A SOYBEAN SUCROSE BINDING-PROTEIN INDEPENDENTLY MEDIATES NONSATURABLESUCROSE UPTAKE IN YEAST

Heterologous expression of a cDNA encoding a 62-kD soybean sucrose bin ding protein in yeast demonstrates that this protein, independent of o ther plant proteins, mediates sucrose uptake across the plasma membran e. Sucrose binding protein-mediated sucrose uptake is nonsaturable up to 30 mM sucrose, is specific for sucrose, and is relatively insensiti ve to treatment with sulfhydryl-modifying reagents. Alteration of the external pH or pretreatment of the yeast cells with protonophores did not significantly affect the rate of C-14-sucrose uptake. This demonst rates that sucrose binding protein-mediated sucrose uptake is not depe ndent on H+ movement and delineates it from other plant sucrose transp orters, Physiological characterization of sucrose uptake into higher p lant cells has shown the presence of both saturable and nonsaturable u ptake components. The nonsaturable mechanism is relatively insensitive to external pH, pretreatment with protonophores, and treatment with s ulfhydryl-modifying reagents. Sucrose binding protein-mediated sucrose uptake in yeast mimics this physiologically described, but mechanisti cally undefined, nonsaturable sucrose uptake mechanism in higher plant s, Functional characterization of the sucrose binding protein thus def ines both a novel component of sucrose uptake and provides important i nsight into this nonsaturable sucrose uptake mechanism, which has rema ined enigmatic since its physiological description.