Characterization of AGT1 encoding a general α‐glucoside transporter from Saccharomyces

Abstract

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.

Maltose Transport by Brewer's Yeasts in Brewer's Wort

“…Maltose transport by AGT1-encoded transporters is therefore expected to be competitively inhibited by maltotriose and trehalose. In contrast, MAL11-and MAL61-encoded transporters are highly specific for maltose 8 , and this is probably true for the other MALx1-encoded transporters (they have almost identical sequences). This suggests that the dominant maltose transporter of the ale strain is that encoded by AGT1, since trehalose causes essentially complete (well over 85%) inhibition of maltose transport by this yeast.…”

“…In addition, maltose and maltotriose compete for the same transporter in strains containing the AGT1-coded transporter, which carries several ␣-glucosides 8,24 , whereas glucose and maltose inhibit each other's transport in some strains, although they are not carried by the same transporters 3,23 . Jespersen et al 11 screened 5 ale yeasts and 25 lager yeasts and found that all contained MALx1 genes and all but one also contained AGT1 genes.…”

“…The maltose transporter encoded by AGT1 carries also other ␣-glucosides including maltotriose, sucrose and trehalose 8,24 . Maltose transport by AGT1-encoded transporters is therefore expected to be competitively inhibited by maltotriose and trehalose.…”

Expression Patterns of Genes and Enzymes Involved in Sugar Catabolism in IndustrialSaccharomyces cerevisiaeStrains Displaying Novel Fermentation Characteristics

“…Expression of the MAL11 (AGT1) gene, whose product is important for maltotriose uptake 22 , was able to be specifically detected by the probe used, and in turn, its pattern of expression monitored. Comparison of patterns of sugar utilisation and expression of sequences detectable by the MAL31 or AGT1 probes highlighted significant strain differences.…”

“…The strong expression of AGT1, especially for strains NCYC 1681 and NCYC 1309, which were capable of constitutive maltose utilisation, suggests the involvement of this gene product in the phenotype. This role is in keeping with the function of Agt1p as a transporter of ␣-glucosides, maltose and maltotriose included 22,49,54,55 . Further work will be required to determine whether attribution of this constitutive behaviour to the AGT1 gene product alone is correct.…”