Abstract
CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1) is a highly conserved E3 ubiquitin ligase from plants to animals and acts as a central repressor of photomorphogenesis in plants. SUPPRESSOR OF PHYA-105 1 family members (SPA1-SPA4) directly interact with COP1 and enhance COP1 activity. Despite the presence of a kinase domain at the N-terminus, no COP1-independent role of SPA proteins has been reported. Here we show that SPA1 acts as a serine/threonine kinase and directly phosphorylates PIF1 in vitro and in vivo. SPAs are necessary for the light-induced phosphorylation, ubiquitination and subsequent degradation of PIF1. Moreover, the red/far-red light photoreceptor phyB interacts with SPA1 through its C-terminus and enhances the recruitment of PIF1 for phosphorylation. These data provide a mechanistic view on how the COP1-SPA complexes serve as an example of a cognate kinase-E3 ligase complex that selectively triggers rapid phosphorylation and removal of its substrates, and how phyB modulates this process to promote photomorphogenesis.
Highlights
CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1) is a highly conserved E3 ubiquitin ligase from plants to animals and acts as a central repressor of photomorphogenesis in plants
To test whether the kinase domain is necessary for the kinase activity of SPA1, we created a point mutation in an amino acid (R517E) on the SPA1 kinase domain, which is conserved in SPA1 sequences from multiple plants (Supplementary Fig. 1)
This study provides a mechanistic view on how PIF1 is phosphorylated and ubiquitinated by a cognate kinase-E3 ubiquitin ligase (COP1-SPA) for rapid lightinduced degradation
Summary
CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1) is a highly conserved E3 ubiquitin ligase from plants to animals and acts as a central repressor of photomorphogenesis in plants. The ubiquitin-26S proteasome system (UPS) plays critical roles in controlling growth and development of all eukaryotes[1,2] and is involved in multiple developmental and environmental responses, including photomorphogenesis in Arabidopsis[2,3] This system targets specific proteins by selectively attaching a polyubiquitin chain by the action of three enzymes: E1, E2, and E3, and the polyubiquitin chain serves as a tag for specific recognition and degradation of the substrate by the UPS pathway[1,2]. CONTITUTIVE PHOTOMORPHOGENIC 1 (COP1) is a RING finger protein containing WD40 repeats and a coiled-coil domain and is highly conserved from plants to mammals[4] It is one of the best characterized E3 ubiquitin ligases with broad roles as a central repressor of light signaling in plants to cancer biology in mammals[5,6,7].
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