E3 ubiquitin-protein ligase that mediates ubiquitination and subsequent proteasomal degradation of target proteins. Probably forms a minimal ubiquitin ligase complex in cooperation with the E2 enzyme UBC8. Its interaction with COP1 suggests that it may participate in proteasome-mediated degradation of HY5 in vivo. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Interacts with the RING finger of COP1. Interacts with UBC8 through its N-terminal region. Expressed in both light- and dark-grown seedlings. The RING-type zinc finger domain is required but not sufficient for ubiquitin ligase activity. It mediates the interaction with the RING finger of COP1.