Component of a SCF (SKP1-CUL1-F-box protein) E3 ubiquitin ligase complex which is required for the restriction and/or maturation of synapses in GABAergic neuromuscular junction (NMJ) presynaptic neurons (PubMed:15208641, PubMed:23665919). Promotes NRJ synapse development and synaptic transmission by negatively regulating the daf-2/InsR pathway in muscles (PubMed:23665919). By targeting convertase egl-3 for degradation, negatively modulates insulin-like protein ins-4 and ins-6 processing (PubMed:23665919). May stabilize synapse formation by promoting the down-regulation of scd-2 (PubMed:15208641). Regulates axon termination in PLM and ALM neurons (PubMed:21968191). Protein modification; protein ubiquitination. 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). Interacts (via SPRY domain) with scd-2 (via cytoplasmic domain) (PubMed:15208641). Interacts (via SPRY domain) with convertase egl-3 (via C-terminus) (PubMed:23665919). Expressed in GABAergic neuromuscular junctions (NMJs). Synaptic defects that include both overdevelopment and underdevelopment of presynaptic and postsynaptic termini (PubMed:15208641). Abnormal morphology of cholinergic and GABAergic neuromuscular junctions (NMJs) characterized by uneven distribution of presynaptic markers snb-1 and unc-17 and post-synaptic markers GABA receptor unc-49 and AChR unc-38. Also, formation of abnormally large and small synapses (PubMed:23665919). Frequency of spontaneous miniature synaptic release (mPSC) from cholinergic and GABAergic NMJs is reduced (PubMed:23665919). In a daf-2 e1370 mutant background, but not in a daf-2 e1370 and daf-16 mu86 mutant background, the morphology of NMJs is normal and normal synaptic transmission is partially restored (PubMed:23665919). Nuclear translocation of daf-16 (isoform a) is impaired in muscles but not in the intestine (PubMed:23665919). Belongs to the FBXO45/Fsn family.