Characterization of the Saccharomyces bayan us-type AGT1 transporter of lager yeast

Transport of maltose and maltotriose into the yeast cell is thought to be rate-limiting in the utilization of these sugars. The maltose and maltotriose transporters Malx1, Agt1, Mtt1 and Mphx are present in different combinations in brewer's yeast strains, conferring different maltose and maltotriose transport characteristics on the strains. A new putative maltose/maltotriose transporter ORF was identified during whole genome sequencing of the lager strain WS34/70 (Y. Nakao et al., DNA Res., 2009, 16, 115-129). Sequence comparisons suggested that this putative alpha -glucoside transporter might be a Saccharomyces bayanus counterpart of the Agtl (Saccharomyces cerevisiae type) transporter. In the present work, the transporter coded by a SbAGTI gene from a lager strain, A15 (and with the same sequence as the corresponding gene in WS34/70) was characterized. It is shown that this SbAGTI encodes a functional alpha -glucoside transporter with a wide-substrate range, including maltose and maltotriose. Trehalose, alpha -methylglucoside and sucrose were inhibitors, suggesting they are also substrates. The SbAgtl transporter had similar affinities for maltose and maltotriose (17 plus or minus 7 and 22 plus or minus 2 mM, respectively) and a higher V(max) for maltose than maltotriose (21 plus or minus 7 and 12 plus or minus 2 mu mol min(-1) g dry yeast(-1) respectively).