Dynamic recruitment of Munc13 primes docked secretory granules for exocytosis

Munc13 proteins are essential for regulated exocytosis in neurons and endocrine cells. They consist of an elongated MUN domain that templates SNARE complex formation during priming, flanked by regulatory membrane-associated C1 and C2 domains. Here, we show, using quantitative high-resolution imaging, that priming of insulin granules coincides with recruitment of on average six copies of Munc13 to individual docked granules, similar to estimates of SNARE complexes formed during exocytosis. Intracellular Ca²⁺- or lipid-signaling accelerates granule priming by promoting C2B-dependent translocation of Munc13 to the plasma membrane, followed by slower (tens of seconds) C2A-domain dependent accumulation at docked granules. Exocytosis in human β-cells also exhibits rapid Ca²⁺-dependent short-term facilitation that involves Ca²⁺/C2B-dependent activation of Munc13 but not further accumulation at the release site. Thus, Munc13 controls secretory granule release probability by two separate C2B-dependent mechanisms that affect its recruitment to the release site and its subsequent activation by Ca²⁺.