Differential scanning calorimetry and Fourier transform infrared analysis of lipid-protein interactions involving the nicotinic acetylcholine receptor.

Lipid-protein interactions were studied using Torpedo californica acetylcholine receptor (AChR) as a model system by reconstituting purified AChR into dielaidoylphosphatidylcholine (DEPC, 18:1 trans-9,10) membranes. The structural and thermodynamic behavior of lipids in the vicinity of the protein were studied by differential scanning calorimetry and Fourier transform infrared spectroscopy. The effects of AChR on the thermodynamic parameters associated with lipid phase transitions were to reduce the enthalpy change, lower the transition temperature and reduce the cooperative behavior of the lipid molecules. A stoichiometry of approx. 95 lipids per AChR molecule was found by simulating the decrease in enthalpy in terms of a simple model in which a fixed number of lipid molecules are prevented from undergoing a cooperative phase transition. In parallel, the vibrational spectra of pure DEPC and AChR reconstituted in DEPC membranes at various lipid to protein ratios were examined. Profiles of the 3000-2800 cm-1 C-H stretching region and 1350-950 cm-1 characteristic of the headgroup region of the lipid exhibit little sensitivity to protein/lipid ratio reflecting weak interaction of AChR with DEPC. The lipid carbonyl on the other hand appear to be increasingly hydrogen bonded in the presence of AChR. The results provide new information about the size and physical state of the motionally restricted lipid environment that surrounds the acetylcholine receptor. The results are discussed in the context of lipid-mediated alterations in acetylcholine receptor function.