GAL2 encodes a high affinity galactose permease that is also able to transport glucose (5, 4). Gal2p is an integral plasma membrane protein belonging to a superfamily of sugar transporters that are predicted to contain 12 transmembrane domains separated by charged residues (6). Structurally and functionally similar sugar transporters have been identified in bacteria, rat, and humans (6, 7). Gal2p is integral to the process of galactose catabolism, and loss of its permease activity renders cells unable to utilize galactose as a sole carbon source (1).

All the galactose structural genes (GAL1, GAL10, GAL7, GAL2) are coordinately regulated at the level of transcription in response to galactose by Gal4p, Gal80p, and Gal3p (8, 9, and reviewed in 10). Regardless of carbon source, the Gal4p transcriptional activator is bound as a dimer to upstream activation sites found in the promoters of the GAL genes. In the presence of galactose, Gal3p sequesters the transcriptional repressor Gal80p in the cytoplasm, thereby relieving inhibition of Gal4p and resulting in GAL gene expression (11). In the absence of galactose, Gal80p remains bound as a dimer, to Gal4p, preventing Gal4p from recruiting other factors of the Pol II transcription machinery (reviewed in 10).

When cells are grown on glucose, GAL2 is negatively regulated by catabolite repression at both the levels of transcription and protein degradation. Downregulation of GAL2 expression is mediated by Sip1p, a subunit of the Snf1p kinase complex (12). Grr1p-dependent protein degradation of Gal2p occurs through ubiquitination at several lysine residues, followed by endocytosis and vacuolar proteolysis (13, 14, 15).

Last updated: 2006-10-12