Question

A complex signaling pathway in yeast allows the cells to accumulate high concentrations of glycerol if they are exposed to high extracellular concentration of salt or glucose. The increased osmolarity of the extracellular medium activates Ras, which in turn activates adenylate cyclase. A second pathway, the HOG (high osmolarity glycerol) pathway activates the MAP kinase pathway. The target protein is the enzyme PFK2, which is activated by phosphorylation. (PFK2 produces an allosteric regulator that activates glycolysis, which ultimately produces glycerol.) In a cell biochemistry study, yeast mutants lacking components of the HOG pathway were first exposed to high concentrations of glucose and then the PFK2 activity was measured. How does the PFK2 activity in the mutants compare when measured in isotonic and hypertonic media?

I.	Ras-cAMP pathway would not operate in the yeast mutants.
II.	Ras-cAMP pathway would potentially function in the yeast mutants.
III.	In the yeast mutants, PFK2 would be activated to similar levels in isotonic and hypotonic media.
IV.	PFK2 would not be activated in the yeast mutants.
V.	In the yeast mutants, PFK2 would only be activated in hypotonic media.
VI.	In the yeast mutants, glucose activation of glycolysis through the MAP kinase pathway would be similar in isotonic and hypotonic media.
VII.	In the yeast mutants, glucose would not activate glycolysis through the MAP kinase pathway.

Answer 1

Answer: The PFK2 activity in the mutants compare when measured in isotonic and hypertonic media:

II. Ras-cAMP pathway would potentially function in the yeast mutants.

VII. In the yeast mutants, glucose would not activate glycolysis through the MAP kinase pathway.

Explanation: Ras-cAMP pathway would be functional in the yeast mutants, which could increase PFK2 activity multifold in hypertonic media. PFK2 would inactivated in hypotonic media. HOG-MAPK pathway is essential for the expression of glycerol-3-phosphate dehydrogenase, an enzyme required for the activation of glycolysis.

Reference: Dihazi, H., Kessler, R., & Eschrich, K. (2004). High osmolarity glycerol (HOG) pathway-induced phosphorylation and activation of 6-phosphofructo-2-kinase are essential for glycerol accumulation and yeast cell proliferation under hyperosmotic stress. Journal of Biological Chemistry, 279(23), 23961-23968.

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