By interference of yeast pheromone-induced MAPK pathway (which leads to growth arrest in yeast) with an alfalfa cDNA library the MP2C gene was isolated (Meskiene et al. 1998). MP2C inactivates the MAPK SIMK through threonine dephosphorylation of the pTEpY motif. MP2C and its Ara-bidopsis orthologue AP2C1 dephosphorylate and inactivate stress-responsive MAPKs SIMK from alfalfa and MPK6 or MPK4 from Arabidopsis, respectively. Affinity towards MAPK is mediated through functional KIM domain at the N-terminal non-catalytic part in MP2C/AP2C1. Alfalfa SIMK and Ara-bidopsis MPK6 or MPK4 interact with MP2C and AP2C1, respectively, and form protein complexes (Meskiene et al. 2003; Schweighofer et al. 2007). The catalytic activity of the phosphatases or MAPK is not imperative for their interaction in yeast, but the N-terminal part and intact KIM are prerequisites. AP2C1 regulates ethylene and modulates plant innate immunity against a fungal necrotrophic pathogen. AP2C1 also negatively regulates wound jas-monates, which correlates with enhanced resistance to herbivores in its absence. MP2C/AP2C1 is expressed in roots, flowers, young leaves and cultured suspension cells, but not in adult leaves. However, upon wounding gene expression is rapidly induced and correlates with the timing of MAPK inacti-vation, suggesting that MP2C/AP2C1 may be a part of a negative feedback responsible for resetting the MAPK signalling pathway (Bogre et al. 1997; Meskiene et al. 2003).
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