Abstract
Plants sense different parts of the sun's light spectrum using distinct photoreceptors, which signal through the E3 ubiquitin ligase COP1. Here, we analyze why many COP1‐interacting transcription factors and photoreceptors harbor sequence‐divergent Val‐Pro (VP) motifs that bind COP1 with different binding affinities. Crystal structures of the VP motifs of the UV‐B photoreceptor UVR8 and the transcription factor HY5 in complex with COP1, quantitative binding assays, and reverse genetic experiments together suggest that UVR8 and HY5 compete for COP1. Photoactivation of UVR8 leads to high‐affinity cooperative binding of its VP motif and its photosensing core to COP1, preventing COP1 binding to its substrate HY5. UVR8–VP motif chimeras suggest that UV‐B signaling specificity resides in the UVR8 photoreceptor core. Different COP1–VP peptide motif complexes highlight sequence fingerprints required for COP1 targeting. The blue‐light photoreceptors CRY1 and CRY2 also compete with transcription factors for COP1 binding using similar VP motifs. Thus, our work reveals that different photoreceptors and their signaling components compete for COP1 via a conserved mechanism to control different light signaling cascades.
Highlights
Flowering plants etiolate in darkness, manifested by the rapid elongation of the embryonic stem, the hypocotyl, and closed and underdeveloped embryonic leaves, the cotyledons
In order to compare how the VP peptide motifs from different plant light signaling components bind COP1, we quantified the interaction of the UVR8 and HYPOCOTYL 5 (HY5) VP peptides with the recombinant Arabidopsis COP1349–675 WD40 domain using isothermal titration calorimetry (ITC)
Our findings suggest that UVR8 requires both its UV-B-sensing core and its VP peptide motif for high-affinity COP1 binding and that the UVR8 VP peptide can inhibit the interaction of HY5 with COP1 (Figs 1 and 2; Yin et al, 2015)
Summary
Flowering plants etiolate in darkness, manifested by the rapid elongation of the embryonic stem, the hypocotyl, and closed and underdeveloped embryonic leaves, the cotyledons. The constitutively photomorphogenic 1 (cop1) mutant displays a light-grown phenotype in the dark, including a short hypocotyl, and open and expanded cotyledons. COP1 can act as a substrate adaptor in CULLIN4–DAMAGED DNA BINDING PROTEIN 1 (CUL4-DDB1)based heteromeric E3 ubiquitin ligase complexes (Chen et al, 2010). These different complexes may finetune COP1’s activity toward different substrates (Ren et al, 2019). COP1 regulates gene expression and plays a central role as a repressor of photomorphogenesis by directly modulating the stability of transcription factors that control the expression of light-regulated genes (Lau & Deng, 2012; Podolec & Ulm, 2018). COP1 binding to HY5 leads to its subsequent degradation via the 26S proteasome in darkness, a process that is inhibited by light (Osterlund et al, 2000)
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