Fluorescence and molecular dynamics studies of the acetylcholine receptor γM4 transmembrane peptide in reconstituted systems

A combination of fluorescence spectroscopy and molecular dynamics ( MD) is applied to assess the conformational dynamics of a peptide making up the outermost ring of the nicotinic acetylcholine receptor ( AChR) transmembrane region and the effect of membrane thickness and cholesterol on the hydrophobic matching of this peptide. The fluorescence studies exploit the intrinsic fluorescence of the only tryptophan residue in a synthetic peptide corresponding to the fourth transmembrane domain of the AChR gamma subunit (gamma M4-Trp(6)) reconstituted in lipid bilayers of varying thickness, and combine this information with quenching studies using depth-sensitive phosphatidylcholine spin-labeled probes and acrylamide, polarization of fluorescence, and generalized polarization of Laurdan. A direct correlation was found between bilayer width and the depth of insertion of Trp(6). We further extend our recent MD study of the conformational dynamics of the AChR channel to focus on the crosstalk between M4 and the lipid-belt region. The isolated gamma M-4 peptide is shown to possess considerable orientational flexibility while maintaining a linear alpha-helical structure, and to vary its tilt depending on bilayer width and cholesterol (Chol) content. MD studies also show that gamma M4 also establishes contacts with the other TM peptides on its inner face, stabilizing a shorter TM length that is still highly sensitive to the lipid environment. In the native membrane the topology of the M4 ring is likely to exhibit a similar behavior, dynamically modifying its tilt to match the hydrophobic thickness of the bilayer.