Postsynaptic Complexin Mediates Constitutive Exocytosis of Nicotinic Acetylcholine Receptor

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.