Functions as a E3 ubiquitin-protein ligase in cooperation with the E2 ubiquitin conjugating enzymes UBC5A and UBC5B. Involved in root development. Required for the maintenance of cell viability after the initiation of root primordial formation. May mediate the degradation of cytotoxic proteins produced in root cells after the actions of auxin, cytokinin and jasmonic acid. Mediates 'Lys-48'-linked polyubiquitination of MBP in vitro. 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. By N-acetylchitooligosaccharide elicitor and by protein phosphatase inhibitor calyculin A. Induction by N-acetylchitooligosaccharide elicitor is inhibited by the protein kinase inhibitor K-252a. The RING-type zinc-finger domain is required for E3 ubiquitin ligase activity.