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FIG. 2. Oxidant signaling through the Sty1 pathway. In the absence of oxidative stress, Sty1 (MAPK) is in a low-activity state and localizes to the cytoplasm as a complex with Wis1, the MAPKK. The MAPKKK Wis4 exists as a complex with the Mcs4 protein in the absence of stress. Upon oxidant challenge, the Mak histidine kinases exhibit alterations in their activity, possibly due to modulation of their associated PAS domains. It is presently unknown whether the degree of kinase activity increases or decreases upon oxidant exposure. Mpr1 is then also modified in a yet-unknown fashion to associate with the Mcs4-Wis4 complex. This association likely leads to an increase in Wis4 kinase activity and triggers the sequential phosphorylations of Wis1 and Sty1. Once Sty1 is phosphorylated, it dissociates from Wis1 and moves to the nucleus, where it binds and phosphorylates Atf1. Atf1 can now activate gene expression to restore normal redox potential.
