Component of an extracellular matrix cue that is involved in the guidance of dorsoventral midline migrations and in the specification of postsynaptic domains at neuromuscular junctions (NMJs) (PubMed:22014523, PubMed:24896188, PubMed:25122090, PubMed:26028575, PubMed:26028574). Acts as a ligand for the netrin receptor unc-40 and the neuroligin receptor nlg-1 (PubMed:22014523, PubMed:26028575, PubMed:25122090, PubMed:24896188). Secreted by the dorsal and ventral nerve cords to attract sensory axons and muscle membrane extensions called muscle arms (PubMed:22014523). In parallel with unc-6 and slt-1, involved in the netrin receptor unc-40 dependent guidance of the AVM and PVM mechanosensory axons along the dorsal-ventral axis (PubMed:22014523). The unc-40 coreceptor eva-1 is enhancing the responsiveness of unc-40 to the madd-4 guidance cue to attract the muscle arm extensions and AVM mechanosensory axons towards the dorsoventral midline (PubMed:25122090). Acts as a synaptic organizer and is required for the specification of inhibitory GABAergic and excitatory cholinergic identities of postsynaptic domains at neuromuscular junctions (NMJs) (PubMed:24896188, PubMed:26028575, PubMed:26028574). Required for the recruitment of unc-40 to both cholinergic and GABAergic NMJs (PubMed:26028575). Promotes the clustering of ACh receptors and GABA(A) receptors at postsynaptic sites during synaptogenesis (PubMed:24896188). The binding to the presynaptic adhesion protein nrx-1 and to the neuroligin nlg-1 at postsynaptic sites promotes clustering of GABAergic receptors at postsynaptic NMJs, thereby contributing to normal GABAergic synaptic transmission (PubMed:26028574). Isoform a and isoform c: Promotes the clustering of acetylcholine receptors (AChR) at excitatory cholinergic synapses of NMJs via the netrin receptor unc-40. Acts as a guidance cue in the attraction of muscle membrane extensions (muscle arms) to the dorsal cord and in cooperation with unc-6 to the ventral cord via the netrin receptor unc-40 and via the unc-40 coreceptor eva-1 (PubMed:22014523, PubMed:25122090). Together with nrx-1, clusters netrin receptor unc-40 and neuroligin nlg-1 at postsynaptic sites of GABAergic NMJs, thereby promoting the recruitment of GABA(A) receptors at GABAergic synapses (PubMed:24896188, PubMed:26028575, PubMed:26028574). Prevents the recruitment of GABAergic receptors to cholinergic synapses (PubMed:24896188). Interacts with eva-1 (via the SUEL-type lectin domain) (PubMed:25122090). Interacts with unc-5 (PubMed:25122090). Interacts with unc-40; the interaction is required for the localization of unc-40 to postsynaptic domains (PubMed:25122090, PubMed:26028575). Isoform a forms homodimers and heterodimers with isoform b (PubMed:24896188). Isoform b forms homodimers and heterodimers with isoform a (PubMed:24896188). Isoform b interacts with nlg-1 (via extracellular domain); the interaction is required for nlg-1 localization to postsynaptic domains (PubMed:26028574, PubMed:26028575). Isoform b interacts (via the Ig-like C2-type domain) with nrx-1 (via C-terminus) (PubMed:26028574). Isoform b is secreted by cholinergic and GABAergic motoneurons and localizes to excitatory cholinergic and inhibitory GABAergic NMJs, whereas isoform a and isoform c are exclusively secreted by cholinergic motoneurons and localize to excitatory cholinergic NMJs. Isoform a: Expressed in the commissural GABAergic and cholinergic motor neurons in the first larval stage but only in the cholinergic motor neurons in later larval stages and in adult animals (PubMed:22014523). At the L1 larval stage, mainly localized at the nerve ring and at the dorsal cord (PubMed:24896188). Isoform b: Expressed in the commissural GABAergic and cholinergic motor neurons whose cell bodies reside in the ventral nerve cord and which extend axons into the ventral and dorsal nerve cord (PubMed:22014523, PubMed:24896188). Also expressed in the head neurons RIA, RIC, lateral IL1s, lateral IL2s, OLLs, RMEs and SABs, all of which extend axons into the nerve ring (PubMed:22014523, PubMed:24896188). Expressed in the embryogenic blast cells and the corresponding terminally differentiated ventral cord motor neurons and head neurons (PubMed:22014523). Isoform a: Expressed in larval stages L1 and L2 and in adult animals. Isoform b: Expressed in embryos, larval stages L1 and L2 and in adult animals. The Ig-like C2-type domain is required for the attraction of the muscle arm extensions. Disrupts the clustering of the cholinergic receptor subunits unc-29, unc-38 and acr-16 and the GABAergic receptor subunit unc-49 at postsynaptic domains of neuromuscular junctions (NMJs), and the receptors are redistributed to extrasynaptic areas (PubMed:24896188, PubMed:26028575). Loss of neuroligin receptor nlg-1 and netrin receptor unc-40 localization to NMJs (PubMed:26028575). Isoform a and isoform c: Defects in AChR localization to cholinergic synapses (PubMed:24896188). Decreased AChR-dependent currents triggered by motoneuron stimulation (PubMed:24896188). Isoform b: Extensive dorsal muscle arm extension defects and weaker ventral muscle arm extension defects (PubMed:24896188, PubMed:22014523). Relocalization of GABAergic receptors from GABAergic to cholinergic synapses (PubMed:24896188, PubMed:26028575). Redistribution of the neuroligin receptor nlg-1 from GABAergic to cholinergic NMJs (PubMed:26028575). In a unc-6 mutant background, extensive ventral muscle arm extension defects (PubMed:22014523). Loss of unc-40 localization at GABAergic NMJs (PubMed:26028575). Produced by alternative promoter usage. Produced by alternative splicing of isoform a.