Postsynaptic Complexin Mediates Constitutive Exocytosis of Nicotinic Acetylcholine Receptor

ABSTRACT Complexin is a small, soluble protein that binds SNARE complexes to both promote Ca ^{2 +} ‐triggered vesicle fusion and suppress spontaneous release at presynaptic terminals. In cultured hippocampal neurons, postsynaptic complexin has been implicated in activity‐dependent receptor exocytosis. Here, we identify a constitutive postsynaptic role of complexin at the C. elegans excitatory neuromuscular junction. Loss of CPX‐1—the major complexin homolog—markedly increases the amplitude of spontaneous miniature postsynaptic current (mPSC) in an extracellular Ca ^{2 +} ‐independent manner. This phenotype is rescued by postsynaptic, but not presynaptic, expression of CPX‐1. Moreover, disruption of nicotinic acetylcholine receptor subunit acr‐16 abolishes the enhanced mPSC amplitude, and the endogenous expression levels of postsynaptic ACR‐16 are elevated in cpx‐1 mutants. This process requires the dense‐core vesicle regulator UNC‐31/CAPS, as unc‐31 null mutations eliminate both the increased mPSC amplitude and elevated ACR‐16 expression in cpx‐1 mutants. In addition, CPX‐1 is also required for activity‐dependent exocytosis of postsynaptic ACR‐16. Together, these findings reveal that complexin regulates both constitutive and activity‐dependent exocytosis of postsynaptic receptors.