The αM1 transmembrane segment of the nicotinic acetylcholine receptor interacts strongly with model membranes

Abstract

The transmembrane domain of the nicotinic acetylcholine receptor (nAChR) plays a role in the regulation of the activity of this important ligand-gated ion channel. The lipid composition of the host membrane affects conformational equilibria of the nAChR and several classes of inhibitors, most notably anaesthetics, interact directly or indirectly with the four transmembrane M-segments, M1-M4, of the nAChR subunits. It has proven difficult to gain insight into structure-function relationships of the M-segments in the context of the entire receptor and the biomembrane environment. However, model membrane systems are well suited to obtain detailed information about protein-lipid interactions. In this solid-state NMR study, we characterized interactions between a synthetic alphaM1 segment of the T. californica nAChR and model membranes of different phosphatidylcholine (PC) lipids. The results indicate that alphaM1 interacts strongly with PC bilayers: the peptide orders the lipid acyl chains and induces the formation of small vesicles, possibly through modification of the lateral pressure profile in the bilayer. The multilamellar vesicle morphology was stabilized by the presence of cholesterol, implying that either the rigidity or the bilayer thickness is a relevant parameter for alphaM1-membrane interactions, which also has been suggested for the entire nAChR. Our results suggest that the model systems are to a certain extent sensitive to peptide-bilayer hydrophobic matching requirements, but that the lipid response to hydrophobic mismatch alone is not the explanation. The effect of alphaM1 on different PC bilayers may indicate that the peptide is conformationally flexible, which in turn would support a membrane-mediated modulation of the conformation of transmembrane segments of the nAChR.

Structure of the First Transmembrane Domain of the Neuronal Acetylcholine Receptor β2 Subunit

“…Although the helical structure of TM domains has been suggested by many experiments, it remains controversial as to whether every TM segment is a linear a-helix (7)(8)(9). Among all TM domains, TM1 appears to be most susceptible to possessing a substantial amount of nonhelical elements (10)(11)(12)(13)(14)(15)(16)(17)(18)(19). Because TM1 is the only TM segment that links covalently to the ligand-binding domain, it can potentially play an important role in the regulation of channel functions.…”

Interfacial properties of the M1 segment of the nicotinic acetylcholine receptor

“…The peptide was characterized by mass spectrometry and a value of 3973.23 was found compared to the calculated average mass of [M+ H]+ 3973.85. The details are described in [14].…”

“…The M1 peptide was synthesized using Fmoc chemistry as described earlier [14]. The peptide was purified by size exclusion chromatography on a Sephadex LH20 column with dichloromethane/methanol (1 : 1 v/v) as eluent.…”

“…At this site, the peptide could be found with almost equal probability in a helical or a non-helical conformation, suggesting that the conserved Pro residue imparts conformational flexibility on M1 [14,15].…”

Specific Membrane Binding of Neurotoxin II Can Facilitate Its Delivery to Acetylcholine Receptor

“…However, it was found that for many membrane receptors, the lipid membrane could influence local concentration, diffusion, conformation, and orientation of a ligand, facilitating its recognition by the receptor, i.e., the membrane could optimize the ligand/receptor interaction through several different effects via the so-called ''membrane catalysis'' (also known as the ''membrane-compartment'') mechanism (8,9). In the case of nAChR, it is well known that some membrane properties (i.e., fluidity, surface charge density, lipid packing, and composition) affect functioning of the receptor and can influence its ligand binding (10)(11)(12)(13)(14)(15)(16). In particular, it was proposed that an approach of a-neurotoxins to their binding sites is sensitive to the physical state of the plasma membrane surrounding nAChR (13).…”