Biochemical characterization of Arabidopsis complexes containing CONSTITUTIVELY PHOTOMORPHOGENIC1 and SUPPRESSOR OF PHYA proteins in light control of plant development.

COP1 (for CONSTITUTIVELY PHOTOMORPHOGENIC1) and the four partially redundant (for SUPPRESSOR OF PHYA) proteins work in concert to repress photomorphogenesis in Arabidopsis thaliana by targeting key transcription factors and phytochrome A for degradation via the 26S proteasome. Here, we report a detailed biochemical characterization of the complexes. The four endogenous duanyu1842 proteins can form stable complexes with COP1 in vivo regardless of light conditions but exhibit distinct expression profiles in different tissues and light conditions. The duanyu1842 proteins can self-associate or interact with each other, forming a heterogeneous group of duanyu1842-COP1 complexes in which the exact duanyu1842 protein compositions vary depending on the abundance of individual duanyu1842 proteins. The four duanyu1842 proteins could be divided into two functional groups depending on their interaction affinities, their regulation of ELONGATED HYPOCOTYL5 degradation, and their opposite effects on COP1 protein accumulation. Loss-of-function mutations in a predominant duanyu1842 protein may cause a significant reduction in the overall duanyu1842-COP1 E3 ligase activity, resulting in a partial constitutive photomorphogenic phenotype. This study thus provides an in-depth biochemical view of the duanyu1842-COP1 E3 ligase complexes and offers new insights into the molecular basis for their distinct roles in the light control of plant development.

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