Atypical E3 ubiquitin-protein ligase which specifically mediates ubiquitination of threonine and serine residues on target proteins, instead of ubiquitinating lysine residues (By similarity). Shows esterification activity towards both threonine and serine, with a preference for threonine, and acts via two essential catalytic cysteine residues that relay ubiquitin to its substrate via thioester intermediates (By similarity). Mediates ubiquitination and subsequent proteasomal degradation of target proteins, including dlk-1 (PubMed:10839353, PubMed:15707898). Negatively regulates a p38 MAP kinase pathway composed of dlk-1, mkk-4, and pmk-3 that functions presynaptically to regulate synaptic architecture (PubMed:15707898). Has a role during synaptogenesis; regulates neuronal morphology, specifically presynaptic differentiation. May regulate the spatial arrangement of presynaptic terminals or restrict their development (PubMed:10839354, PubMed:15208641, PubMed:15707898, PubMed:21968191). By targeting both dlk-1 and mlk-1 for degradation, may negatively regulate mlk-1/mek-1/kgb-1 and dlk-1/mkk-4/pmk-3 pathways which are involved in axon regeneration after injury (PubMed:21670305). Regulates axon termination in PLM and ALM neurons (PubMed:21968191). May play a role in the formation of muscle connections between the body wall and the motor axons in the dorsal and ventral cord (PubMed:27123983). [E2 ubiquitin-conjugating enzyme]-S-ubiquitinyl-L-cysteine + [acceptor protein]-L-threonine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-3-O-ubiquitinyl-L-threonine. Protein modification; protein ubiquitination. Interacts with rae-1 (PubMed:22357847). Component of an SCF (SKP1-CUL1-F-box protein) E3 ubiquitin ligase complex composed of cul-1, fsn-1, rpm-1 and skr-1 (PubMed:15208641). Localizes in periactive zone along dorsal and lateral nerve cords. Expression is seen in the pharynx, coelomocytes and distal tip cell. Most abundant expression is in axons of the nerve ring neuropil. Expression is not seen in body wall muscle, hypodermis or intestine. First expressed in comma stage embryos and persists through to adulthood. The PHR domains are compact beta-sandwich folds composed of 11 antiparallel strands and decorated with conserved apical loops. They are likely to play a structural role and mediate interactions with substrates or partners (By similarity). The tandem cysteine domain region confers threonine specificity and contains the two essential catalytic cysteine residues that relay ubiquitin. It binds four zinc ions in a C5HC7HC2 configuration. In mutants, mechanosensory neurons fail to accumulate tagged vesicles, retract synaptic branches and ectopically extend axons (PubMed:10839354). Some motor neurons branch and overgrow, others show altered synaptic organization (PubMed:10839354). Fewer body wall muscle connections to target motor neurons (PubMed:27123983). Belongs to the RING-Cys relay (RCR) family.