Solubilization, reconstitution, and attempted affinity chromatography of the sugar transporter of the erythrocyte membrane

Characterization of glucose transport activity reconstituted from heart and other tissues

We examined several aspects of glucose transport reconstituted in liposomes, with emphasis on transporters of rat heart (mostly GLUT4) compared to those of human erythrocytes (GLUT1), and on effects of agents that modulate transport in intact cells. Several types of samples gave higher reconstituted activity using liposomes of egg lipids rather than soybean lipids. Diacylglycerol, proposed to activate transporters directly as part of the mechanism of insulin action, increased the intrinsic activity of heart transporters by only 25%, but increased the size of the reconstituted liposomes by 90%. The dipeptide Cbz-Gly-Phe-NH2 inhibited GLUT4 with a Ki of 0.2 mM, compared to 2.5 mM for GLUT1, which explains its preferential inhibition of insulin-stimulated glucose transport in adipocytes. Verapamil, which inhibits insulin- and hypoxia-stimulated glucose transport in muscle, had no effect on reconstituted transporters. Heart transporters had a higher Km for glucose uptake (13.4) than did GLUT1 (1.6 mM), in agreement with a recent study of GLUT1 and GLUT4 expressed in yeast and reconstituted in liposomes. Transporters reconstituted from heart and adipocytes were 40-70% inactivated by external trypsin, suggesting the presence of trypsin-sensitive sites on the cytoplasmic domain of GLUT4. NaCl and KCl both reduced reconstituted transport activity, but KCl had a much smaller effect on the size of the liposomes.

Purification of proteins of the Na/Cl cotransporter from membranes of Ehrlich ascites cells using a bumetanide-sepharose affinity column

Bumetanide-binding proteins were isolated from membranes of Ehrlich ascites tumor cells by affinity chromatography. An affinity column was constructed with the active moiety of bumetanide as a ligand using 4'-azidobumetanide, a photoactive analogue which inhibits Na/Cl cotransport in Ehrlich cells with high specificity. Covalent binding of the 4'-azidobumetanide with Sepharose was promoted by photolysis. Membranes isolated from Ehrlich cells were solubilized with n-octylglucoside. Solubilized proteins retarded by the affinity column were readily eluted by bumetanide. In reducing gels the major proteins eluted by bumetanide were approximately 76 kDa and 38-39 kDa. There were also two proteins of 32 to 35 kDa eluted in lesser amounts. No proteins retarded by the affinity column were eluted with extensive washing without bumetanide. Furthermore, bumetanide eluted no proteins from a "control" column lacking the specific ligand. Upon rechromatography with bumetanide in solution, bumetanide-eluted proteins were not retarded, but their purity was increased by the retardation of contaminating proteins. Bumetanide-binding protein purified in this manner were characterized further by electrophoresis in nonreducing, nondenaturing gels.

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.

Reconstitution of glucose transport activity from erythrocyte membranes without detergent and its use in studying effects of ATP depletion

The direct reconstitution of unsolubilized membrane proteins by the freeze-thaw procedure avoids possible changes in properties produced by detergent solubilization and fractionation. Glucose transport activity was reconstituted using human erythrocyte membranes, with about 2/3 of the glucose uptake being stereo-specific. The highest specific activity occurred at low ratios of protein to lipid in the reconstitution, where most transport was due to liposomes containing single transporter molecules. Transporters were reconstituted with a scrambling of orientations, indicated by a 50% inactivation by added trypsin. Separation of unreconstituted protein doubled the specific activity. Similar results were obtained using the purified transporter (Wheeler, T.J. and Hinkle, P.C. (1981) J. Biol. Chem. 256, 8907-8914). The same ratio of net uptake to equilibrium exchange was observed for the two preparations. Their relative reconstituted transport activities and cytochalasin B binding activities were equal, indicating that the two were reconstituted with similar efficiencies. The decrease in glucose transport in erythrocytes produced by ATP depletion and the stimulation produced by resealing with ATP (Jacquez, J.A. (1983) Biochim. Biophys. Acta 727, 367-378) were confirmed. However, no difference was observed in reconstituted transport activity using ghosts resealed with or without ATP, indicating that ATP produces indirect effects rather than modifications of the transporter.