Galactose-dependent reversible interaction of Gal3p with Gal80p in the induction pathway of Gal4p-activated genes of Saccharomyces cerevisiae

When galactose is added to logarithmically growing culture of the yeast Saccharomyces cerevisiae, a set of genes encoding galactose-metabolizing enzymes (GAL genes) starts to be transcribed within a few minutes. This rapid induction involves a serial interplay of Gal3p, Gal80p, and Gal4p. Recent experiments have indicated that a direct interaction between Gal3p and Gal80p plays a pivotal role in an early step of GAL induction. Here we demonstrate that complex of Gal3p and Gal80p, otherwise unstable, is stabilized in the presence of 0.1 mM galactose and 0.5 mM ATP. The requirement for galactose and ATP for stable complex formation is also observed by using highly purified Gal3p and Gal80p from yeast. We further show that thus formed Gal3p/Gal80p complex can easily be dissociated when it is washed with buffer lacking galactose. Finally, we show that mutant proteins encoded by GAL80^S or GAL80DE21, which confer galactose-uninducible phenotype, fail to interact with Gal3p. These results strongly suggest that Gal3p functions as the sensor and transducer of galactose signal in the induction pathway of Gal4p-activated genes.