Molecular Determinants of GLUT1: Structure and Function: A Dissertation

Hebert and Carruthers (1992) showed that the human erythrocyte glucose transporter is an allosteric complex of four GLUT1 proteins whose structure and substrate binding properties are stabilized by reductant-sensitive noncovalent subunit interactions. The GLUT1 tetramer dissociates into dimers upon exposure to reductant but subunits are not associated via disulfide bridges. Each subunit of SDS-denatured tetrameric GLUT1 exposes only two thiols while reduced denatured GLUT1 exposes all six sulfhydryl groups. They hypothesized that glucose transporter oligomeric structure and cooperative catalytic function resulted from noncovalent subunit interactions promoted or stabilized by intramolecular disulfide bridges. These interactions give rise to an antiparallel arrangement of substrate binding sites within the transporter complex. In the present studies, we tested aspects of this model. Specifically, we wanted 1) to understand why the native, noncovalent, homotetrameric GLUT1 complex is sensitive to reductant, 2) to determine whether the tetramer is more catalytically efficient than the dimer in situ, and 3) to test the hypothesis that it is the antiparallel arrangement of substrate binding sites between subunits that provides the transporter with its catalytic advantage. We used biochemical and molecular biological approaches to isolate specific determinants of transporter oligomeric structure and/or transport function in purified isolated transporter preparations, in intact red cells and in CHO cells. We have also examined the hypothesis that net sugar transport in the human erythrocyte is rate limited by reduced cytosolic diffusion of sugars and/or by reversible sugar association with intracellular macromolecules. Our findings support the hypothesis that each subunit of the parental glucose transporter contains a single intramolecular disulfide bridge located between cysteine residues 347 and 421. This disulfide seems to be necessary for GLUT1 tetramerization. Our findings suggest that GLUT1 N-terminal residues 1 through 199 provide contact surfaces for subunit dimerization but are insufficient for subunit tetramerization. Our studies also show that in situ disulfide disruption by cell impermeant reductants results in the loss of cooperative subunit interactions and a 3 to 15-fold reduction in the transport efficiency of the transporter. We further find that in situ GLUT1 is susceptible to exofacial proteolysis. Exofacial trypsin cleavage eliminates cooperativity between subunits but does not affect transporter oligomeric structure or transport activity. Thus catalytic efficiency does not derive directly from cooperative interactions between substrate binding sites on adjacent subunits. We have confirmed that 30MG transport in human erythrocytes is a diffusion limited process. We find that steady-state sugar uptake in red cells and K562 cells measures two processes - sugar translocation and intracellular sugar binding. We propose a model for native GLUT1 structure and function