Heteropentameric architecture predisposes functional inequivalence of acetylcholine receptor agonist sites

Muscle-type acetylcholine receptors are heteropentameric ion channels composed of four different but evolutionarily related subunits. These subunits assemble with a precise stoichiometry and arrangement, such that two distinct agonist-binding sites are formed at interfaces between a principle α-subunit and a complementary δ or ε/γ-subunit. Chemical differences between the two complementary subunits are presumed to confer functional inequivalence to the two agonist sites. This interpretation, however, neglects the asymmetric architecture of the acetylcholine receptor, which places each subunit, and therefore each agonist site, at unique positions within the heteropentamer. The extent to which functional inequivalence of the agonist sites stems from this structural asymmetry, as opposed to chemical differences, remains unexplored. Here, we reconstruct an ancestral subunit capable of substituting for both complementary subunits, thereby engineering hybrid ancestral/human acetylcholine receptors with chemically identical agonist sites. Despite being chemically identical, these agonist sites remain functionally inequivalent. Leveraging a human disease-causing mutation, we reveal that functional inequivalence stems from distinct intersubunit interactions, rather than chemical differences alone. This insight challenges the conventional understanding that chemical differences are the primary cause of agonist site functional inequivalence. Instead, our findings highlight the importance of the heteropentameric architecture in predisposing the agonist sites for functional inequivalence, offering new perspectives on acetylcholine receptor structure, function, and evolution.