Temperature-induced expression of yeast FKS2 is under the dual control of protein kinase C and calcineurin.

FKS1 and FKS2 are alternative subunits of the glucan synthase complex, which is responsible for synthesizing 1,3-beta-glucan chains, the major structural polymer of the Saccharomyces cerevisiae cell wall. Expression of FKS1 predominates during growth under optimal conditions. In contrast, FKS2 expression is induced by mating pheromone, high extracellular [Ca2+], growth on poor carbon sources, or in an fks1 mutant. Induction of FKS2 expression in response to pheromone, CaCl2, or loss of FKS1 function requires the Ca2+/calmodulin-dependent protein phosphatase calcineurin. Therefore, a double mutant in calcineurin (CNB1) and FKS1 is inviable due to a deficiency in FKS2 expression. To identify novel regulators of FKS2 expression, we isolated genes whose overexpression obviates the calcineurin requirement for viability of an fks1 mutant. Two components of the cell integrity signaling pathway controlled by the RHO1 G protein (MKK1 and RLM1) were identified through this screen. This signaling pathway is activated during growth at moderately high temperatures. We demonstrate that calcineurin and the cell integrity pathway function in parallel, through separable promoter elements, to induce FKS2 expression during growth at 39 degrees C. Because RHO1 also serves as a regulatory subunit of the glucan synthase, our results define a regulatory circuit through which RHO1 controls both the activity of this enzyme complex and the expression of at least one of its components. We show also that FKS2 induction during growth on poor carbon sources is a response to glucose depletion and is under the control of the SNF1 protein kinase and the MIG1 transcriptional repressor. Finally, we show that FKS2 expression is induced as cells enter stationary phase through a SNF1-, calcineurin-, and cell integrity signaling-independent pathway.