A release of local subunit conformational heterogeneity underlies gating in a muscle nicotinic acetylcholine receptor.

Synaptic receptors respond to neurotransmitters by opening an ion channel across the post-synaptic membrane to elicit a cellular response. Here we use recent Torpedo acetylcholine receptor structures and functional measurements to delineate a key feature underlying allosteric communication between the agonist-binding extracellular and channel-gating transmembrane domains. Extensive mutagenesis at this inter-domain interface re-affirms a critical energetically coupled role for the principal α subunit ß1-ß2 and M2-M3 loops, with agonist binding re-positioning a key ß1-ß2 glutamate/valine to facilitate the outward motions of a conserved M2-M3 proline to open the channel gate. Notably, the analogous structures in non-α subunits adopt a locally active-like conformation in the apo state even though each L9' hydrophobic gate residue in each pore-lining M2 α-helix is closed. Agonist binding releases local conformational heterogeneity transitioning all five subunits into a conformationally symmetric open state. A release of conformational heterogeneity provides a framework for understanding allosteric communication in pentameric ligand-gated ion channels.

Origin of acetylcholine antagonism in ELIC, a bacterial pentameric ligand-gated ion channel.

ELIC is a prokaryotic homopentameric ligand-gated ion channel that is homologous to vertebrate nicotinic acetylcholine receptors. Acetylcholine binds to ELIC but fails to activate it, despite bringing about conformational changes indicative of activation. Instead, acetylcholine competitively inhibits agonist-activated ELIC currents. What makes acetylcholine an agonist in an acetylcholine receptor context, and an antagonist in an ELIC context, is not known. Here we use available structures and statistical coupling analysis to identify residues in the ELIC agonist-binding site that contribute to agonism. Substitution of these ELIC residues for their acetylcholine receptor counterparts does not convert acetylcholine into an ELIC agonist, but in some cases reduces the sensitivity of ELIC to acetylcholine antagonism. Acetylcholine antagonism can be abolished by combining two substitutions that together appear to knock out acetylcholine binding. Thus, making the ELIC agonist-binding site more acetylcholine receptor-like, paradoxically reduces the apparent affinity for acetylcholine, demonstrating that residues important for agonist binding in one context can be deleterious in another. These findings reinforce the notion that although agonism originates from local interactions within the agonist-binding site, it is a global property with cryptic contributions from distant residues. Finally, our results highlight an underappreciated mechanism of antagonism, where agonists with appreciable affinity, but negligible efficacy, present as competitive antagonists.

Distinct functional roles for the M4 α-helix from each homologous subunit in the heteropentameric ligand-gated ion channel nAChR.

The outermost lipid-exposed α-helix (M4) in each of the homologous α, ß, δ, and γ/ε subunits of the muscle nicotinic acetylcholine receptor (nAChR) has previously been proposed to act as a lipid sensor. However, the mechanism by which this sensor would function is not clear. To explore how the M4 α-helix from each subunit in human adult muscle nAChR influences function, and thus explore its putative role in lipid sensing, we functionally characterized alanine mutations at every residue in αM4, ßM4, δM4, and εM4, along with both alanine and deletion mutations in the post-M4 region of each subunit. Although no critical interactions involving residues on M4 or in post-M4 were identified, we found that numerous mutations at the M4-M1/M3 interface altered the agonist-induced response. In addition, homologous mutations in M4 in different subunits were found to have different effects on channel function. The functional effects of multiple mutations either along M4 in one subunit or at homologous positions of M4 in different subunits were also found to be additive. Finally, when characterized in both Xenopus oocytes and human embryonic kidney 293T cells, select αM4 mutations displayed cell-specific phenotypes, possibly because of the different membrane lipid environments. Collectively, our data suggest different functional roles for the M4 α-helix in each heteromeric nAChR subunit and predict that lipid sensing involving M4 occurs primarily through the cumulative interactions at the M4-M1/M3 interface, as opposed to the alteration of specific interactions that are critical to channel function.

Recent Insight into Lipid Binding and Lipid Modulation of Pentameric Ligand-Gated Ion Channels.

Pentameric ligand-gated ion channels (pLGICs) play a leading role in synaptic communication, are implicated in a variety of neurological processes, and are important targets for the treatment of neurological and neuromuscular disorders. Endogenous lipids and lipophilic compounds are potent modulators of pLGIC function and may help shape synaptic communication. Increasing structural and biophysical data reveal sites for lipid binding to pLGICs. Here, we update our evolving understanding of pLGIC-lipid interactions highlighting newly identified modes of lipid binding along with the mechanistic understanding derived from the new structural data.

Ion channels as lipid sensors: from structures to mechanisms.

Ion channels play critical roles in cellular function by facilitating the flow of ions across the membrane in response to chemical or mechanical stimuli. Ion channels operate in a lipid bilayer, which can modulate or define their function. Recent technical advancements have led to the solution of numerous ion channel structures solubilized in detergent and/or reconstituted into lipid bilayers, thus providing unprecedented insight into the mechanisms underlying ion channel-lipid interactions. Here, we describe how ion channel structures have evolved to respond to both lipid modulators and lipid activators to control the electrical activities of cells, highlighting diverse mechanisms and common themes.

The functional role of the αM4 transmembrane helix in the muscle nicotinic acetylcholine receptor probed through mutagenesis and coevolutionary analyses.

The activity of the muscle-type Torpedo nicotinic acetylcholine receptor (nAChR) is highly sensitive to lipids, but the underlying mechanisms remain poorly understood. The nAChR transmembrane α-helix, M4, is positioned at the perimeter of each subunit in direct contact with lipids and likely plays a central role in lipid sensing. To gain insight into the mechanisms underlying nAChR lipid sensing, we used homology modeling, coevolutionary analyses, site-directed mutagenesis, and electrophysiology to examine the role of the α-subunit M4 (αM4) in the function of the adult muscle nAChR. Ala substitutions for most αM4 residues, including those in clusters of polar residues at both the N and C termini, and deletion of up to 11 C-terminal residues had little impact on the agonist-induced response. Even Ala substitutions for coevolved pairs of residues at the interface between αM4 and the adjacent helices, αM1 and αM3, had little effect, although some impaired nAChR expression. On the other hand, Ala substitutions for Thr422 and Arg429 caused relatively large losses of function, suggesting functional roles for these specific residues. Ala substitutions for aromatic residues at the αM4-αM1/αM3 interface generally led to gains of function, as previously reported for the prokaryotic homolog, the Erwinia chrysanthemi ligand-gated ion channel (ELIC). The functional effects of individual Ala substitutions in αM4 were found to be additive, although not in a completely independent manner. Our results provide insight into the structural features of αM4 that are important. They also suggest how lipid-dependent changes in αM4 structure ultimately modify nAChR function.

Structural basis for the modulation of pentameric ligand-gated ion channel function by lipids.

Pentameric ligand-gated ion channels (pLGICs) play a central role in synaptic communication and are implicated in a plethora of neurological disorders leading to human disease. Membrane lipids are known to modulate pLGIC function, but the mechanisms underlying their effects are poorly understood. Recent structures reveal sites for the binding of membrane lipids to pLGICs, thus providing a structural basis for interpreting functional data on pLGIC-lipid interactions. Here, we review the literature describing the known functional effects of membrane lipids on different members of the pLGIC superfamily and highlight pLGIC structures that exhibit bound lipids. We discuss new insight into the mechanisms of pLGIC-lipid interactions that has been derived from these recent structures.