Interactions between gangtioside G_(M3) and glucose transporter. GLUTl were studied by measuring the effect of G_(M3) on steady-state and time-resolved fluorescence of purified GLUT1 in synthetic lipids and on the 3-O-methylglucose uptake by human erythrocytes. The intrinsic tryptophan fluorescence showed a GLUT 1 emission maximum of 335 nm, and increased in the presence of G_(M3) by 12% without shifting the emission maximum. The fluorescence lifetimes of intrinsic tryptophan on GLUT1 consisted of a long component of 7.8 ns and a short component of 2.3 ns and G_(M3) increased both lifetime components. Lifetime components were quenched by acrylamide and KI. Acrylamide-induced quenching of long-lifetime components was partly recovered by G_(M3). However, KI-induced quenching of short- and long-lifetime components was not rescued by G_(M3). The anisotropy of 1.6-diphenyl-1,3,5-hexatriene (DPH)-probed dimyristoylphosphatidylcholine (DMPC) model membrane was also increased with G_(M3) incorporation. The transport rate of 3-O-methylglucose increased by 20% with G_(M3) incorporation on the erythrocytes. Therefore. G_(M3) altered the environment of lipid membrane and induced the conformational change of GLUT1.