SUMMARY
        
        Nicotiana tabacum transcription factor DBP1 is the first representative of a novel family of transcriptional regulators that, besides exhibiting sequence-specific DNA-binding, also possess protein phosphatase activity of the 2C type. In this work we have accomplished the functional characterization of DBP1. DNA-binding ability resides in the N-terminal region and involves a highly conserved domain that we have designated DNC motif. Using the yeast two-hybrid system we have identified 14-3-3 isoform G as the first interactor of a DBP factor. The interaction site lies in the N-terminal region, close to the DNC motif. By means of transient expression assays in Nicotiana benthamiana we have shown that expression of 14-3-3 G promotes DBP1 nuclear exclusion, and that the modulation of DBP1 nucleo-cytoplasmic shuttling mediated by 14-3-3 G is actually involved in the regulation of the expression of DBP1 target genes like CEVI1.
	AtDBP1 is the closest structural homologue in the model species Arabidopsis thaliana among the four we have identified. AtDBP1 also binds DNA with the same sequence specificity as DBP1 and shows protein phosphatase activity in vitro. Moreover, it is able to interact with GRF6, a 14-3-3 G homologue, through the N-terminal region. With the aim of identifying both transcriptional and possible post-transcriptional AtDBP1 targets we performed a comparative proteomic analysis between wild-type Col-0 and atdbp1 loss-of-function mutant plants that resulted in the detection of lower accumulation levels in the atdbp1 mutant of a plant specific isoform of the translation intiation factor eIF4E named eIF(iso)4E. The reduction in the amount of eIF(iso)4E protein seems to obey to a post-trascriptional mechanism, and very likely occurrs post-translationally since we have demonstrated a direct protein-protein interaction between AtDBP1 and eIF(iso)4E that suggests that the latter could be a substrate of AtDBP1 protein phosphatase activity.
	eIF(iso)4E and eIF4E play a key role in the life cycle of potyvirus. The lower protein level of eIF(iso)4E in the atdbp1 mutant hampers progression of the infection by the potyvirus PPV (Plum Pox Virus), resulting in a significant delay in viral accumulation and movement. Therefore, loss of AtDBP1 function leads to partial resistance against potyvirus. We have also shown that viral infection promotes stabilization of eIF(iso)4E protein, and that application of the proteosome inhibitor MG-132 results in an increased accumulation of eIF(iso)4E in atdbp1 mutant plants, suggesting that the phosphorylated form of eIF(iso)4E is more susceptible to degradation via proteasome and that AtDBP1-mediated dephosphorylation might contribute to stabilization of eIF(iso)4E. These results assign a biological role to the protein phosphatase activity of AtDBP1 and point to this regulatory factor as a novel component of recessive resistance mechanisms against potyvirus.