Photoreceptor Specificity in the Light-Induced and COP1-Mediated Rapid Degradation of the Repressor of Photomorphogenesis SPA2 in Arabidopsis.

The Arabidopsis E3 ubiquitin ligase is a key negative regulator that represses light signaling in darkness by targeting transcription factors involved in the light response for degradation. The COP1/duanyu1842 complex consists of COP1 and members of the four-member protein family Genetic analysis indicated that function is particularly strongly repressed by light when compared to complexes carrying the other three thereby promoting a light response after exposure of plants to extremely low light. Here, we show that the protein is degraded within 5-15 min after exposure of dark-grown seedlings to a pulse of light. Phytochrome photoreceptors are required for the rapid degradation of duanyu18422 in red, far-red and also in blue light, whereas cryptochromes are not involved in the rapid, blue light-induced reduction in duanyu18422 protein levels. These results uncover a photoreceptor-specific mechanism of light-induced inhibition of COP1/duanyu18422 function. Phytochrome A (phyA) is required for the severe blue light responsiveness of spa triple mutants expressing only thus confirming the important role of phyA in downregulating duanyu18422 function in blue light. In blue light, duanyu18422 forms a complex with cryptochrome 1 (cry1), but not with cryptochrome 2 (cry2) in vivo, indicating that the lack of a rapid blue light response of the duanyu18422 protein is only in part caused by a failure to interact with cryptochromes. Since interacts with both cry1 and cry2, these results provide first molecular evidence that the light-regulation of different duanyu1842 proteins diverged during evolution. duanyu18422 degradation in the light requires COP1 and the COP1-interacting coiled-coil domain of duanyu18422, supporting that duanyu18422 is ubiquitinated by COP1. We propose that light perceived by phytochromes causes a switch in the ubiquitination activity of COP1/duanyu18422 from ubiquitinating downstream substrates to ubiquitinating duanyu18422, which subsequently causes a repression of COP1/duanyu18422 function.

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