Three Agt1 transporters from brewer's yeasts exhibit different temperature dependencies for maltose transport over the range of brewery temperatures (0-20°C)

Zero-trans rates of maltose transport by brewer's yeasts exert strong control over fermentation rates and are strongly temperature-dependent over the temperature range (20-0 degrees C) of brewery fermentations. Three -glucoside transporters, ScAgt1(A60) (a Saccharomyces cerevisiae version of Agt1 from an ale strain), ScAgt1-A548V (a variant of ScAgt1(A60) with a single amino acid change in a transmembrane domain), and SbAgt1 (a Saccharomyces (eu)bayanus version from a lager strain), were compared. When expressed in the same laboratory yeast, grown at 24 degrees C and assayed at 0, 10, and 20 degrees C, SbAgt1 had the lowest absolute maltose uptake activity at 20 degrees C but smallest temperature dependence, ScAgt1-A548V had the highest activity but greatest temperature dependence, and ScAgt1(A60) had intermediate properties. ScAgt1(A60) exhibited higher absolute rates and smaller temperature dependencies when expressed in laboratory rather than brewer's strains. Absolute rates closely reflected the amounts of GFP-tagged ScAgt1(A60) transporter in each host's plasma membrane. Growth at 15 degrees C instead of 24 degrees C decreased the absolute activities of strains expressing ScAgt1(A60) by two- to threefold. Evidently, the kinetic characteristics of at least ScAgt1(A60) depended on the nature of the host plasma membrane. However, no consistent correlation was observed between transport activities and fatty acid or ergosterol compositions.