Required in the presynaptic motoneuron to down-regulate the levels of wnd and restrain synaptic terminal growth at the neuromuscular junction (NMJ). Negatively regulates the localization of vas to the posterior pole of the oocyte. Involved in primordial germ cell formation. Protein modification; protein ubiquitination. Component of an E3 ubiquitin ligase complex composed of hiw and Fsn (PubMed:17697379, PubMed:21874015). Interacts with Rae1, probably as part of the hiw-Fsn complex (PubMed:21874015). Interacts (via B30.2/SPRY domain) with vas (PubMed:20123973). Interacts with Cul1 (PubMed:20123973). In mid- to late-stage oocytes, recruited to the pole plasm. This posterior distribution is not maintained in later developmental stages. Some cortical accumulation is observed at mid- to late-stage oocytes. In mature eggs, present at high levels throughout the ooplasm. Expressed in nurse cells and oocytes (at protein level) (PubMed:20123973). Expressed in the brain (PubMed:21874015). Expressed in the neuromuscular junction (NMJ) (PubMed:17697379). Expressed in oocytes (at protein level). Dramatic overgrowth of synaptic termini, with a large increase in the number of synaptic boutons and branches. Also impaired synaptic transmission. Belongs to the FBXO45/Fsn family.