Deviation of carbohydrate metabolism by the SIT4 phosphatase in Saccharomyces cerevisiae

A prominent phenotype of the yeast sit4 mutant, which lacks the Ser-Thr phosphatase Sit4, is hyper-accumulation of glycogen and the failure to grow on respiratory substrates. We investigated whether these two phenotypes are linked by studying the metabolic response to SIT4 deletion. Although the sit4 mutant failed to grow on respiratory substrates, in the exponential growth, phase respiration was de-repressed; active respiration was confirmed by measuring oxygen consumption and NADH generation. However, the fermentation rate and the internal glucose 6-phosphate and pyruvate levels were reduced, while glycogen content was high. Respiro-fermentative and respiratory substrates such as galactose, glycerol and ethanol were directed toward glycogen synthesis, which indicates that sit4 mutant deviates metabolism to glycogenesis by activating a glycogen futile cycle and depleting cells of Krebs cycle intermediates. An important feature of the sit4 mutant was the lack of growth under anaerobic conditions, suggesting that respiration is necessary to meet the energy requirements of the cell. Addition of aspartic acid, which can restore Krebs cycle intermediates, partially restored growth on ethanol. Our findings suggest that inhibition of Sit4 activity may be essential for redirecting carbohydrate flux to gluconeogenesis and glycogen storage.