Regulation of Phytochrome B Nuclear Localization through Light-Dependent Unmasking of Nuclear-Localization Signals

Abstract

Phytochromes are red and far-red photoreceptors that regulate plant growth and development in response to environmental light cues. Phytochromes exist in two photo-interconvertible conformational states: an inactive Pr form and an active Pfr form. The alteration of phytochromes' subcellular location functions as a major regulatory mechanism of their biological activities. Whereas phytochromes in the Pr form localize in the cytoplasm, phytochromes in the Pfr form accumulate in the nucleus, where they interact with transcription factors to regulate gene expression. The molecular details of the regulation of phytochrome translocation by light are poorly understood. Using Arabidopsis phyB as a model, we demonstrate that the C-terminal PAS-related domain (PRD) is both necessary and sufficient for phyB nuclear import and that the entire C terminus is required for nuclear-body (NB) localization. We also show that phyB's N-terminal bilin lyase domain (BLD) and PHY domain interact directly with the PRD in a light-dependent manner. In vivo localization studies indicate that BLD-PHY is sufficient to regulate phyB's nuclear accumulation. For phyB nuclear localization, our results suggest a molecular mechanism in which the nuclear-localization signal in the PRD is masked by interactions with phyB's chromophore-attachment domains and unmasked by light-dependent conformational changes.

Transcriptional and posttranscriptional regulation of transcription factor expression in Arabidopsis roots

“…Besides regulation of intercellular movement, TF activity might also be regulated via modulation of subcellular localization, as has been previously reported in plants (34,35) and animals (12, 13). For 10 of the 39 (25%) translational fusions that had detectable GFP, the GFP-fusion protein was not found to be nuclear-localized in any of the root tissues (Table 3).…”

Photobody Localization of Phytochrome B Is Tightly Correlated with Prolonged and Light-Dependent Inhibition of Hypocotyl Elongation in the Dark

“…Studies of photobody localization using Arabidopsis phyB have shown that the C-terminal domain of phyB is involved in dimerization and is sufficient for both nuclear and photobody localization (Matsushita et al, 2003;Chen et al, 2005). When the C-terminal domain of phyB is replaced with a dimerization domain, a Simian Vacuolating Virus40 nuclear localization signal, and GFP, the chimeric protein, NGB (for the N-terminal half of phyB fused to GFP), does not localize to photobodies (Matsushita et al, 2003).…”

The Structure of Phytochrome: A Picture Is Worth a Thousand Spectra

“…While bacterial and cyanobacterial phytochromes typically employ classical two-component phosphotransfer relays, the mechanism of plant phytochrome signaling appears considerably more complex, involving lightmediated nuclear translocation and regulation of transcription factor function (Chen et al, 2004;Nagatani, 2004;Huq and Quail, 2005;Schä fer and Nagy, 2005). The C-terminal regulatory domains of plant phytochromes have been shown to mediate homodimerization and light-modulated nuclear targeting, both of which are required for signal transmission (Matsushita et al, 2003;Chen et al, 2005). Signaling by plant phytochromes also involves protein-protein interactions with the N-terminal part of the protein (Ni et al, 1999;Oka et al, 2004), although it is not yet known whether these interactions are actually mediated via the conserved photosensory core (P2-P3-P4), via the N-terminal Ser/Thr-rich extension specific to plant phytochromes, or both.…”