Component of light signal transduction machinery. Involved in repression of photomorphogenesis in darkness by participating in the CDD complex, a complex probably required to regulate the activity of ubiquitin conjugating enzymes (E2s). Repression of photomorphogenesis is probably mediated by ubiquitination and subsequent degradation of photomorphogenesis-promoting factors such as HY5, HYH and LAF1. Although strongly related to ubiquitin-conjugating enzyme, it has no catalytic activity by itself due to the absence of the conserved Cys active site at position 120. It can however enhance the activity of E2 conjugating enzymes. Component of the CDD complex, at least composed of COP10, DET1 and DDB1A (PubMed:15342494, PubMed:24563205). Interacts with E3 ubiquitin ligase COP1. Interacts with E2 ubiquitin conjugating UBC5. Interacts with CSN3, CSN4 and CSN8 subunits of the COP9 complex. A number of isoforms are produced. According to EST sequences. Expressed in flower, leaf, stem and seedling. Expressed at lower level in root. Belongs to the ubiquitin-conjugating enzyme family.