The evolutionarily conserved CONSTITUTIVE PHOTOMORPHOGENESIS 1 (COP1) is a RING and WD40 protein that functions as a substrate receptor of CULLIN4-DAMAGED DNA BINDING PROTEIN 1 (CUL4-DDB1)-based E3 ubiquitin ligases in both plants and animals. In Arabidopsis, COP1 is a central repressor of photomorphogenesis in the form of COP1-SUPPRESSOR OF PHYA (SPA) complex(es). CUL4-DDB1-COP1-SPA suppresses the photomorphogenic program by targeting the transcription factor ELONGATED HYPOCOTYL 5 for degradation. Intriguingly, under photomorphogenic UV-B light, COP1 reverses its repressive role and promotes photomorphogenesis. However, the mechanism by which COP1 is functionally switched is still obscure. Here, we demonstrate that UV-B triggers the physical and functional disassociation of the COP1-SPA core complex(es) from CUL4-DDB1 and the formation of a unique complex(es) containing the UV-B receptor UV RESISTANCE LOCUS 8 (UVR8). The establishment of this UV-B-dependent COP1 complex(es) is associated with its positive modulation of ELONGATED HYPOCOTYL 5 stability and activity, which sheds light on the mechanism of COP1's promotive action in UV-B-induced photomorphogenesis.light signaling | protein complex | posttranscriptional regulation I n response to light and darkness, plant seedlings establish lightgrown and dark-grown phenotypes via a series of developmental changes, termed photomorphogenesis and skotomorphogenesis, respectively. CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) is a known RING E3 ubiquitin ligase that has been evolutionally conserved from plants to humans (1, 2). It was originally identified by genetic screens for seedlings of Arabidopsis thaliana that exhibit constitutive photomorphogenesis in darkness (1, 3), as a key member of the pleiotropic CONSTITUTIVE PHOTOMOR-PHOGENIC/DE-ETIOLATED/FUSCA (COP/DET/FUS) gene family. These COP/DET/FUS proteins biochemically contribute to three entities: the COP1-SUPRESSOR OF PHYA (SPA) complex(es), the COP9 signalosome (CSN), and the COP10-DET1-Damaged DNA Binding Protein 1 (DDB1) (CDD) complex. COP1-SPA, independent of CDD but in concert with CULLIN4-DDB1 (CUL4-DDB1), targets photomorphogenesis promoting transcription factors including ELONGATED HYPOCOTYL 5 (HY5) for the ubiquitin-proteasome system-mediated degradation, so as to repress the traditional photomorphogenesis triggered by far-red and visible light (4, 5).Intriguingly, in contrast to their antagonistic roles in the traditional photomorphogenesis, COP1 and HY5 both take positive parts in low-fluence and long-wavelength UV-B-induced photomorphogenesis. This response is initiated by the UV-B receptor UV RESISTANCE LOCUS 8 (UVR8) which absorbs UV-B through its internal chromophore tryptophan residues (6, 7). UVR8 then monomerizes to interact with the UV-B-inducible protein COP1 for downstream signaling (8-10). The physical manifestations of this process include hypocotyl shortening, anthocyanin accumulation, and tolerance against damaging UV-B. The loss of either COP1 or HY5 has previously been shown to result in de...

The Arabidopsis thaliana protein UVR8 is a photoreceptor for ultraviolet-B. Upon ultraviolet-B irradiation, UVR8 undergoes an immediate switch from homodimer to monomer, which triggers a signalling pathway for ultraviolet protection. The mechanism by which UVR8 senses ultraviolet-B remains largely unknown. Here we report the crystal structure of UVR8 at 1.8 Å resolution, revealing a symmetric homodimer of seven-bladed β-propeller that is devoid of any external cofactor as the chromophore. Arginine residues that stabilize the homodimeric interface, principally Arg 286 and Arg 338, make elaborate intramolecular cation-π interactions with surrounding tryptophan amino acids. Two of these tryptophans, Trp 285 and Trp 233, collectively serve as the ultraviolet-B chromophore. Our structural and biochemical analyses identify the molecular mechanism for UVR8-mediated ultraviolet-B perception, in which ultraviolet-B radiation results in destabilization of the intramolecular cation-π interactions, causing disruption of the critical intermolecular hydrogen bonds mediated by Arg 286 and Arg 338 and subsequent dissociation of the UVR8 homodimer.

As sessile organisms, higher plants have evolved the capacity to sense and interpret diverse light signals to modulate their development. In Arabidopsis thaliana, low-intensity and long-wavelength UV-B light is perceived as an informational signal to mediate UV-B-induced photomorphogenesis. Here, we report that the multifunctional E3 ubiquitin ligase, CONSTITUTIVE PHOTOMORPHOGENESIS1 (COP1), a known key player in UV-B photomorphogenic responses, is also a UV-B-inducible gene. Two transcription factors, FAR-RED ELONGATED HYPOCOTYL3 (FHY3) and ELONGATED HYPOCOTYL5 (HY5), directly bind to distinct regulatory elements within the COP1 promoter, which are essential for the induction of the COP1 gene mediated by photomorphogenic UV-B signaling. Absence of FHY3 results in impaired UV-B-induced hypocotyl growth and reduced tolerance against damaging UV-B. Thus, FHY3 positively regulates UV-B-induced photomorphogenesis by directly activating COP1 transcription, while HY5 promotes COP1 expression via a positive feedback loop. Furthermore, FHY3 and HY5 physically interact with each other, and this interaction is diminished by UV-B. Together, our findings reveal that COP1 gene expression in response to photomorphogenic UV-B is controlled by a combinatorial regulation of FHY3 and HY5, and this UV-B-specific working mode of FHY3 and HY5 is distinct from that in far-red light and circadian conditions.