STRUCTURAL EFFECTS OF NEUTRAL AND ANIONIC LIPIDS ON THE NICOTINIC ACETYLCHOLINE-RECEPTOR - AN INFRARED DIFFERENCE SPECTROSCOPY STUDY

The effects of both neutral and anionic lipids on the structure of the nicotinic acetylcholine receptor (nAChR) have been probed using infra red difference spectroscopy, The difference between infrared spectra o f the nAChR recorded using the attenuated total reflectance technique in the presence and absence of the neu rotransmitter analog, carbamylc holine, exhibits a complex pattern of positive and negative bands that provides a spectral map of the structural changes that occur in the n AChR upon ligand binding and subsequent desensitization. This spectral map is essentially identical in difference spectra recorded from nati ve, native alkaline-extracted, and affinity purified nAChR reconstitut ed into either soybean asolectin or egg phosphatidylcholine membranes containing both neutral and anionic lipids, This result suggests both a similar structure of the nAChR and a similar resting to desensitized conformational change in each membrane environment. In contrast, diff erence spectra recorded from the nAChR reconstituted into egg phosphat idylcholine membranes lacking neutral and/or anionic lipids all exhibi t an essentially identical pattern of band intensity variations, which is similar to the pattern of variations observed in difference spectr a recorded in the continuous presence of the desensitizing local anest hetic, dibucaine, The difference spectra suggest that the main effect of both neutral and anionic lipids in a reconstituted egg phosphatidyl choline membrane is to help stabilize the nAChR in a resting conformat ion, In the absence of neutral and/or anionic lipids, the nAChR is con verted into an alternate conformation that appears to be analogous to the local anesthetic-induced desensitized state, Significantly, the pr oportion of receptors found in the resting versus the putative desensi tized state appears to be dependent upon the final lipid composition o f the reconstituted membrane, A lipid-dependent modulation of the equi librium between a channel-active resting and channel-inactive desensit ized state may account for the modulations of nAChR activity that are observed in different lipid membranes.