Lipid-Protein Interactions at the Nicotinic Acetylcholine Receptor

Abstract

The structural and functional properties of reconstituted nicotinic acetylcholine receptor membranes composed of phosphatidyl choline either with or without cholesterol and/or phosphatidic acid have been examined to test the hypothesis that receptor conformational equilibria are modulated by the physical properties of the surrounding lipid environment. Spectroscopic and chemical labeling data indicate that the receptor in phosphatidylcholine alone is stabilized in a desensitizedlike state, whereas the presence of either cholesterol or phosphatidic acid favors a resting-like conformation. Membranes that effectively stabilize a resting-like state exhibit a relatively large proportion of non-hydrogen-bonded lipid ester carbonyls, suggesting a relatively tight packing of the lipid head groups and thus a well ordered membrane. Functional reconstituted membranes also exhibit gel-to-liquid crystal phase transition temperatures that are higher than those of nonfunctional reconstituted membranes composed of phosphatidylcholine alone. Significantly, incorporation of the receptor into phosphatidic acid-containing membranes leads to a dramatic increase in both the lateral packing densities and the gel-to-liquid crystal phase transition temperatures of the reconstituted lipid bilayers. These results suggest a functional link between the nicotinic acetylcholine receptor and the physical properties of phosphatidic acid-containing membranes that could underlie the mechanism by which this lipid preferentially enhances receptor function.

Molecular Dynamics Simulations of Ternary Membrane Mixture: Phosphatidylcholine, Phosphatidic Acid, and Cholesterol

“…The correlation between bulk lipid ordering and nAChR function is not obvious based on previous experimental studies 9,50 . The nAChR in 3:2 POPC/POPA appears to be predominately in a resting like state that is capable of undergoing agonist-induced desensitization.…”

“…The prepared large ternary lipid patch has the 3:1:1 molar ratio of POPC, POPA, and CHOL to mimic the composition requirement of the lipid mixture for functional nAChRs 9 . Sodium and chloride ions were added into the system by replacing water molecules to produce ionic concentrations of 150 mM.…”

“…Hence, the lipid ordering is unlikely a dominant factor that affects the protein functions. The requirement of the presence of both PA and CHOL for fully functional nAChR 9,11 implies the importance of potential interplay between PA and CHOL. Although it is impossible to assess synergetic effects of POPA and CHOL on nAChR in a system without the presence of nAChR, the close interactions of POPA and CHOL observed in our current system implicated the possibility for producing their synergetic effects and the necessity for further investigation along the line.…”

“…In this study, we generated a ternary lipid system comprising of 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC), 1-palmitoyl-2-oleoyl phosphatidic acid (POPA), and CHOL in a 3:1:1 molar ratio. The composition of the ternary system mimics the lipid requirement for a functional nAChR 9 . The size of the bilayer is large enough to embed a whole nAChR for further studies.…”

Tryptophan-scanning Mutagenesis in the αM3 Transmembrane Domain of the Muscle-type Acetylcholine Receptor

“…Different experimental approaches have revealed dynamic information about agonist-induced conformational changes of the pore forming domains (M1 and M2) (8,(11)(12)(13)(14). It is noteworthy that a recent study suggested that there is no significant movement of the pore lining domains during channel activation of the AChR (14).…”