E3 ubiquitin-protein ligase involved in the regulation of root growth. Acts as positive regulator of root gravitropism. Possesses E3 protein ligase activity 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. Interacts with SINAT1, SINAT2, SINAT3, SINAT4, SINAT5, TOR1/SPR2 and FIP2. Expressed in root tips and leaf primordia. No visible phenotype under normal growth conditions, but mutant seedlings exhibit enhanced root wavy growth and curvature in response to gravitropism.