Identification of putative negative regulators of yeast signaling through a screening for protein phosphatases acting on cell wall integrity and mating MAPK pathways.

The lack of signaling through MAPK pathways leads to a defective cellular response to the corresponding stimulus, but an improper hyperactivation of these routes results in deleterious effects as well. Protein phosphorylation is an activating modification for signal transmission through components of MAPK pathways and thus, protein phosphatases are key negative regulators of these cellular routes by limiting excessive signaling activity. However, in contrast to most of the protein kinases operating in MAPK pathways, protein phosphatases usually exhibit redundancy and promiscuity, which has limited the identification of their function. In order to identify new putative phosphatases operating in Saccharomyces cerevisiae MAPK signaling, we have taken advantage of growth inhibition promoted by overproduction of constitutively active components of the mating and cell wall integrity (CWI) pathways to perform a screen with a collection of 43 protein phosphatases or phosphatase-regulatory proteins. The phosphatases able to alleviate the induced growth inhibition when overproduced were further studied by testing their capacity to downregulate expression of mating and CWI responsive promoters and the consequences of their removal on MAPK signaling. Epistasis analysis placed the Ser/Thr protein phosphatase Ppq1 as a regulator of the mating MAPK module downstream the MAPKKK Ste11. The dual specificity phosphatase Yvh1 was found to be important for the maintenance of cell wall integrity and appropriate signaling through the CWI pathway. Moreover, we have found that Ptc2 and Ptc4 bind to the CWI MAPK Slt2. Together with known phosphatases of the mating and CWI pathway, as Msg5 or Ptp2, other putative negative regulators of both pathways that came up in the screening were Ptc2, Oca2 and Ptp1. We show that Ptp1 physically interacts with Slt2 and the mating MAPK Fus3. Elimination of Ptp1 results in increased signaling through these pathways, suggesting that this tyrosine phosphatase, like Ptp2 and Ptp3, plays a downregulatory role on both MAPKs.

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