Tobacco DNA-binding protein phosphatase (DBP1) was isolated by interaction with the promoter region of tomato citrus exocortis viroid (CEVI1) gene, which is induced during the course of compatible plant-virus interactions. The C-terminal part of DBP1 shows homology to the PP2C catalytic domain, whereas the N-terminal region contains DNA binding sequences (Carrasco et al. 2003). Correspondingly, DBP1 shows Mg2+-dependent phosphatase activity and contains a functional nuclear localization signal. DBP1 negatively controls transcription of CEVI1 as demonstrated by constitutive up-regulation of the CEVI1 gene in DBP1-antisense transgenic tobacco plants. DBP1 and its distantly related Arabidopsis AtDBP1 possess in vitro DNA binding activity, mediated by the N-terminal region via the conserved DNC (DBP N-terminal core) motif (Carrasco et al. 2005). Interestingly, the 14-3-3 isoform G from tobacco and the 14-3-3 X/GRF6 from Arabidopsis were identified by screenings in yeast using DBP1 and AtDBP1 as baits, respectively (Carrasco et al. 2006). The N-terminal region of DBP1 is necessary and sufficient for 14-3-3 G binding and confers nucleocytoplasmic shuttling in vivo. It is proposed that 14-3-3 isoform G positively regulates CEVI1 expression through interaction with DBP1 followed by nuclear export of the complex, thereby relieving repression of CEVI1 promoter (Carrasco et al. 2006).
Recently, important findings in the functional characterization of plant members of the PTP and PPM families have provided new perspectives on the regulation of signal transduction in stress and developmental pathways. It will be important to investigate the regulation of PTP and PP2C phos-phatases by oxidation-related signalling in planta. Of particular significance would be to define the links between the disruption of PTP and/or PP2C function and the MAPK activities. Genetic analyses of knockout mutants, plant tilling lines and application of additional tools (such as the split ubiquitine system for protein interactions and protein complex analysis by proteomics) will certainly facilitate identification of new pathways and characterization of substrates that are targeted by these phosphatases in vivo. The united efforts of different groups in this respect will accelerate uncovering the roles of plant protein phosphatases in signalling.
Acknowledgements We thank Patricia T. W. Cohen, Alison DeLong and Pedro L. Rodriguez Egea for comments and David Kerk for providing the latest update of Arabidopsis phosphatase sequence prediction.
Our research is supported by grants from the Austrian Science Fund FWF.
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