A C1-C2 Module in MUNC13 Inhibits Calcium-Dependent Neurotransmitter Release

Abstract

Almost all known forms of fast chemical synaptic transmission require the synaptic hub protein Munc13, but the mechanisms by which it controls vesicle fusion are not well understood. Using the C. elegans Munc13 ortholog UNC-13, we show that deletion of the C2B domain, the most highly conserved domain of Munc13, enhances calcium-dependent exocytosis downstream of vesicle priming, revealing a novel autoinhibitory role for the C2B. Furthermore, C2B inhibition is relieved by calcium-mediated binding of C2B to the plasma membrane. Finally, DAG binding to the neighboring C1 domain or its complete deletion also disengages C2B inhibition. Inhibition by the C1C2B module requires a conserved linker sequence leading into the MUN domain, and disruption of this linker structure relieves autoinhibition. Therefore, C1-C2B exerts a basal inhibitory effect on Munc13 in the primed state, while elevated calcium and DAG release this inhibition. Calcium-dependent relief of C1-C2B inhibition may contribute to superpriming, short-term plasticity, and modulation by lipid signaling pathways.