Perturbation of the interaction between Gal4p and Gal80p of the Saccharomyces cerevisiae GAL switch results in altered responses to galactose and glucose

In S. cerevisiae, following the Whole Genome Duplication (WGD), GAL1‐encoded galactokinase retained its signal transduction function but lost basal expression. On the other hand, its paralogue GAL3, lost kinase activity but retained its signalling function and basal expression, thus making it indispensable for the rapid induction of the S. cerevisiae GAL switch. However, a gal3Δ strain exhibits delayed growth kinetics due to the redundant signalling function of GAL1. The subfunctionalization between the paralogues GAL1 and GAL3 is due to expression divergence and is proposed to be due to the alteration in the Upstream Activating Sequences (UAS_G). We demonstrate that the GAL switch becomes independent of GAL3 by altering the interaction between Gal4p and Gal80p without altering the configuration of UAS_G. In addition to the above, the altered switch of S. cerevisiae loses ultrasensitivity and stringent glucose repression. These changes caused an increase in fitness in the disaccharide melibiose at the expense of a decrease in fitness in galactose. The above altered features of the ScGAL switch are similar to the features of the GAL switch of K. lactis that diverged from S. cerevisiae before the WGD.     

Analysis of the galactose signal transduction pathway in Saccharomyces cerevisiae: interaction between Gal3p and Gal80p.

The GAL3 gene plays a critical role in galactose induction of the GAL genes that encode galactose- metabolizing enzymes in Saccharomyces cerevisiae. Defects in GAL3 result in a long delay in GAL gene induction, and overproduction of Gal3p causes constitutive expression of GAL. Here we demonstrate that concomitant overproduction of the negative regulator, Gal80p, and Gal3p suppresses this constitutive GAL expression. This interplay between Gal80p and Gal3p is direct, as tagged Gal3p coimmunoprecipitated with Gal80p. The amount of coprecipitated Gal80p increased when GAL80 yeast cells were grown in the presence of galactose. When both GAL80 and GAL3 were overexpressed, the amount of coprecipitated Gal80p was not affected by galactose. Tagged gal3 mutant proteins bound to purified Gal80p, but only poorly in comparison with the wild type, suggesting that formation of the Gal80p-Gal3p complex depends on the normal function of Gal3p. Gal3p appeared larger in Western blots (immunoblots) than predicted by the published nucleic acid sequence. Reexamination of the DNA sequence of GAL3 revealed several mistakes, including an extension at the 3' end of another predicted 97 amino acids.     

Autoregulation of regulatory proteins is key for dynamic operation of GAL switch in Saccharomyces cerevisiae

Autoregulation and nucleocytoplasmic shuttling play important roles in the operation of the GAL regulatory system. However, the significance of these mechanisms in the overall operation of the switch is unclear. In this work, we develop a dynamic model for the GAL system and further validate the same using steady-state and dynamic experimental expression data. Next, the model is used to delineate the relevance of shuttling and autoregulation in response to inducing, repressing, and non-inducing-non-repressing media. The analysis indicates that autoregulation of the repressor, Gal80p, is key in obtaining three distinct steady states in response to the three media. In particular, the analysis rationalizes the intuitively paradoxical observation that the concentration of repressor, Gal80p, actually increases in response to an increase in the inducer concentration. On the other hand, although nucleocytoplasmic shuttling does not affect the dynamics of the system, it plays a dominant role in obtaining a sensitive response to galactose. The dynamic model was also used to obtain insights on the preculturing effect on the system behavior.     

Identification of two proteins encoded by the Saccharomyces cerevisiae GAL4 gene.

We placed the Saccharomyces cerevisiae GAL4 gene under control of the galactose regulatory system by fusing it to the S. cerevisiae GAL1 promoter. After induction with galactose, GAL4 is now transcribed at about 1,000-fold higher levels than in wild-type S. cerevisiae. This regulated high-level expression has enabled us to tentatively identify two GAL4-encoded proteins.     

