Induction and catabolite repression of alpha-glucosidase synthesis in protoplasts of Saccharomyces carlsbergensis

Inhibition by 2-deoxy-D-glucose of synthesis of glycoprotein enzymes by protoplasts of Saccharomyces: relation to inhibition of sugar uptake and metabolism

The synthesis of the glycoprotein enzymes, invertase and acid phosphatase, by protoplasts of Saccharomyces mutant 1016, is inhibited by 2-deoxy-d-glucose (2-dG) after a 20- to 30-min lag period under conditions (external sugar to 2-dG ratio of 40:1) which cause only a slight decrease in total protein synthesis. Formation of one intracellular enzyme, alpha-glucosidase, is also sensitive, but production of another, alkaline phosphatase, is unaffected. A nonmetabolized glucose analogue, 6-deoxy-d-glucose, had no inhibitory effect. The total uptake of external fructose and maltose was decreased by 2-dG after a lag period of about the same duration as that before the inhibition of synthesis of enzymes or of mannan and glucan; during this time 2-dG was taken up by the protoplasts and accumulated primarily as 2-dG-6-phosphate (2-dG-6-P). Studies in vitro showed that 2-dG-6-P inhibits both yeast phosphoglucose isomerase and phosphomannose isomerase. The intracellular levels of the 6-phosphates of glucose, fructose, and mannose did not increase in the presence of 2-dG. We suggest that the high internal level of 2-dG-6-P blocks synthesis of the cell wall polysaccharides and glycoproteins in two ways. It directly inhibits the conversion of fructose-6-P to glucose-6-P and to mannose-6-P. At the same time, it restricts the transport of fructose and maltose into the cell; however, the continuing limited uptake of the sugars still provides sufficient energy for protein synthesis. The cessation of alpha-glucosidase synthesis is probably a result of depletion of the internal pool of maltose (the inducer). Our findings support the suggestion that restriction of synthesis of the carbohydrate moiety of glycoproteins reduces formation of the active enzyme.

Induction of galactokinase in Saccharomyces cerevisiae: kinetics of induction and glucose effects

The induced synthesis of galactokinase and the repressing effects of glucose on this synthesis have been investigated in whole yeast cells rendered permeable by treatment with dimethyl sulfoxide. It was found that the induction response of uninduced cells to galactose is clearly dependent on the nature of the carbon source upon which the culture was grown prior to exposure to galactose. Glucose-grown cells exhibited a long lag before induction, whereas lactate-grown cells exhibited induced synthesis within 8 min. A concentration of 0.5% galactose was found to be optimal for induction. The addition of glucose to yeast cultures growing on galactose resulted in a severe transient repression of synthesis which was followed by a resumed rate of synthesis characteristic of a weaker permanent catabolite repression. Neither 2-deoxygalactose nor fucose acted as gratuitous inducers of the pathway, nor did they serve as a substrates for galactokinase.

Differential sensitivities of the two malate dehydrogenases and the maltose permease to the effect of glucose in Saccharomyces carlsbergensis

In Saccharomyces carlsbergensis the two malate dehydrogenase activities, which are localized in different compartments of the cell, were found to differ in their response to glucose. The cytoplasmic malate dehydrogenase activity appears to be sensitive to inactivation by very low concentrations of glucose. The mitochondrial malate dehydrogenase activity is only repressed at a higher glucose concentration. Maltose permease is also sensitive to inactivation by glucose. Conditions were found such that the maltose permease was present while the cytoplasmic malate dehydrogenase was inactivated. The different sensitivities of the two malate dehydrogenases and maltose permease to the effect of glucose may explain the preferential use of glucose, maltose, and products of glucose metabolism (2- and 3-carbon skeletons) as carbon sources for growth in the order as mentioned.

Osmotic regulation of invertase formation and secretion by protoplasts of Saccharomyces

Osmotic regulation of invertase formation and secretion by protoplasts of Saccharomyces was examined using sorbitol, KCl, NaCl, or magnesium sulfate as the osmotic support. The synthesis and secretion of the enzyme is remarkably sensitive to the osmolarity of the supporting medium irrespective of the particular support employed. Invertase formation was inhibited at high osmolarity and was maximal at 0.65 to 0.75 osmolal, even though some leakage of the intracellular enzyme alpha-glucosidase and of ultraviolet (UV)-absorbing materials occurred under these conditions. The reduction of invertase formation and secretion due to high osmolarity was eliminated promptly when protoplasts were transferred into a medium of lower osmolarity. The rate of fructose uptake and of threonine incorporation into protein was decreased by high osmolarity; also reduction of invertase formation could be partially reversed by increasing the level of sugar supplied as energy source. Thus changes in the permeability of the plasma membrane (and presumably also in its structure) are important factors in the response of protoplasts to high osmolarity, though certainly not the complete explanation. Protoplasts suspended in 0.8 m sorbitol, with 10mm fructose as the energy source, increased their invertase level 5- to 10-fold during a 2-hr incubation without substantial release of alpha-glucosidase or UV-absorbing materials. Both the large and small forms of invertase were present in the protoplasts, but only the large form was released into the medium when enzyme was being actively synthesized. Formation and secretion of newly formed invertase and the release of enzyme initially present were inhibited by cycloheximide.