MECHANISM OF GLUCOSE AND MALTASE TRANSPORT IN PLASMA-MEMBRANE VESICLES FROM THE YEAST CANDIDA-UTILIS

Transport of glucose and maltose was studied in plasma-membrane vesicl es from Candida utilis. The yeast was grown on a mixture of glucose an d maltose in aerobic carbon-limited continuous cultures which enabled transport to be studied for both sugars with the same vesicles. Vesicl es were prepared by fusion of isolated plasma membranes with proteolip osomes containing bovine heart cytochrome c oxidase as a protonmotive- force-generating system. Addition of reduced cytochrome c generated a proton-motive force, consisting of a membrane potential, negative insi de, and a pH gradient, alkaline inside. Energization led to accumulati on of glucose and maltose in these vesicles, reaching accumulation rat ios of about 40-50. Accumulation also occurred in the presence of vali nomycin or nigericin, but was prevented by a combination of the two io nophores or by uncoupler, showing that glucose and maltose transport a re dependent on the proton-motive force. Comparison of sugar accumulat ion with quantitative data on the proton-motive force indicated a 1:1 H+/sugar stoichiometry for both transport systems. Efflux of accumulat ed glucose was observed on dissipation of the proton-motive force. Exc hange and counterflow experiments confirmed the reversible character o f the Hf-glucose symporter. In contrast, uncoupler or a mixture of val inomycin plus nigericin induced only a slow efflux of accumulated malt ose. Moreover under counterflow conditions, the expected transient acc umulation was small. Thus the H+-maltose symporter has some characteri stics of a carrier that is not readily reversible. It is concluded tha t in C. utilis the transport systems for glucose and maltose are both driven by the proton-motive force, but the mechanisms are different.