Quantitative analysis of GAL genetic switch of Saccharomyces cerevisiae reveals that nucleocytoplasmic shuttling of Gal80p results in a highly sensitive response to galactose

The nucleocytoplasmic shuttling of the repressor Gal80p is known to play a pivotal role in the signal transduction process of GAL genetic switch of Saccharomyces cerevisiae (Peng, G., and Hopper, J. E. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 8548-8553). We have developed a comprehensive model of this GAL switch to quantify the expression from the GAL promoter containing one or two Gal4p-binding sites and to understand the biological significance of the shuttling process. Our experiments show that the expression of proteins from the GAL promoter containing one and two binding sites for Gal4p is ultrasensitive (a steep response to a given input). Furthermore, the model revealed that the shuttling of Gal80p is the key step in imparting ultrasensitive response to the inducer. During induction, free Gal80p concentration is altered by sequestration, without any change in the distribution coefficient across the nuclear membrane. Furthermore, the estimated concentrations of Gal80p and Gal3p allow basal expression of alpha-galactosidase, but not beta-galactosidase, from the GAL promoter containing one and two binding sites for Gal4p, respectively. Conversely, the expression from genes with two binding sites is more sensitive to inducer concentration as compared with one binding site. We show that autoregulation of Gal80p is coincidental to the autoregulation of Gal3p, and it does not impart ultrasensitivity. We conclude from our analysis that the ultrasensitivity of the GAL genetic switch is solely because of the shuttling phenomena of the repressor Gal80p across the nuclear membrane.     

Mutational hypersensitivity of a gene regulatory protein: Saccharomyces cerevisiae Gal80p

The inhibitor of galactose catabolic (GAL) gene expression in Saccharomyces cerevisiae, Gal80p, interacts with the activator Gal4p and the signal transducer Gal3p and self-associates. Selection for loss of Gal80p inhibitor function yielded gal80 mutants at an extremely high rate. Out of these, 21 nonoverlapping point mutants were identified; each were due to a single-amino-acid exchange in conserved residues. Semiquantitative biochemical analysis of the corresponding mutant proteins revealed that each of the 21 amino acid alterations caused simultaneous defects in every single protein-protein interaction and in Gal80's structural integrity. Thus, Gal80 provides an unprecedented example for a protein's structural sensitivity to minimal sequence alterations.     

Iron requirement for GAL gene induction in the yeast Saccharomyces cerevisiae

Iron is an essential nutrient. Its deficiency hinders the synthesis of ATP and DNA. We report that galactose metabolism is defective when iron availability is restricted. Our data support this connection because 1) galactose-mediated induction of GAL promoter-dependent gene expression was diminished by iron limitation, and 2) iron-deficient mutants grew slowly on galactose-containing medium. These two defects were immediately corrected by iron replacement. Inherited defects in human galactose metabolism are characteristic of the disease called galactosemia. Our findings suggest that iron-deficient galactosemic individuals might be more severely compromised than iron-replete individuals. This work shows that iron homeostasis and galactose metabolism are linked with one another.     

GAL3 gene product is required for maintenance of the induced state of the GAL cluster genes in Saccharomyces cerevisiae.

The activities of the first three enzymes for galactose catabolism normally become detectable within 15 min after the addition of galactose into a culture of the yeast Saccharomyces cerevisiae. In S. cerevisiae with a recessive mutation termed gal3, a longer-than-normal lag is observed before the appearance of the enzyme activities (O. Winge and C. Roberts, C. R. Trav. Lab. Carlsberg Ser. Physiol. 24:263-315, 1948). I isolated two S. cerevisiae mutants with temperature-sensitive defects in the GAL3 gene. Temperature shift experiments with one of those mutants led to the conclusion that the GAL3 function is required not only for the initiation of enzyme induction but also for the maintenance of the induced state in galactose-nonfermenting S. cerevisiae because of a defect in any of the genes for the galactose-catabolizing enzymes, such as gal1 or gal10. In contrast, the GAL3 function is phenotypically dispensable in galactose-metabolizing S. cerevisiae. Thus, the normal catabolism of galactose can substitute for the GAL3 function.