Regulation of maltose transport in Saccharomyces cerevisiae
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Solute transport in Saccharomyces cerevisiae can be regulated through mechanisms such as trans-inhibition and/or catabolite inactivation by nitrogen or carbon sources. Studies in hybrid membranes of S. cerevisiae suggested that the maltose transport system Mal61p is fully reversible and capable of catalyzing both influx and efflux transport. This conclusion has now been confirmed by studies in a S. cerevisiae strain lacking the maltase enzyme. Whole cells of this strain, wherein the orientation of the maltose transporter is fully preserved, catalyze fully reversible maltose transport. Catabolite inactivation of the maltose transporter Mal61p was studied in the presence and absence of maltose metabolism and by the use of different glucose analogues. Catabolite inactivation of Mal61p could be triggered by maltose, provided the sugar was metabolized, and the rate of inactivation correlated with the rate of maltose influx. We also show that 2-deoxyglucose, unlike 6-deoxyglucose, can trigger catabolite inactivation of the maltose transporter. This suggests a role for early glycolytic intermediates in catabolite inactivation of the Mal61 protein. However, there was no correlation between intracellular glucose-6-phosphate or ATP levels and the rate of catabolite inactivation of Mal61p. On the basis of their identification in cell extracts, we speculate that (dideoxy)-trehalose and/or (deoxy)-trehalose-6-phosphate trigger catabolite inactivation of the maltose transporter.