In Arabidopsis, several dual-specificity phosphatases (DSPs) were described to control plant growth under stress conditions. A gene disruption of mitogen-activated protein kinase phosphatase MKP1 in Arabidopsis (mkpl) results in plant hypersensitivity to stress by UV-C and methyl methanesul-fonate (Ulm et al. 2001). These factors activate repair mechanisms of DNA damage and cell recovery. UV-C activates the Arabidopsis MAPK, MPK6 but not MPK4. Though MPK6, MPK3 and MPK4 were all found to interact with MKP1 in yeast two hybrid assays, only the activity of MPK6 was effected in the mkpl mutant plants, suggesting that only MPK6 is a substrate for MKP1 in plants. The role of MKP1 in salt stress responses was demonstrated by expression profiling of wild-type vs. mkpl mutant lines and increased resistance to salinity of mkpl plants, both implicating functions for MKP1 in regulation of plant responses to environmental challenges (Ulm et al. 2002).
MKP1 contains the conserved catalytic part and a long carboxy-terminal extension with a domain similar to the actin-binding protein gelsolin, suggesting an association with the regulation of the cytoskeleton. Calmodulin (CaM) binding domain and interaction with CaM was identified in the or-thologue of Arabidopsis MKP1 from tobacco NtMKP1 (Yamakawa et al. 2004). Its expression is regulated in response to cell death, pathogen infection and wounding. Wound-activation of the MAP kinase SIPK and cell death is compromised by NtMKP1 overexpression in tobacco. NtMKP1 binds SIPK through the N-terminal non-catalytic region. This binding strongly increases phosphatase activity and is partially dependent on the putative MAPK common docking domain of SIPK (Katou et al. 2005). Another Ara-bidopsis dual-specificity phosphatase DsPTP1 hydrolyzes both phosphor-threonine and phosphor-tyrosine and dephosphorylates/inactivates MPK4 in vitro. Phosphatase activity of DsPTP1 depends on the conserved catalytic cys-
teine (Gupta et al. 1998). DsPTPl contains two Ca2+-dependent CaM-binding domains, and binding to CaM inhibits its activity to dephosphorylate tyrosine on myelin basic protein (MBP) (Yoo et al. 2004).
Another DSP from Arabidopsis, PHS1 (propyzamide hypersensitive) controls microtubule organization and embryonic development (Naoi and Hashimoto 2004), as well as plant growth under stress conditions and regulation of ABA signalling (Quettier et al. 2006). The semidominant phs1-1 mutation, which has a substitution of Arg to Cys amino acid in a putative MAPK interaction motif (KIM) in the N-terminal part of the protein, disrupts microtubule organization in roots. Similar KIM is present in MKPs from mammals and mediates binding to MAPKs (Farooq and Zhou 2004; Martin et al. 2005; Pulido et al. 1998). Whether putative KIM indeed mediates phosphatase binding to plant MAPKs and whether phs1-1 mutation influences this binding has still to be tested.
T-DNA null plants phs1-2 are recessive lethal embryonic mutants (Naoi and Hashimoto 2004). phs1-3 mutant plants with T-DNA insertion in the promoter region, resulting in abridged gene expression, show inhibited seed germination in the presence of ABA and stronger inhibition of the light-induced opening of stomata by ABA, thereby suggesting PHS1 as a negative regulator of ABA signalling (Quettier et al. 2006). These examples demonstrate that plant dual-specificity phosphatases counteract the activity of MAPKs and control plant responses during stress and in development.
The DSP phosphatase IBR5 was identified as an Arabidopsis indole-3-butyric acid (IBA)-response mutant ibr5 (Monroe-Augustus et al. 2003). The phenotype of this mutant is similar to other auxin-response mutants: a long root and a short hypocotyl when grown in the light, aberrant vascular patterning, increased leaf serration, and reduced accumulation of an auxin-inducible reporter. However, overexpression of IBR5 did not dramatically alter the auxin sensitivity of plants. No phosphatase activity in vitro was shown for IBR5, which could be due to the requirement of substrate binding to activate this enzyme (like for NtMKPl or certain mammalian DSPs). The substrates of IBR5 remain to be identified.
AtPTEN1 encodes a dual-specificity phosphatase closely related to PTEN, a tumor suppressor in animals. The recombinant AtPTENl demonstrates phosphatase activity to dephosphorylate phosphotyrosine and phosphatidyli-nositol substrates, like its homologues in animals. AtPTEN1 is expressed exclusively in pollen and is essential for pollen development, as shown by suppression of AtPTEN1 expression by RNA interference that causes pollen cell death after mitosis (Gupta et al. 2002).
Another Arabidopsis DSP phosphatase is PTPKIS1/SEX4/DSP4, previously nominated PTPKIS1 (protein-tyrosine phosphatase kinase interaction sequence) due to the presence of a kinase interaction sequence (KIS), which mediates interaction with the plant SNF1-related kinase (SnRK), AKIN11 (Fordham-Skelton et al. 2002) and encodes a carbohydrate-binding domain, allowing it to bind starch granules (Kerk et al. 2006; Niittyla et al. 2006; Sokolov et al. 2006). In addition, SEX4/DSP4 protein has a plastid targeting sequence and is localized to chloroplasts, where it controls starch metabolism, primarily by regulation of starch breakdown. It associates with starch granules in a light-dependent manner and represents a major starch granule-bound phosphatase activity during the day. Phosphatase activity and the starch-binding capacity of SEX4/DSP4 are controlled by redox and pH. SEX4/DSP4 has close orthologues in other plant species and resembles the animal DSP laforin (Alonso et al. 2004) that regulates glycogen accumulation, indicating striking parallels in the regulation of starch metabolism in plants and glycogen metabolism in mammals. A future task will be to investigate whether the downstream targets of SEX4/DSP4 are glucans or proteins in plants (Kerk et al. 2006; Niittyla et al. 2006; Sokolov et al. 2006).
The animal Eyes Absent proteins (Eya) represent a novel family of dual-function enzymes with transcription factor and phosphatase activities (Jemc and Rebay 2007). Eya proteins are transcription factors with intrinsic phos-phatase activity capable of modulating transcriptional complexes and are responsible for organ formation (Li et al. 2003). They also represent a mechanistically new class of tyrosine phosphatases (PTPs), which does not contain the cysteine-containing signature motif and includes an aspartate as a nu-cleophile in a metal-dependent reaction similar to the phosphoserine phos-phatases of the haloacid dehalogenase (HAD) family (Lunn 2002). Eyes Absent homologues have been identified in rice, alfalfa and Arabidopsis (Takeda et al. 1999). Although the Arabidopsis homologue of animal Eya, AtEYA is a tyrosine-specific phosphatase, as demonstrated in vitro, the role of AtEYA-mediated dephosphorylation in plant biology remains to be elucidated (Raya-pureddi et al. 2005).
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