Galactokinase Encoded by GAL1 Is a Bifunctional Protein Required for Induction of the GAL Genes in Kluyveromyces lactis and Is Able To Suppress the gal3 Phenotype in Saccharomyces cerevisiae

Abstract

We have analyzed a GAL1 mutant (gal1-r strain) of the yeast Kluyveromyces lactis which lacks the induction of beta-galactosidase and the enzymes of the Leloir pathway in the presence of galactose. The data show that the K. lactis GAL1 gene product has, in addition to galactokinase activity, a function required for induction of the lactose system. This regulatory function is not dependent on galactokinase activity, as it is still present in a galactokinase-negative mutant (gal1-209). Complementation studies in Saccharomyces cervisiae show that K. lactis GAL1 and gal1-209, but not gal1-r, complement the gal3 mutation. We conclude that the regulatory function of GAL1 in K. lactis soon after induction is similar to the function of GAL3 in S. cerevisiae.

Regulation of primary carbon metabolism in Kluyveromyces lactis

“…The K. lactis proteins Gal1p, a galactokinase, and Gal80p, an inhibitor of Gal4p have been instrumental in showing, for the first time, that activation of Gal4p is caused by galactose-and ATP-dependent Gal1p-Gal80p interaction that relieves Gal80p-mediated inhibition [72]. This regulatory activity of KlGal1p is separable by mutation from its enzymatic activity [73].…”

“…The reason for this difference lies again in a redundancy in S. cerevisiae that is not found in K. lactis. In S. cerevisiae two related proteins Gal3p and ScGal1p fulfil the same regulatory function as KlGal1p whereas no Gal3p homolog exists in K. lactis [73]. ScGal1p is less efficient than Gal3p or KlGal1p in activating ScGal4p explaining why gal3 mutants show a delayed induction.…”

Yeast galactokinase in closed conformation can switch between catalytic and signal transducer states

“…S.cerevisiae GAL 1 ( ScGAL1 ) encoded galactokinase ( Sc Gal1p),the first enzyme of Leloir pathway of galactose metabolism, is one of the earliest proteins to be identified as moonlighting [27, 28, 29]. The canonical biochemical activity of Sc Gal1p is to catalyse the phosphorylation of galactose using ATP as the phosphate donor, resulting in the formation of galactose 1-phosphate [30].…”

“…The second biochemical activity is to sequester Sc Gal80p, the repressor of the GAL switch, in response to galactose and ATP [26, 31], which serve as ligands for allosteric activation of Sc Gal1p. The function of this biochemical activity is to alleviate the Sc Gal80p mediated transcriptional repression, thereby allowing Sc Gal4p to activate the transcription of galactose metabolizing genes [27, 28, 29].…”

“…Over the past three decades, several independent studies have introduced substitutions in ScGAL1 [29, 40], ScGAL3 [41] and KlGAL1 ( K. lactis GAL1 , an orthologue of ScGAL1 in Kluyveromyces lactis is also a moonlighting protein and has been studied extensively) [28, 42] to understand the mechanistic basis of their biochemical activities (see Table 1 for details). However, these studies do not shed light on how galactose and ATP serve as substrates for catalysis as well as ligands for allosteric activation of Sc Gal1p.…”