Study on the mechanism to regulate the yeast osmo-regulatory MAPK pathway activation via phosphorylation and dephosphorylation

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Membrane localization of the Ste11 MAPKKK is essential for activation of both the filamentous growth/invasive growth (FG/IG) MAP kinase (MAPK) pathway and the SHO1 branch of the osmoregulatory HOG MAPK pathway, and is mediated by binding of the Ste50 scaffold protein to the Opy2 membrane anchor. We found that Opy2 has two major (CR-A and CR-B), and a minor (CR-D), binding sites for Ste50. CR-A binds Ste50 constitutively, and can transmit signals to both the HOG and the FG/IG pathways. CR-B, in contrast, binds Ste50 only when Opy2 is phosphorylated by Yck1/Yck2 under glucose-rich conditions, and transmits the signal preferentially to the HOG pathway. Ste50 phosphorylation by the MAPKs activated by the HOG or the mating pathway dissociates Ste50 from Opy2, thereby preventing excessive activation of the HOG pathway, or reduces the basal activity of the mating MAPK pathway. Thus, dynamic regulation of Ste50-Opy2 interaction fine-tunes the MAPK signaling network.