Light-dependent, Dark-promoted Interaction between Arabidopsis Cryptochrome 1 and Phytochrome B Proteins*♦

Background: Phytochrome B (phyB) and cryptochrome 1 (CRY1) are plant photoreceptor proteins that mediate red and UV/blue light signaling responses, respectively. Results: CRY1 binds phyB in the dark, and light stimulation of either photoreceptor disrupts this interaction. Conclusion: CRY1 interacts specifically with the Pr (dark) state of phyB. Significance: The interaction may provide a mechanism for cross-talk between these photoreceptor pathways in plants. Plant photoreceptors transduce environmental light cues to downstream signaling pathways, regulating a wide array of processes during growth and development. Two major plant photoreceptors with critical roles in photomorphogenesis are phytochrome B (phyB), a red/far-red absorbing photoreceptor, and cryptochrome 1 (CRY1), a UV-A/blue photoreceptor. Despite substantial genetic evidence for cross-talk between phyB and CRY1 pathways, a direct interaction between these proteins has not been observed. Here, we report that Arabidopsis phyB interacts directly with CRY1 in a light-dependent interaction. Surprisingly, the interaction is light-dissociated; CRY1 interacts specifically with the dark/far-red (Pr) state of phyB, but not with the red light-activated (Pfr) or the chromophore unconjugated form of the enzyme. The interaction is also regulated by light activation of CRY1; phyB Pr interacts only with the unstimulated form of CRY1 but not with the photostimulated protein. Further studies reveal that a small domain extending from the photolyase homology region (PHR) of CRY1 regulates the specificity of the interaction with different conformational states of phyB. We hypothesize that in plants, the phyB/CRY1 interaction may mediate cross-talk between the red/far-red- and blue/UV-sensing pathways, enabling fine-tuning of light responses to different spectral inputs.

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