E3 ubiquitin-protein transferase, which catalyzes ubiquitination of target proteins together with ubiquitin-conjugating enzyme E2 ubc-18 (By similarity). Acts with ubc-18 to regulate pharyngeal development (PubMed:16457801, PubMed:19521497). [E2 ubiquitin-conjugating enzyme]-S-ubiquitinyl-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-N(6)-ubiquitinyl-L-lysine. Autoinhibited by the ariadne domain, which masks the second RING-type zinc finger that contains the active site and inhibits the E3 activity. Interacts with ubiquitin-conjugating enzyme E2 ubc-18. Expression is dynamic during early embryonic development, with ubiquitous somatic expression occurring between the 50- and 200-cell stage (PubMed:16457801). By the late proliferative phase of embryogenesis, expression is reduced, but maintained at high levels in muscle precursors (PubMed:16457801). Later in embryogenesis, moderate expression occurs in the lateral ectoderm (PubMed:16457801). Pharyngeal expression is very low at both the comma and 1.5-fold embryonic stages (PubMed:16457801). Expression is highest in muscle and neuronal cells in larvae and adults, including many of the amphid neurons that are proximal to the posterior bulb of the pharynx (PubMed:16457801). Consistent expression in cells of the somatic gonad including distal tip, sheath, and spermathecal cells, as well as in vulval cells undergoing morphogenesis (PubMed:16457801). Neuronal expression in the midbody includes the CAN, HSN, and ALM cells; neurons of the ventral and dorsal cords; and a number of posterior deirid neurons (PubMed:16457801). Expressed in all pairs of coelomocytes (PubMed:16457801). Members of the RBR family are atypical E3 ligases. They interact with an E2 conjugating enzyme and function like HECT-type E3 enzymes: they bind E2s via the first RING-type zinc finger, but require an obligate trans-thiolation step during the ubiquitin transfer, requiring a conserved active site Cys residue in the second RING-type zinc finger. The active site probably forms a thioester intermediate with ubiquitin taken from the active-site cysteine of the E2 before ultimately transferring it to a Lys residue on the substrate. RNAi-mediated knockdown on a pha-1 or on a lin-35;ubc-18 mutant background produces a high percentage of animals with the Pun (pharyngeal unattached) phenotype, whereby the pharynx fails to elongate and form an attachment to the anterior alimentary opening or buccal cavity. Belongs to the RBR family. Ariadne subfamily.