Adapter protein that provides a link between vesicular membrane traffic and the actin assembly machinery. Acts together with Cdc42 to stimulate actin nucleation mediated by WASp and the ARP2/3 complex (PubMed:18701694, PubMed:27601635). Binds to membranes enriched in phosphatidylinositol 4,5-bisphosphate and causes local membrane deformation (PubMed:23761074, PubMed:26686642). Required for normal structure and function of synapses at the neuromuscular junction (PubMed:18701694, PubMed:14980202, PubMed:18498733, PubMed:21464232, PubMed:26686642, PubMed:26567222, PubMed:27601635, PubMed:29568072). Plays a role in synaptic vesicle trafficking (PubMed:29568072). Required for the release of a normal number of synaptic vesicles per action potential (PubMed:26567222). Homodimer (Probable) (PubMed:27601635). Interacts (via SH3 domain 1) with WASp (PubMed:14980202, PubMed:18701694, PubMed:27601635). Interacts (via SH3 domain 1) with shi/dynamin (PubMed:18701694, PubMed:18498733). Interacts (via SH3 domain 2) with Dap160 (PubMed:18701694, PubMed:18498733, PubMed:26686642). Interacts (via F-BAR domain) with SH3PX1 (PubMed:26567222). Interacts (via SH3 domain 2) with Snx16 (PubMed:21464232). Identified in a complex with Syn and Syt1 (PubMed:29568072). Detected at presynaptic axon terminals at the neuromuscular junction (PubMed:14980202, PubMed:18701694, PubMed:21464232, PubMed:26686642, PubMed:26567222, PubMed:27601635, PubMed:29568072). Colocalizes with Cdc42 and Rab11 (PubMed:18701694, PubMed:21464232). Detected in larval body wall muscle (PubMed:29568072). Detected at the neuromuscular junction, on motoneuron axons and axon terminals, at synaptic boutons in the periactive zone surrounding the synapse (at protein level) (PubMed:14980202, PubMed:29568072). Detected on motoneuron axons and axon terminals, at synaptic boutons in the periactive zone surrounding the synapse (PubMed:26686642). Detected in embryonic brain, ventral nerve cord and sensory neurons, but not in muscle. In the autoinhibited state, the SH3 domains are bound to the concave surface of the F-BAR domain and prevent promiscuous membrane binding. Upon heterologous expression, the isolated F-BAR domain is localized at the cell membrane, and causes the formation of cellular protrusions (PubMed:23761074, PubMed:26686642). Contrary to F-BAR domains from other proteins, causes membrane flattening on giant unilamellar vesicles (in vitro) (PubMed:23761074, PubMed:26686642). Binds to membranes enriched in phosphatidylserine and phosphatidylinositides, such as phosphatidylinositol 3-phosphate, phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 4,5-bisphosphate (PubMed:23761074, PubMed:26686642, PubMed:27601635). No visible phenotype in adults at room temperature, but flies rapidly loose coordination and become paralyzed within three minutes at 38 degrees Celsius (PubMed:14980202). Larvae display overgrowth of neuromuscular junctions, with increased numbers of synaptic boutons and increased frequency and complexity of axon branching (PubMed:14980202, PubMed:18701694, PubMed:18498733, PubMed:21464232, PubMed:26686642, PubMed:29568072). The synaptic area and the number of reserve and readily releasable synaptic vesicles are decreased (PubMed:29568072).