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 SPA (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 SPA-COP1 complexes. The four endogenous SPA 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 SPA proteins can self-associate or interact with each other, forming a heterogeneous group of SPA-COP1 complexes in which the exact SPA protein compositions vary depending on the abundance of individual SPA proteins. The four SPA 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 SPA protein may cause a significant reduction in the overall SPA-COP1 E3 ligase activity, resulting in a partial constitutive photomorphogenic phenotype. This study thus provides an in-depth biochemical view of the SPA-COP1 E3 ligase complexes and offers new insights into the molecular basis for their distinct roles in the light control of plant development.