Currently 127 protein phosphatase candidates are predicted in the Arabidopsis genome (Kerk 2006; Kerk et al. 2002; Kerk, personal communication). Among them only one classical PTP, one low molecular weight LMW PTP and 23 DSP members were identified. The hallmark that defines the PTPs is a cysteine-containing signature motif HC(X)5R(S/T), which is the active site in the catalytic domain that hydrolyzes phosphor-tyrosine via a thiol phosphate intermediate (Farooq and Zhou 2004; Tonks 2005). A new group of dual-specificity tyrosine phosphatases that do not have the cysteine-containing signature but use an aspartate as a nucleophile in a metal-dependent reaction (Alonso et al. 2004), has also been recently described in plants by the Arabidopsis homologue of the Drosophila Eyes Absent (EYA) (Rayapureddi et al. 2005). Low numbers of plant PTPs compared to 107 PTPs encoded in the human genome (Alonso et al. 2004) may imply that tyrosine phosphorylation in higher plants is less common than in mammals; however, studies with anti-phosphotyrosine antibodies and tyrosyl-phosphorylation inhibitors suggested that plant proteins are phos-phorylated on tyrosine residues at levels comparable to those seen in animals (Barizza et al. 1999). Tyrosine phosphorylation is necessary for activity of mitogen-activated protein kinases (MAPKs), as shown by phosphorylation of essential TEY motif and phosphor-amino acid analysis of phosphorylated MAPKs (Gupta and Luan 2003; Huang et al. 2000; Kiegerl et al. 2000). Both ty-rosine and threonine must be phosphorylated to get the kinase into an active conformation (Canagarajah et al. 1997). Conversely, MAPKs are completely inactivated by dephosphorylation of either phosphor-amino acid residue. Thus, tyrosine-specific, dual-specificity and threonine-specific phosphatases are capable of inactivating the MAPKs (Camps et al. 2000; Cohen 2004; Fa-rooq and Zhou 2004; Martin et al. 2005).
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