Following light-induced nuclear translocation, specific members of the phytochrome (phy) photoreceptor family (phyA to phyE) interact with bHLH transcription factors, such as PIF3, and induce changes in target-gene expression. The biochemical mechanism comprising signal transfer from phy to PIF3 has remained undefined but results in rapid degradation of PIF3. We provide evidence that photoactivation of phy induces rapid in vivo phosphorylation of PIF3 preceding degradation. Both phyA and phyB redundantly induce this PIF3 phosphorylation, as well as nuclear speckle formation and degradation, by direct interaction with PIF3 via separate binding sites. These data suggest that phy-induced phosphorylation of proteins such as PIF3 may represent the primary intermolecular signaling transaction of the activated photoreceptor, tagging the target protein for proteosomal degradation, possibly in nuclear speckles.

Following light-induced nuclear translocation, phytochrome photoreceptors interact with and induce rapid phosphorylation and degradation of bHLH transcription factors, such as PHYTOCHROME-INTERACTING FACTOR 3 (PIF3), to regulate gene expression. Concomitantly this interaction triggers feedback reduction of phytochrome B (phyB) levels. Light-induced phosphorylation of PIF3 is necessary for the degradation of both proteins. We report that this PIF3 phosphorylation induces, and is necessary for, recruitment of LRB (Light-Response Bric-a-Brack/Tramtrack/Broad (BTB)) E3 ubiquitin ligases to the PIF3-phyB complex. The recruited LRBs promote concurrent polyubiqutination and degradation of both PIF3 and phyB in vivo. These data reveal a linked signal-transmission and attenuation mechanism involving mutually assured destruction of the receptor and its immediate signaling partner.