The presynaptic active zone protein Munc13 is essential for neurotransmitter
release, playing key roles in vesicle docking and priming. Mechanistically, it
is thought that the C2A domain of Munc13 inhibits the priming function by
homodimerization, and that RIM disrupts the autoinhibitory homodimerization
forming monomeric priming-competent Munc13. However, it is unclear whether the
C2A domain mediates other Munc13 functions in addition to this
inactivation–activation switch. Here, we utilize mutations that modulate the
homodimerization and heterodimerization states to define additional roles of
the Munc13 C2A domain. Using electron microscopy and electrophysiology in
hippocampal cultures, we show that the C2A domain is critical for additional
steps of vesicular release, including vesicle docking. Optimal vesicle docking
and priming is only possible when Munc13 heterodimerizes with RIM via its C2A
domain. Beyond being a switching module, our data suggest that the Munc13-RIM
heterodimer is an active component of the vesicle docking, priming and release
complex
