The activities of the E3 ubiquitin ligase COP1/SPA, a key repressor in light signaling

•The COP1/SPA complex suppresses light signaling in darkness.•Functions of the protein domains in COP1 and SPA proteins.•The structure of the COP1 WD-repeat domain was solved.•The regulation of COP1/SPA activity is complex and integrates multiple signals.•Novel functions of COP1 were revealed.

Light is a critical signal to integrate plant growth and development with the environment. Downstream of photoreceptors, the E3 ubiquitin ligase COP1/SPA is a key repressor of photomorphogenesis which targets many positive regulators of light signaling, mainly transcription factors, for degradation in darkness. In light-grown plants COP1/SPA activity is repressed, allowing light responses to occur. This review provides an overview on our current knowledge on COP1/SPA repressor function, focusing in particular on the roles of the respective protein domains and the mechanisms of light-induced inactivation of COP1/SPA. Moreover, we summarize how COP1 activity is regulated by other interacting proteins, such as a SUMO E3 ligase and Phytochrome-Interacting Factors (PIFs), as well as by hormones. At last, several novel functions of COP1 that were recently revealed are included.