Functional asymmetry of transmembrane segments in nicotinic acetylcholine receptors
Nicotinic acetylcholine receptors are heteropentameric ion channels that open upon activation to a single conducting state. The second transmembrane segments of each subunit were identified as channel-forming elements, but their respective contribution in the gating process remains unclear. Moreover, the detailed impact of variations of the membrane potential, such as occurring during an action potential, on the transmembrane domains, is unknown. Residues at the 12' position, close to the center of each second transmembrane segment, play a key role in channel gating. We examined their functional symmetry by substituting a lysine to that position of each subunit and measuring the electrical activity of single channels. For 12' lysines in the alpha, gamma and delta subunits rapid transitions between an intermediate and large conductance appeared, which are interpreted as single lysine protonation events. From the kinetics of these transitions we calculated the pK (a) values of respective lysines and showed that they vary differently with membrane hyperpolarization. Respective mutations in beta or epsilon subunits gave receptors with openings of either intermediate or large conductance, suggesting extreme pK (a) values in two open state conformations. The results demonstrate that these parts of the highly homologous transmembrane domains, as probed by the 12' lysines, sense unequal microenvironments and are differently affected by physiologically relevant voltage changes. Moreover, observation of various gating events for mutants of alpha subunits suggests that the open channel pore exists in multiple conformations, which in turn supports the notion of functional asymmetry of the channel.