Cations Mediate Interactions between the Nicotinic Acetylcholine Receptor and Anionic Lipids

Interactions between the nicotinic acetylcholine receptor (nAChR) and phosphatidic acid (PA) are bidirectional in that membranes containing PA are effective at stabilizing an agonist-responsive nAChR, whereas incorporation of the nAChR into the same membranes leads to a substantial increase in lipid lateral packing density. A previous study suggested that the ability of PA to adopt a dianionic ionization state is key. We monitored the ionization state of PA in both reconstituted and protein-free membranes. In model membranes composed of PA and 3:2 (mol/mol) phosphatidylcholine (PC)/PA, the monoanionic-to-dianionic transition of PA was detected with a pKa of 8.7 and 6.5, respectively. In the reconstituted 3:2 PC/PA membranes, however, PA was stabilized in a monoanionic state at pH values up to 10. Although dianionic PA does not play a role in nAChR function, we found that both the stabilization of monoanionic PA and the concentration of other cations at the bilayer surface can account for changes in bilayer physical properties that are observed upon incorporation of the nAChR into 3:2 PC/PA membranes. A nAChR-induced concentration of cations at the bilayer surface likely mediates interactions between the nAChR and the anionic lipids in its membrane environment.