Ligand-modulation of the stability of the glucose transporter GLUT 1
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The glucose transporter GLUT 1 was isolated from human erythrocytes and reconstituted into endogenous membrane lipids. Results from thermal denaturation studies, using differential scanning calorimetry, indicate that the thermal denaturation temperature of GLUT 1 is significantly lower in the presence of ATP. The lowering of this transition temperature is very dependent on pH. At more acidic pH, ATP has a greater effect of lowering the thermal denaturation temperature of the protein. For example, with 4.8 mM ATP, the denaturation endotherm is lowered by over 10 degrees at pH 4.3, whereas at pH 7.4, ATP does not alter this transition temperature. However, a change in pH alone, in the absence of ATP, has very little effect on the denaturation temperature. Both glucose and salt partially reverse the lowering of the temperature of thermal denaturation caused by ATP. Studies of acrylamide quenching of the Trp residues of GLUT 1 indicate that at neutral pH, ATP increases the Stern-Volmer quenching constant, while glucose lowers it. The results indicate that ATP binds to GLUT 1 and destabilizes the native structure, leading to a lowering of the thermal denaturation temperature and an increase in acrylamide quenching. The effects of ATP are reversed in part by glucose and are also partly electrostatic in nature.
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