DET1-mediated COP1 regulation avoids HY5 activity over second-site targets to tune plant photomorphogenesis

Abstract

DE-ETIOLATED1 (DET1) is a negative regulator of plant photomorphogenesis acting as a component of the C3D complex, which can further associate to CULLIN4 to form a CRL4C3D E3 ubiquitin ligase. CRL4C3D is thought to act together with CRL4COP1SPA ubiquitin ligase, to promote the ubiquitin-mediated degradation of the master regulatory transcription factor ELONGATED HYPOCOTYL5 (HY5), thereby controlling photomorphogenic gene regulatory networks. Yet, functional links between COP1 and DET1 have long remained elusive. Here, upon mass spectrometry identification of DET1 and COP1-associated proteins, we provide in vivo evidence that DET1 associates with COP1 to promote its destabilization, a process necessary to dampen HY5 protein abundance. By regulating HY5 over-accumulation, DET1 is critical to avoid its association to second-site loci, including many PIF3 target genes. Accordingly, excessive HY5 levels result in an increased HY5 repressive activity and are sufficient to trigger fusca-like phenotypes otherwise observed typically in COP1 and COP9 signalosome mutant seedlings. This study therefore identifies that DET1-mediated regulation of COP1 stability tunes down HY5 cistrome and avoids hyper-photomorphogenic responses that might compromise plant viability.

Illuminating the COP1/SPA Ubiquitin Ligase: Fresh Insights Into Its Structure and Functions During Plant Photomorphogenesis

“…Since Arabidopsis COP1 interacted with DDB1 independently of DET1, it was proposed that the CUL4-DDB1-COP1/SPA complex may be structurally distinct from the CUL4-C3D complex ( Chen et al, 2010 ; Lau and Deng, 2012 ; Irigoyen et al, 2014 ). However, a recent preprint demonstrated an in vivo association of COP1/SPA proteins with the C3D complex in dark-grown Arabidopsis cell cultures that constitutively express DET1 ( Cañibano et al, 2020 ). This argues for the possibility that COP1/SPA proteins may form a part of the DET1-containing C3D complex in Arabidopsis.…”

“…Another mechanism of COP1 regulation may be via the alternatively spliced COP1b, which potentially interferes with COP1 activity, at least in certain tissues ( Zhou et al, 1998 ). Recently, the C3D component DET1 was also shown to regulate the levels of COP1 toward fine-tuning HY5 activity in Arabidopsis ( Cañibano et al, 2020 ). However, the mechanisms underlying DET1-mediated destabilization of COP1 are not yet known.…”

“…However, unlike in Arabidopsis, the association between hCOP1 and DDB1 in the CUL4-E3 complex is not direct, but bridged by DET1 ( Wertz et al, 2004 ). Work in a recent preprint demonstrated that AtCOP1 co-purifies with AtDET1 in vivo , raising the possibility that AtCOP1 acts together with AtDET1 as members of the same complex or of interacting complexes ( Cañibano et al, 2020 ). Depending on the ubiquitination substrates, hCOP1 could act as an E3 ligase independently or in association with the CUL4-DDB1-RBX-DET1 (CDD) complex.…”