MP2C, A PLANT PROTEIN PHOSPHATASE 2C, FUNCTIONS AS A NEGATIVE REGULATOR OF MITOGEN-ACTIVATED PROTEIN-KINASE PATHWAYS IN YEAST AND PLANTS

By interference of the yeast pheromone mitogen-activated protein kinas e (MAPK) pathway with an alfalfa cDNA expression library, we have isol ated the MP2C gene encoding a functional protein phosphatase type 2C. Epistasis analysis in yeast indicated that the molecular target of the MP2C phosphatase is Ste11, a MAPK kinase kinase that is a central reg ulator of the pheromone and osmosensing pathways. In plants, MP2C func tions as a negative regulator of the stress-activated MAPK (SAMK) path way that is activated by cold, drought, touch, and wounding. Although activation of the SAMK pathway occurs by a posttranslational mechanism , de novo transcription and translation of protein factor(s) are neces sary for its inactivation. MP2C is likely to be this or one of these f actors, because wound-induced activation of SAMK is followed by MP2C g ene expression and recombinant glutathione S-transferase-MP2C is able to inactivate extracts containing wound-induced SAMK. Wound-induced MP 2C expression is a transient event and correlates with the refractory period, i.e., the time when restimulation of the SAMK pathway is not p ossible by a second stimulation. These data suggest that MP2C is part of a negative feedback mechanism that is responsible for resetting the SAMK cascade in plants.