A full-length construct of the glucose transporter isoform GLUT1 has been expressed in Sf9 (Spodoptera frugiperida Clone 9) insect cells, and a photolabelling approach has been used to show that the expressed protein binds the bismannose compound 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis-(D-mannos- 4-yloxy)-2-propylamine (ATB-BMPA) and cytochalasin B at its exofacial and endofacial binding sites respectively. Constructs of GLUT1 which produce either the N-terminal (amino acids 1-272) or C-terminal (amino acids 254-492) halves are expressed at levels in the plasma membrane which are similar to that of the full-length GLUT1 (approximately 200 pmol/mg of membrane protein), but do not bind either ATB-BMPA or cytochalasin B. When Sf9 cells are doubly infected with virus constructs producing both the C- and N-terminal halves of GLUT1, then the ligand labelling is restored. Only the C-terminal half is labelled, and, therefore, the labelling of this domain is dependent on the presence of the N-terminal half of the protein. These results suggest that the two halves of GLUT1 can assemble to form a stable complex and support the concept of a bilobular structure for the intact glucose transporters in which separate C- and N-domain halves pack together to produce a ligand-binding conformation.

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