had been isolated on DEAE-cellulose partially resolved the transporter and the phospholipid fractions. In this case, the resolution was better with 50 than with 30 mM detergent. The maximum glucose transport activity was approximately one-tenth of that observed before fractionation and appeared in two main fractions, at the main transporter fraction as well as at the overlap between the transporter and the lipids. The activity level was the same in both fractions, although the protein concentration was much lower in the second one. Addition of 2 mM egg-yolk phospholipids to the eluent did not increase the activity. This strongly indicates that the glucose transporter needs some specific membrane lipids to retain high transport activity. At the concentration of ca. 0.3 mg/ml used, the glucose transporter was probably eluted as a dimer in the absence of phospholipids and as a monomer in their presence.

High performance agarose gel chromatography in sodium dodecyl sulfate of integral membrane proteins from human red cells, with special reference to the glucose transporter

Integral membrane proteins from human red cells were fractionated in sodium dodecyl sulfate solutions by high performance gel filtration on the small-bead cross-linked agarose gel Superose 6. The components were identified by acrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The combination of Superose chromatography with electrophoresis afforded high resolution. As expected the gel filtration elution volumes depended essentially on the molecular mass, but the elution volumes decreased stepwise as the detergent concentration was increased from 0.6 to 100 mM, with the largest decrease for the glucose transporter. The resolution increased as the flow rate was decreased from 60 to 1 ml X cm-2 X h-1. The Mr values for the anion and glucose transporters as estimated by Superose 6-chromatography at 50 mM detergent were 75-80% of the corresponding Mr values obtained by electrophoresis. At 50 mM dodecyl sulfate the proteins were resolved into four fractions (a-d) which mainly contained: (a) dimer and (b) monomer of the anion transporter, (c) the glucose transporter and (d) components of Mr below 40 000. Monoclonal antibodies that possibly are directed against the glucose transporter (Lundahl, P., Greijer, E., Cardell, S., Mascher, E. and Andersson, L. (1986) Biochim. Biophys. Acta 855, 345-356) interacted only with part of the 4.5-material in fraction c in immunoblotting (Western blotting). Superose 6-chromatography of red cell glucose transporter that had been partially purified on DEAE-cellulose and Mono Q resolved one major and two minor fractions. Electrophoretic analysis showed that components of Mr 90,000, 50,000, and 25,000 had been separated from the major Mr-55,000-4.5-material and revealed size heterogeneity within the major chromatographic fraction. Heating of the glucose transporter in the presence of dodecyl sulfate caused an unexpected retardation of monomeric transporter on Superose 6. The apparent Mr decreased from 44,000 to 29,000.

Improved preparation of the integral membrane proteins of human red cells, with special reference to the glucose transporter

Human red cell membranes were isolated and partially stripped of peripheral proteins by gel filtration of hemolysates on a Sepharose CL-4B column at pH 8 connected in tandem to a Sepharose CL-6B column at pH 10.5. The eluted material was washed by centrifugations, once at pH 10.5 and twice at pH 12. In this way, water-soluble proteins and peripheral membrane proteins were thoroughly removed, and 0.2 g of integral membrane proteins could be prepared within 10 h from 0.2 litre of red cells. The exposure to high pH did not lower the D-glucose transport activity, and electrophoretically pure glucose transport protein could be isolated from this preparation. Gel filtration in sodium dodecyl sulfate separated the integral membrane components into four fractions, one of them containing 4.5-material; gel electrophoresis showed about 14 zones and two-dimensional electrophoresis resolved up to 100 mostly minor components, among which the glucose transporter focused around pH 7. However, purified glucose transporter focused around pH 8. Glucose and nucleoside transport proteins were co-purified in active form on DEAE-cellulose and a fraction isolated by adsorption to Mono Q was used for immunization of mice and production of monoclonal antibodies. One hybridoma produced antibodies that reacted with material in the 4.5-region, possibly the glucose transport protein, and not with band 3-material. Upon two-dimensional electrophoresis of integral membrane components that had been solubilized with octyl glucoside the immunoreactive and the silver-stained 4.5-material focused in a broad range from pH 6 to pH 9. A possible explanation for this heterogeneity might be interaction between the glucose and nucleoside transport proteins and negatively charged lipids.

Sequence and structure of a human glucose transporter

The amino acid sequence of the glucose transport protein from human HepG2 hepatoma cells was deduced from analysis of a complementary DNA clone. Structural analysis of the purified human erythrocyte glucose transporter by fast atom bombardment mapping and gas phase Edman degradation confirmed the identity of the clone and demonstrated that the HepG2 and erythrocyte transporters are highly homologous and may be identical. The protein lacks a cleavable amino-terminal signal sequence. Analysis of the primary structure suggests the presence of 12 membrane-spanning domains. Several of these may form amphipathic alpha helices and contain abundant hydroxyl and amide side chains that could participate in glucose binding or line a transmembrane pore through which the sugar moves. The amino terminus, carboxyl terminus, and a highly hydrophilic domain in the center of the protein are all predicted to lie on the cytoplasmic face. Messenger RNA species homologous to HepG2 glucose transporter messenger RNA were detected in K562 leukemic cells, HT29 colon adenocarcinoma cells, and human kidney tissue.

Purification of oestrogen synthetase by high-performance liquid chromatography. Two membrane-bound enzymes from the human placenta

Human placental NADPH-cytochrome P-450 reductase, obtained by 2',5'-ADP-Sepharose affinity chromatography, was separated into two reductase-active peaks on a Pharmacia Mono-Q column. By this short, two-step chromatographic procedure, the two reductases were obtained in a homogeneous state with high retention of activity and in over 900-fold purification. Aromatase-reconstituting activity was present only in the higher-molecular-weight reductase (79 000 D), not in the smaller, 70 000 D reductase, which turned out to be a proteolysis product of the former. Both proteins were eluted as a single peak in reversed-phase high-performance liquid chromatography on a Protesil-diphenyl column. Similar results were obtained with bovine hepatic NADPH-cytochrome P-450 reductases. On the other hand, starting from a reductase-free preparation, we have obtained by high-performance ion-exchange chromatography and high-performance size-exclusion chromatography, only partial purification of the aromatase cytochrome P-450, which showed the following values: aromatase activity, 3.995 nmol/min/mg protein (60-fold purification); cytochrome P-450 content, 1.376 nmol/mg protein (23-fold purification); molecular weight, 165 000 D (estimated as an aggregate by size-exclusion chromatography). Although complete purification of the aromatase component has yet to be accomplished, our results suggest that high-performance ion-exchange chromatography on a Mono-Q column is very useful for the purification of acidic, membrane-bound enzymes with good retention of activity.