Characterization of the saccharomycesbayanus-type Agt1 transporter of lager yeast

Abstract

Transport of maltose and maltotriose into the yeast cell is thought to be rate-limiting in the utilization of these sugars. Themaltose and maltotriose transporters Malx1, Agt1, Mtt1and Mphx are present in different combinations in brewer's yeaststrains, conferring differentmaltose and maltotriose transport characteristics on the strains. A new putative maltose/maltotriose transporter ORF was identified during whole genome sequencing of the lager strain WS4/70 (Y. Nakao et al., DNA Res., 2009, 16, 115-129). Sequence comparisons suggested that this putative Α-glucoside transporter might be aSaccharomyces bayanus counterpart of the Agt1 (Sacharomyces cerevisiae type) transporter. In the present work, thetransporter coded by a SbAGT1gene from a lager strain, A15 (and with the same sequence as the corresponding gene inWS34/70)was characterized. It is shown that this SbAGT1 encodes a functional Α-glucoside transporterwith a wide-substraterange, including maltose and maltotriose. Trehalose, a-methylglucoside and sucrose were inhibitors, suggesting they arealso substrates. The SbAgt1 transporter had similar affinities for maltose and maltotriose (17±7 and 22±2mM, respectively)and a higher Vmax for maltose than maltotriose (21±7 and 12±2μmol min-1g dry yeast-1, respectively). © 2012 The Institute of Brewing & Distilling.