Glucose-induced degradation of the MDH2 isozyme of malate dehydrogenase in yeast.

MDH2, the nonmitochondrial isozyme of malate dehydrogenase in Saccharomyces cerevisiae, was determined to be a target of glucose-induced proteolytic degradation. Shifting a yeast culture growing with acetate to medium containing glucose as a carbon source resulted in a 25-fold increase in turnover of MDH2. A truncated form of MDH2 lacking amino acid residues 1-12 was constructed by mutagenesis of the MDH2 gene and expressed in a haploid yeast strain containing a deletion disruption of the corresponding chromosomal gene. Measurements of malate dehydrogenase specific activity and determination of growth rates with diagnostic carbon sources indicated that the truncated form of MDH2 was expressed at authentic MDH2 levels and was fully active. However, the truncated enzyme proved to be less susceptible to glucose-induced proteolysis, exhibiting a 3.75-fold reduction in turnover rate following a shift to glucose medium. Rates of loss of activity for other cellular enzymes known to be subject to glucose inactivation were similarly reduced. An extended lag in attaining wild type rates of growth on glucose measured for strains expressing the truncated MDH2 enzyme represents the first evidence of a selective advantage for the phenomenon of glucose-induced proteolysis in yeast.