Dissecting the functions of COP1 in the UVR8 pathway with a COP1 variant in Arabidopsis.

Abstract

COP1 is a critical repressor of plant photomorphogenesis in darkness. However, COP1 plays distinct roles in the photoreceptor UVR8 pathway in Arabidopsis thaliana. COP1 interacts with UV-B-activated UVR8 monomers and promotes their retention and accumulation in the nucleus. Moreover, COP1 has UV-B signaling function, which involves the binding of its WD40 domain to UVR8 and HY5 via conserved VP motifs of these proteins. UV-B-activated UVR8 interacts with COP1 via both the core domain and the VP motif, leading to the displacement of HY5 from COP1 and HY5 stabilization. However, whether the UV-B signaling function of COP1 is solely dependent on its VP motif binding capacity and whether UV-B regulates COP1 subcellular localization remain unclear. Based on published structures of COP1 WD40 domain, we generated a single amino acid substitution COP1 variant, COP1C509S , which cannot bind to VP motifs but retains the ability to interact with the UVR8 core domain. UV-B only marginally increased nuclear YFP-COP1 levels but promoted YFP-COP1 accumulation significantly in the cytosol, but not with YFP-COP1C509S . Thus, the full UVR8-COP1 interaction is important for COP1 accumulation in cytosol. Notably, UV-B signaling including activation of HY5 transcription was obviously retained in the Arabidopsis lines expressing YFP-COP1C509S , which cannot bind VP motifs. We conclude that the full binding of UVR8 to COP1 leads to the predominant accumulation of COP1 in the cytosol and COP1 has additional UV-B signaling function besides VP binding-mediated protein destabilization.