The PP2C family comprises a group of serine threonine phosphatases that are dependent on Mg2+ or Mn2+ for their activity, are active as monomers, insensitive to known phosphatase inhibitors and share no apparent amino acid sequence homology to the other types of phosphatases (Cohen 1989, 2004). However, similarities in protein architecture suggest a similar catalytic mechanism of these enzymes to other PPP proteins, whereby a metal-bound water molecule acts as a nucleophile to directly transfer phosphate from a specific amino acid of the substrate in an acid-base catalytic reaction (Barford et al. 1998). Arabidopsis PP2Cs represent the largest known protein phosphatase family in plants with 76 discrete members (Kerk 2006; Kerk et al. 2002; Schweighofer et al. 2004) (http://www.arabidopsis.org/browse/genefamily/PP2C.jsp). Bioinfor-matics aligned this family in clusters, where cluster A contains genes predominantly associated with ABA signal transduction, such as ABI1, ABI2, AtPP2CA, HAB1, HAB2 (Schweighofer et al. 2004) and Fagus sylvatica FsPP2C1 (Gonzalez-Garcia et al. 2003). Cluster B is characterized by the phosphatases containing MAPK interaction motif (KIM) in the N-terminal extensions. KIM [(K/R)3-4X1-6(L/I)X(L/I)] is similar to the ones found in animal MAPK-interacting proteins, such as MAPK phosphatases, MAPK kinases (Farooq and Zhou 2004; Martin et al. 2005; Pulido et al. 1998) or transcription factors (Biondi and Nebreda 2003; Ho et al. 2003). Interestingly, plant PP2Cs have evolved similar MAPK interaction motifs, which otherwise are found in evolutionally unrelated animal or yeast PTP/DSP phosphatases. Ara-bidopsis AP2C1 and alfalfa MP2C regulate stress MAPK signalling. Cluster C includes the POLTERGEIST-type phosphatases (POL) that are involved in development. The most phylogenetically distant kinase-associated protein phosphatase (KAPP) represents a singleton in the PP2C family and it regulates receptor-like kinases (RLK).
One of the PP2C roles in eukaryotes is to reverse stress-induced protein kinase cascades. Below we will discuss the signalling pathways that plant PP2Cs participate in.
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