The maltose transporter of Saccharomyces cerevisiae is subject to rapid, irreversible inactivation in the presence of glucose. Loss of transport function was paralleled by a decrease in amount of transporter protein and most likely involves endocytosis and degradation of the protein in the vacuole. This (catabolite) inactivation of Mal61p was triggered not only by glucose but also by 2-deoxy-D-glucose, which cannot be metabolized beyond 2-deoxy-D-glucose phosphate. The signal that targets membrane proteins specifically for catabolite inactivation is unknown. To investigate whether or not specific modification of Mal61p triggers the inactivation, putative protein kinase A and C phosphorylation sites were removed, and the transport activities and levels of the mutant proteins upon addition of glucose were followed in time. Three Mal61p mutants, i.e. S295A, T363A, and S487A, exhibited significantly reduced rates of inactivation in the presence of glucose. Likewise, in wild-type Mal61p the rate of inactivation and degradation of the protein paralleled each other in the case of T363A. On the contrary, for the S295A and S487A mutants the rates of protein degradation were slowed down more profoundly than was the loss of transport activity. These observations indicate that (i) some form of modification (e.g. phosphorylation) of the protein precedes breakdown, (ii) the modification inactivates Mal61p, and (iii) the inactivation of Mal61p is not necessarily followed by proteolytic degradation.The first step in maltose metabolism in yeast is performed by the maltose transport protein, which catalyzes the uptake of maltose in symport with one proton. Subsequently, 1,4-␣-glucosidase (maltase) hydrolyzes internalized maltose into two molecules of glucose. Maltose fermenting strains of Saccharomyces cerevisiae have one or more MAL loci. Each locus comprises at least three genes, MALX1 encodes the maltose transport protein, MALX2 encodes the maltase, and MALX3 encodes an activator of MALX1 and MALX2 (X denotes one of five MAL loci, with X ϭ 1, 2, 3, 4, or 6) (1). MAL gene expression in maltose-fermenting "wild-type" strains is inducible and glucose-repressed. The latter phenomenon is mediated by the main glucose repression/derepression pathway, which constitutes the transcription factor Mig1p, at least one protein kinase (Snf1p), and several other proteins whose functions are unknown (2). While the main glucose repression/derepression pathway inhibits the synthesis of the proteins involved in the first steps of maltose metabolism, the presence of glucose activates a second mechanism involved in the inactivation (also termed catabolite inactivation) of the maltose transporter but not of maltase (3-7). The inactivation seems to be due to targeting of the protein to the vacuole, where it is proteolyzed (4, 5, 8).If sugar-respiring or -derepressed (grown on nonfermentable carbon sources) cells of S. cerevisiae are fed with glucose or other rapidly fermentable sugars such as fructose or sucrose, a number of metabolic changes occur v...

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