The Saccharomyces cerevisiae type 2C protein phosphatase Ptc1 is required for a wide variety of cellular functions, although only a few cellular targets have been identified. A genetic screen in search of mutations in protein kinase-encoding genes able to suppress multiple phenotypic traits caused by the ptc1 deletion yielded a single gene, MKK1, coding for a MAPK kinase (MAPKK) known to activate the cell-wall integrity (CWI) Slt2 MAPK. In contrast, mutation of the MKK1 paralog, MKK2, had a less significant effect. Deletion of MKK1 abolished the increased phosphorylation of Slt2 induced by the absence of Ptc1 both under basal and CWI pathway stimulatory conditions. We demonstrate that Ptc1 acts at the level of the MAPKKs of the CWI pathway, but only the Mkk1 kinase activity is essential for ptc1 mutants to display high Slt2 activation. We also show that Ptc1 is able to dephosphorylate Mkk1 in vitro. Our results reveal the preeminent role of Mkk1 in signaling through the CWI pathway and strongly suggest that hyperactivation of Slt2 caused by upregulation of Mkk1 is at the basis of most of the phenotypic defects associated with lack of Ptc1 function.KEYWORDS synthetic genetic interactions; cell-wall integrity pathway; protein dephosphorylation; Saccharomyces cerevisiae P ROTEIN kinases play an essential role in nearly every aspect of cell physiology, and the activity of these key players is frequently modulated by phosphorylation. Therefore, protein phosphatases (PPases) are important regulators of protein kinases and cellular homeostasis. Among them, type 2C Ser/Thr PPases constitute an evolutionarily conserved group that, in contrast to other PPase families, are monomeric enzymes apparently lacking regulatory subunits. In the yeast Saccharomyces cerevisiae, seven members (Ptc1-Ptc7) have been identified and at least partially characterized [see Arino et al. (2011) for a review].Ptc1 is the closest homolog of human Wip1, a phosphatase involved in the regulation of stress-induced and DNA damage-induced networks in diverse physiologic and pathologic conditions (Le Guezennec and Bulavin 2010;Zhu and Bulavin 2012) and by far the most widely studied yeast isoform. Both the large number of characteristic phenotypes and the specific changes in the transcriptomic profile (Gonzalez et al. 2006) derived from deletion of the gene suggest that this phosphatase is involved in a large variety of cellular processes not shared by other Ptc family members. Early evidence indicated that Ptc1 was involved in the negative regulation of the high-osmolarity glycerol (HOG) pathway (Maeda et al. 1993;Maeda et al. 1994), and subsequent work demonstrated that Ptc1 could dephosphorylate the Hog1 MAPK in vitro and in vivo (Warmka et al. 2001 interacts with the N-terminal domain of Nbp2, an SH3 domaincontaining protein that serves as an adaptor for the recruitment of Ptc1 to the Pbs2-Hog1 complex, and this interaction is necessary for Ptc1 to participate in the regulation of HOGmediated signaling (Uetz et al. 2000;Ito et al. 2001;Ma...

Background: Dual specificity protein phosphatases (DSPs) bind MAPKs through specific interaction motifs.Results: A novel motif (IYT) in Msg5 mediates a common docking domain-independent interaction with the yeast cell integrity kinase Slt2.Conclusion: Distinct mechanisms allow Msg5 to differentially bind Slt2 and Mlp1 or mating/pseudohyphal MAPKs Fus3 and Kss1.Significance: Elucidation of the mechanisms by which MAPKs interact with key regulators is vital to understanding cell signaling.