Derepression masquerades as activation in a pentameric ligand-gated ion channel
Agonists are ligands that bind to receptors and activate them. In the case of ligand-gated ion channels, such as the muscle-type nicotinic acetylcholine receptor, mechanisms of agonist activation have been studied for decades. Taking advantage of a reconstructed ancestral muscle-type {beta}-subunit that forms spontaneously activating homopentamers, here we show that incorporation of human muscle-type -subunits represses spontaneous activity, and furthermore that the presence of agonist relieves this -subunit-dependent repression. Our results demonstrate that rather than provoking channel activation/opening, agonists may instead inhibit the inhibition of intrinsic spontaneous activity. Thus, agonist activation may be the apparent manifestation of agonist-induced derepression. These results provide insight into intermediate states that precede channel opening and have implications for the interpretation of agonism in ligand-gated ion channels.