RIM and RIM-BP Form Presynaptic Active-Zone-like Condensates via Phase Separation.

Both the timing and kinetics of neurotransmitter release depend on the positioning of clustered Ca²⁺ channels in active zones to docked synaptic vesicles on presynaptic plasma membranes. However, how active zones form is not known. Here, we show that RIM and RIM-BP, via specific multivalent bindings, form dynamic and condensed assemblies through liquid-liquid phase separation. Voltage-gated Ca²⁺ channels (VGCCs), via C-terminal-tail-mediated direct binding to both RIM and RIM-BP, can be enriched to the RIM and RIM-BP condensates. We further show that RIM and RIM-BP, together with VGCCs, form dense clusters on the supported lipid membrane bilayers via phase separation. Therefore, RIMs and RIM-BPs are plausible organizers of active zones, and the formation of RIM and RIM-BP condensates may cluster VGCCs into nano- or microdomains and position the clustered Ca²⁺ channels with Ca²⁺ sensors on docked vesicles for efficient and precise synaptic transmissions.

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