During postembryonic development, functions with endocytic adapter Eps-15 in neurons to restrain synaptic growth, by inhibiting BMP signaling, and to control synaptic endocytosis. Required presynaptically for neuromuscular junction (NMJ) neurotransmission. Inhibits neuronal BMP signaling by promoting endocytic internalization and subsequent endosomal trafficking of the BMP receptor wit. In this way, regulates the Fmr1 translational regulator controlling Futsch expression to modulate neuronal microtubule stability, which controls both synaptogenesis and neuronal survival. Interacts with Eps-15 (via C-terminal region); the interaction is required for spartin localization to the NMJ presynaptic membrane. Colocalizes with Eps-15 at presynaptic cell membrane. Expressed in larval brain, ventral nerve cord and neuropil (at protein level). Partially edited. Target of Adar. Mutant larvae show an overgrowth of the third-instar NMJ, with an overall bouton number and satellite bouton number increase of 40% and 90%, respectively, compared with wild type. Mutant larvae also show a decrease in the amplitude of evoked excitatory junction currents (EJCs), but normal spontaneous miniature EJCs (mEJCs) and axonal transport. Adult mutants display reduced locomotor activity and progressive vacuolization in the brain, which is associated with neurodegeneration.