RIM‐binding proteins recruit BK‐channels to presynaptic release sites adjacent to voltage‐gated Ca(2+)‐channels

The active zone of presynaptic nerve terminals organizes the neurotransmitter release machinery, thereby enabling fast Ca(2+)‐triggered synaptic vesicle exocytosis. BK‐channels are Ca(2+)‐activated large‐conductance K(+)‐channels that require close proximity to Ca(2+)‐channels for activation and control Ca(2+)‐triggered neurotransmitter release by accelerating membrane repolarization during action potential firing. How BK‐channels are recruited to presynaptic Ca(2+)‐channels, however, is unknown. Here, we show that RBPs (for RIM‐binding proteins), which are evolutionarily conserved active zone proteins containing SH3‐ and FN3‐domains, directly bind to BK‐channels. We find that RBPs interact with RIMs and Ca(2+)‐channels via their SH3‐domains, but to BK‐channels via their FN3‐domains. Deletion of RBPs in calyx of Held synapses decreased and decelerated presynaptic BK‐currents and depleted BK‐channels from active zones. Our data suggest that RBPs recruit BK‐channels into a RIM‐based macromolecular active zone complex that includes Ca(2+)‐channels, synaptic vesicles, and the membrane fusion machinery, thereby enabling tight spatio‐temporal coupling of Ca(2+)‐influx to Ca(2+)‐triggered neurotransmitter release in a presynaptic terminal.