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

The Arabidopsis COP1/SPA 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/SPA complex consists of COP1 and members of the four-member SPA protein family (SPA1-SPA4). Genetic analysis indicated that COP1/SPA2 function is particularly strongly repressed by light when compared to complexes carrying the other three SPAs, thereby promoting a light response after exposure of plants to extremely low light. Here, we show that the SPA2 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 SPA2 in red, far-red and also in blue light, whereas cryptochromes are not involved in the rapid, blue light-induced reduction in SPA2 protein levels. These results uncover a photoreceptor-specific mechanism of light-induced inhibition of COP1/SPA2 function. Phytochrome A (phyA) is required for the severe blue light responsiveness of spa triple mutants expressing only SPA2, thus confirming the important role of phyA in downre-gulating SPA2 function in blue light. In blue light, SPA2 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 SPA2 protein is only in part caused by a failure to interact with cryptochromes. Since SPA1 interacts with both cry1 and cry2, these results provide first molecular evidence that the light-regulation of different SPA proteins diverged during evolution. SPA2 degradation in the light requires COP1 and the COP1-interacting coiled-coil domain of SPA2, supporting that SPA2 is ubiquitinated by COP1. We propose that light perceived by phytochromes causes a switch in the ubiquitination activity of COP1/SPA2 from ubiquitinating downstream substrates to ubiquitinating SPA2, which subsequently causes a repression of COP1/SPA2 function.