Interaction of the Arabidopsis E2F and DP Proteins Confers Their Concomitant Nuclear Translocation and Transactivation

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Abstract
E2F transcription factors are required for the progression and arrest of the cell cycle in animals. Like animals, plants have evolved to conserve the E2F family. The Arabidopsis genome encodes E2F and DP proteins that share a high similarity with the animal E2F and DP families. Here, we show that Arabidopsis E2F and DP proteins are not predominantly localized to the nucleus in analyses with green fluorescent protein, and that the complete nuclear localization of some members is driven by the co-expression of their specific partner proteins. Both AtE2F1 and AtE2F3 were translocated to the nucleus and transactivate an E2F reporter gene when co-expressed with DPa but not DPb. In contrast, AtE2F2 was inactive for both nuclear translocation and transactivation even when Dpa or DPb was co-expressed. Because the DNA binding activities of the three E2Fs are equally stimulated by the interaction with DPa or DPb in vitro, the observed transactivation of AtE2F1 and AtE2F3 is DPa specific and nuclear import dependent. A green fluorescent protein fusion with an AtE2F3 mutant, in which a conserved nuclear export signal-like sequence in the dimerization domain was deleted, was localized to the nucleus. Thus, the concomitant nuclear translocation seems to be conferred by the DPa interaction to release an activity that inhibits an intrinsic nuclear import activity of AtE2Fs. Furthermore, the nuclear translocation of AtE2F3 stimulated by DPa was abolished by the deletion of the N-terminal region of AtE2F3, which is conserved among all the E2F proteins identified in plants to date. Replacement of the N-terminal region of AtE2F3 with a canonical nuclear localization signal only partially mimicked the effect of the DPa co-expression, demonstrating the function of plant E2F distinct from that observed for animal E2Fs. These observations suggest that the function of plant E2F and DP proteins is primarily controlled by their nuclear localization mediated by the interaction with specific partner proteins.