COP10 is a ubiquitin-conjugating enzyme variant (UEV), which is thought to act 
together with COP1, DET1, and the COP9 signalosome (CSN) in Arabidopsis to 
repress photomorphogenesis. Here, we demonstrate that COP10 interacts with 
ubiquitin-conjugating enzymes (E2s) in vivo, and can enhance their activity in 
vitro, an activity distinct from previous characterized UEVs such as MMS2 and 
UEV1. Furthermore, we show that COP10 forms a complex with UV-damaged 
DNA-binding protein 1a (DDB1a) and de-etiolated 1 (DET1), and physically 
interacts with COP1 and the CSN. Purified CDD (COP10, DDB1, DET1) complex also 
shows enhancement of E2 activity (UEA) similar to that observed with COP10 
itself. Our data suggests that COP10, along with COP1 and the CSN, promotes the 
degradation of positive regulators of photomorphogenesis, such as the 
transcription factor HY5, via the ubiquitin/26S proteasome system. Thus, the CDD 
complex may act as a ubiquitylation-promoting factor to regulate 
photomorphogenesis.