Munc13 serves as a molecular filter for SNARE sorting and assembly in active zone condensates

Neurotransmitter release requires the precise localization and assembly of the SNARE machinery at presynaptic release sites. Although liquid-liquid phase separation of active zone scaffolds is known to organize these sites, the mechanism for the specific enrichment of the SNARE machinery has remained unclear. In this study, we establish that within RIM1/RIM-BP2 biomolecular condensates, Munc13-1 functions as an organizational hub that spatially sorts and concentrates cognate SNAREs, orchestrating their stepwise assembly. In particular, this condensate environment empowers Munc13-1 to cluster Syb2 into nanodomains on the vesicle membrane, dramatically increasing local SNARE density and ensuring efficient complex formation. Consequently, the introduction of RIM1/RIM-BP2 into PC12 cells enhances spatiotemporally precise dense-core vesicle exocytosis in a Munc13-dependent manner. Our findings suggest a model in which phase separation creates a privileged platform that enables Munc13-1 to direct SNARE complex assembly, thereby ensuring the speed and precision of synaptic vesicle exocytosis.