Effect of membrane lipid composition on the conformational equilibria of the nicotinic acetylcholine receptor

The effects of cholesterol (Chol) and an anionic lipid, dioleoylphosphatidi c acid (DOPA) on the conformational equilibria of the nicotinic acetylcholi ne receptor (nAChR) have been investigated using Fourier transform infrared difference spectroscopy. The difference between spectra recorded in the pr esence and absence of agonist from the nAChR reconstituted into 3:1:1 egg p hosphatidylcholine (EPC)/DOPA/Chol membranes exhibits positive and negative bands that serve as markers of the structural changes associated with the resting to desensitized conformational change. These markers are absent in similar difference spectra recorded from the nAChR reconstituted into EPC m embranes lacking both Chol and DOPA, indicating that the nAChR cannot under go conformational change in response to agonist binding. When low levels of either Chol or DOPA up to 25 mol % of the total lipid are included in the EPC membranes, the markers suggest the predominant stabilization of a confo rmation that is a structural intermediate between the resting and desensiti zed states. At higher levels of either Chol or DOPA, the nAChR is stabilize d in a conformation that is capable of undergoing agonist-induced desensiti zation, although DOPA appears to be required for the nAChR to adopt a confo rmation fully equivalent to that found in native and 3:1:1 EPC/DOPA/Chol me mbranes. The ability of these two structurally diverse lipids, as well as o thers (Ryan, S. E., Demers, C. N., Chew, J. P., Baenziger, J. E. (1996) J. Biol. Chem. 271, 24590-24597), to modulate the functional state of the nACh R suggests that lipids act on the nAChR via an indirect effect on some phys ical property of the lipid bilayer, The data also suggest that anionic lipi ds are essential to stabilize a fully functional nAChR, We propose that mem brane fluidity modulates the relative populations of nAChRs in the resting and desensitized states but that subtle structural changes in the presence of anionic lipids are essential for full activity.