In a "two-component system," extracellular stimuli are transmitted by the transfer of a phosphoryl group from a sensor histidine kinase to a response regulator (RR), a mechanism referred to as phosphorelay. In the fission yeast Schizosaccharomyces pombe, peroxide stress signals are transmitted by phosphorelay to the Mcs4 RR, which activates the Spc1 MAP kinase (MAPK) cascade. We previously demonstrated that a glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) physically interacts with Mcs4 and promotes phosphorelay signaling to Mcs4. Independently of the phosphorelay mechanism, Mcs4 also plays a critical role in osmostress signaling, as a part of the stable ternary complex with the Wis4 and Win1 MAPK kinase kinases (MAPKKKs). Interestingly, GAPDH dissociates from Mcs4 upon osmostress, while oxidative stress promotes their association. The Mcs4 RR may serve as a switching hub that mediates activation of the Wis4-Win1 MAPKKK heteromer in response to different forms of stress. 