Conversion from CUL4-based COP1–SPA E3 apparatus to UVR8–COP1–SPA complexes underlies a distinct biochemical function of COP1 under UV-B

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.