Cryptochrome 1 promotes photomorphogenesis in Arabidopsis by displacing substrates from the <scp>COP1</scp> ubiquitin ligase

In blue light, cryptochrome photoreceptors inhibit the key repressor of light signaling, the COP1/SPA ubiquitin ligase, to promote photomorphogenic responses. This inhibition relies on the direct interaction between COP1 and cryptochromes. Here, we analyzed the molecular mechanism of CRY1-mediated inhibition of COP1. We show that the VP motif in the C-terminal domain of CRY1 is essential for the COP1-CRY1 interaction in Arabidopsis. Phenotypic analysis of transgenic Arabidopsis plants harboring a mutation in the VP motif reveals that the VP motif of CRY1 is required for blue light-induced responses, such as seedling de-etiolation and anthocyanin biosynthesis. Via its VP motif, CRY1 inhibits the interaction between COP1 and the COP1 substrate transcription factors PAP2 and HY5. Replacing the VP motif of CRY1 with that of the human COP1 interactor TRIB1 produces a functional photoreceptor in transgenic plants. Since HY5, PAP2 and CRY1 interact with COP1 through their respective VP motifs, our results demonstrate that CRY1 inhibits the activity of COP1 by competitively displacing substrates from COP1. Taken together with previous results showing VP-dependent substrate displacement by photoactivated CRY2 and UVR8 photoreceptors, our results highlight the conservation of this mechanism across multiple photoreceptors.