Molecular genetic analysis is used to characterize the AGT1 gene encoding an alpha-glucoside transporter. AGT1 is found in many Saccharomyces cerevisiae laboratory strains and maps to a naturally occurring, partially functional allele of the MAL1 locus. Agt1p is a highly hydrophobic, postulated integral membrane protein. It is 57% identical to Mal61p, the maltose permease encoded at MAL6, and is also a member of the 12 transmembrane domain superfamily of sugar transporters. Like Mal61p, Agt1p is a high-affinity, maltose/proton symporter, but Mal61p is capable of transporting only maltose and turanose, while Agt1p transports these two alpha-glucosides as well as several others including isomaltose, alpha-methylglucoside, maltotriose, palatinose, trehalose and melezitose. AGT1 expression is maltose inducible and induction is mediated by the Mal-activator. The sequence of the upstream region of AGT1 is identical to that of the maltose-inducible MAL61 gene over a 469 bp region containing the UASMAL but the 315 bp sequence immediately upstream of AGT1 shows no significant homology to the sequence immediately upstream of MAL61. The evolutionary origin of the MAL1 allele to which AGT1 maps and the relationship of AGT1 to other alpha-glucoside fermentation genes is discussed.

The addition of glucose to maltose-fermenting Saccharomyces cerevisiae cells causes a rapid and irreversible loss of the ability to transport maltose, resulting both from the repression of transcription of the maltose permease gene and from the inactivation of maltose permease. The latter is referred to as glucose-induced inactivation or catabolite inactivation. We describe an analysis of this process in a maltose-fermenting strain expressing a hemagglutinin (HA)-tagged allele of MAL61, encoding maltose permease. The transfer of maltoseinduced cells expressing the Mal61/HA protein to rich medium containing glucose produces a decrease in maltose transport rates which is paralleled by a decrease in Mal61/HA maltose permease protein levels. In nitrogen starvation medium, glucose produces a biphasic inactivation, i.e., an initial, rapid loss in transport activity (inhibition) followed by a slower decrease in transport activity, which correlates with a decrease in the amount of maltose permease protein (proteolysis). The inactivation in both rich and nitrogen-starved media results from a decrease in V max with no apparent change in K m . Using strains carrying mutations in END3, REN1(VPS2), PEP4, and PRE1 PRE2, we demonstrate that the proteolysis of Mal61/HAp is dependent on endocytosis and vacuolar proteolysis and is independent of the proteosome. Moreover, we show that the Mal61/HA maltose permease is present in differentially phosphorylated forms.The addition of glucose to maltose-fermenting Saccharomyces cerevisiae cells causes an irreversible loss of the ability to transport maltose which is more rapid than can be explained by the combined effects of glucose repression of maltose permease gene transcription and cell growth (5,8,16). Similar irreversible inactivation of the high-affinity galactose and glucose transporters and of the gluconeogenic enzymes has been described, and the phenomenon is generally referred to as glucose-induced inactivation or catabolite inactivation (6,13,21,33,37,39,40).Early studies of the mechanism of glucose-induced inactivation of the gluconeogenic enzymes demonstrated glucose-stimulated proteolysis of these cytoplasmic enzymes (35,36,47). The mechanism of fructose-1,6-biphosphatase inactivation has been investigated most extensively. Here, two distinct processes are stimulated in response to glucose: an initial, rapid, reversible phosphorylation, which is correlated with a decrease in enzyme activity but not protein levels, and a slower, irreversible proteolysis (10,34,47). The pathway of the proteolysis is as yet unresolved, with conflicting published reports of both vacuole-dependent and proteosome-dependent proteolysis (10,11,44).MAL61 of the MAL6 locus and its nearly identical homologs at the other MAL loci (MAL11, MAL21, MAL31, and MAL41) encode Saccharomyces maltose permease. Mal61p is a highaffinity (2 to 4 mM) proton/maltose symporter and is a member of the 12-transmembrane-domain family of sugar transporters (8, 9). Expression of MAL61 is maltose induced and glucose repres...

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