%0 Journal Article %A Haneui Bae %A Shirui Chen %A John P. Roche %A Minrong Ai %A Chunlai Wu %A Aaron Diantonio %A Ethan R. Graf %T Rab3-GEF controls active zone development at the Drosophila neuromuscular junction %D 2016 %R 10.1523/ENEURO.0031-16.2016 %J eneuro %P ENEURO.0031-16.2016 %X Synaptic signaling involves the release of neurotransmitter from presynaptic active zones (AZs). Proteins that regulate vesicle exocytosis cluster at AZs, composing the cytomatrix at the active zone (CAZ). At the Drosophila neuromuscular junction (NMJ), the small GTPase Rab3 controls the distribution of CAZ proteins across release sites, thereby regulating the efficacy of individual AZs. Here we identify Rab3-GEF as a second protein that acts in conjunction with Rab3 to control AZ protein composition. At rab3-GEF mutant NMJs, Bruchpilot (Brp) and Ca2+ channels are enriched at a subset of AZs, leaving the remaining sites devoid of key CAZ components in a manner that is indistinguishable from rab3 mutant NMJs. As the Drosophila homologue of mammalian DENN/MADD and C. elegans AEX-3, Rab3-GEF is a Guanine nucleotide Exchange Factor (GEF) for Rab3 that stimulates GDP to GTP exchange. Mechanistic studies reveal that while Rab3 and Rab3-GEF act within the same mechanism to control AZ development, Rab3-GEF is involved in multiple roles. We show that Rab3-GEF is required for transport of Rab3. However, the synaptic phenotype in the rab3-GEF mutant cannot be fully explained by defective transport and loss of GEF activity. A transgenically expressed GTP-locked variant of Rab3 accumulates at the NMJ at wild type levels and fully rescues the rab3 mutant but is unable to rescue the rab3-GEF mutant. Our results suggest that while Rab3-GEF acts upstream of Rab3 to control Rab3 localization and likely GTP-binding, it also acts downstream to regulate CAZ development, potentially as a Rab3 effector at the synapse.Significance Statement: The release of neurotransmitter at synapses is regulated by proteins that form the presynaptic release machine. We show here that Rab3-GEF is required for controlling the protein composition of release sites in the Drosophila neuromuscular junction (NMJ). Our results indicate that loss of Rab3-GEF results in the development of NMJs that are structurally and functionally indistinguishable from NMJs that lack the protein Rab3. Our studies reveal that Rab3 and Rab3-GEF act within the same molecular mechanism and support a model wherein Rab3-GEF serves as a Rab3 effector to control active zone protein composition. This work provides further insight into a novel molecular mechanism that controls the strength of synapses. %U https://www.eneuro.org/content/eneuro/early/2016/03/07/ENEURO.0031-16.2016.full.pdf