Glucose and nucleoside transporters of human erythrocytes: effects of detergents on immunoadsorption of a membrane protein to its monoclonal antibody

Solution NMR of membrane proteins: practice and challenges

This review focuses upon the application of solution NMR methods to multispan integral membrane proteins, particularly with respect to determination of global folds by this approach. Practical methods are described along with the special difficulties that confront the application of solution NMR to proteins that dwell in the netherworld of the lipid bilayer.

Characterization and partial purification of liver glucose transporter GLUT2

GLUT2, the major facilitative glucose transporter isoform expressed in hepatocytes, pancreatic beta-cells, and absorptive epithelial cells, is unique not only with its low affinity and broad substrate specificity as a glucose transporter, but also with its implied function as a glucose-sensor. As a first essential step toward structural and biochemical elucidation of these unique, GLUT2 functions, we describe here the differential solubilization and DEAE-column chromatography of rat hepatocyte GLUT2 protein and its reconstitution into liposomes. The reconstituted GLUT2 bound cytochalasin B in a saturable manner with an apparent dissociation constant (K(d)) of 2.3 x 10(-6) M and a total binding capacity (B(T)) of 8.1 nmol per mg protein. The binding was completely abolished by 2% mercury chloride, but not affected by cytochalasin E. Significantly, the binding was also not affected by 500 mM D-glucose or 3-O-methyl D-glucose (3OMG). The purified GLUT2 catalyzed mercury chloride-sensitive 3OMG uptake, and cytochalasin B inhibited this 3OMG uptake. The inhibition was dose-dependent with respect to cytochalasin B, but was independent of 3OMG concentrations. These findings demonstrate that our solubilized GLUT2 reconstituted in liposomes is at least 60% pure and functional, and that GLUT2 is indeed unique in that its cytochalasin B binding is not affected by its substrate (D-glucose) binding. Our partially purified GLUT2 reconstituted in vesicles will be useful in biochemical and structural elucidation of GLUT2 as a glucose transporter and as a possible glucose sensor.

Interaction of the erythrocyte lactate transporter (monocarboxylate transporter 1) with an integral 70-kDa membrane glycoprotein of the immunoglobulin superfamily

Treatment of intact erythrocytes with 4,4'-diisothiocyanostilbene-2, 2'-disulfonate (DIDS) causes irreversible inhibition and chemical labeling of the lactate transporter, monocarboxylate transporter 1 (MCT1) (Poole, R. C., and Halestrap, A. P. (1992) Biochem. J. 283, 855-862). In rat erythrocytes DIDS also causes cross-linking of MCT1 to another protein in the membrane to give a product of 130 kDa on SDS-polyacrylamide gel electrophoresis. Cross-linking is markedly reduced by those compounds that protect against irreversible inhibition of lactate transport by DIDS and enhanced by imposition of a pH gradient across the plasma membrane to recruit the substrate binding site of MCT1 to an exofacial conformation. These data indicate that DIDS cross-linking is via the same site on MCT1 as is responsible for inhibition of transport. Antibodies raised against the cross-linked conjugate react with proteins of approximately 40 kDa (MCT1) and 70 kDa on Western blots of erythrocyte membranes and an additional band of 130 kDa after treatment of erythrocytes with 100 microM DIDS. The 70-kDa protein that is cross-linked to MCT1 was purified and shown to contain N-linked carbohydrate; the apparent core molecular mass is 40 kDa. Amino acid sequencing showed that the protein is the rat equivalent of the membrane-spanning mouse teratocarcinoma glycoprotein GP-70, a member of the immunoglobulin superfamily related to basigin (Ozawa, M., Huang, R. P., Furukawa, T. , and Muramatsu, T. (1988) J. Biol. Chem. 263, 3059-3062). Possible implications of the specific interaction between MCT1 and this protein are discussed.

Hybrid Fo complexes of the ATP synthases of spinach chloroplasts and Escherichia coli. Immunoprecipitation and mutant analyses

Hybrid Fo complexes of the ATP synthases of spinach chloroplast (CFo) and Escherichia coli (EFo) were investigated. Immunoprecipitations with polyclonal antibodies against the different Fo subunits clearly revealed that hybrid Fo complexes derived from CFo subunit III and EFo subunits a and b were formed in vivo. In addition, the ATPase activities of the hybrid ATP synthase, measured in everted cytoplasmic membranes of an atpE mutant strain transformed with the atpH gene coding for CFo III, were comparable to activities obtained for the same mutant strain complemented with the atpE gene (EFo c). Nevertheless, CFo III was not able to replace EFo c functionally, since the strain containing the hybrid ATP synthase was not able to grow on succinate. In order to investigate the reason for this lack of function, hybrid proteolipids of CFo III and EFo c were constructed. Only a chimaeric protein comprising the seven N-terminal amino acid residues from CFo III and the remaining part of EFo c was able to replace wild-type EFo c, whereas hybrid proteins with 13 and 33 N-terminal amino acids of CFo III were not functional. The results suggested that a network of interactions between the subunits essential for proton translocation and/or coupling of the F1 part exists, which was optimized for each species during evolution, although the overall structure of FoF1 complexes has been conserved.