COP1

COP1, short for constitutive photomorphogenesis 1, is a plant E3 ubiquitin ligase that functions as a central regulator of light signaling. It was identified in Arabidopsis thaliana for its role in suppressing photomorphogenesis in darkness; losses of COP1 lead to light-grown characteristics even when plants are grown in the dark, hence the name.

Molecular architecture of COP1 includes an N-terminal RING finger domain that provides the catalytic activity for ubiquitin transfer, a central coiled-coil region that facilitates oligomerization and complex assembly, and a C-terminal WD40-repeat domain that recognizes substrate proteins. In darkness, COP1 forms a multi-subunit E3 ligase with SUPPRESSOR OF PHYA-105 (SPA) proteins to generate the COP1-SPA complex, which functions as a CRL4-like ubiquitin ligase targeting key transcription factors for degradation.

Primary substrates of COP1-SPA include HY5, HYH, LAF1, and HFR1, transcription factors that promote light responses. Ubiquitination of these factors leads to their proteasomal degradation, effectively repressing photomorphogenesis and maintaining skotomorphogenic growth in the dark. Light signals modulate COP1 activity by inhibiting the COP1-SPA complex: photoreceptors such as phytochromes (phyA/phyB) and cryptochromes (CRY1/CRY2) promote COP1 relocation from the nucleus to the cytoplasm or otherwise suppress its activity, allowing HY5 and related factors to accumulate and drive light-responsive gene expression.

COP1 is conserved among plants and interacts with broader light-signaling networks, including the COP9 signalosome and other ubiquitin-proteasome pathway components. Its regulation is integral to seedling development, shade avoidance, and adaptive responses to environmental light.