Functional dissection of Arabidopsis COP1 reveals specific roles of its three structural modules in light control of seedling development

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Abstract
Arabidopsis COP1 acts as a repressor of photomorphogenesis in darkness, and light stimuli abrogate the repressive ability and nuclear abundance of COP1. COP1 has three known structural modules: an N‐terminal RING‐finger, followed by a predicted coiled‐coil and C‐terminal WD‐40 repeats. A systematic study was undertaken to dissect the functional roles of these three COP1 domains in light control of Arabidopsis seedling development. Our data suggest that COP1 acts primarily as a homodimer, and probably dimerizes through the coiled‐coil domain. The RING‐finger and the coiled‐coil domains can function independently as light‐responsive modules mediating the light‐controlled nucleocytoplasmic partitioning of COP1. The C‐terminal WD‐40 domain functions as an autonomous repressor module since the overexpression of COP1 mutant proteins with intact WD‐40 repeats are able to suppress photomorphogenic development. This WD‐40 domain‐mediated repression can be at least in part accounted for by COP1's direct interaction with and negative regulation of HY5, a bZIP transcription factor that positively regulates photomorphogenesis. However, COP1 self‐association is a prerequisite for the observed interaction of the COP1 WD‐40 repeats with HY5. This work thus provides a structural basis of COP1 as a molecular switch.