STRUCTURAL FEATURES OF ISOLATED M2 HELICES OF NICOTINIC RECEPTORS - SIMULATED ANNEALING VIA MOLECULAR-DYNAMICS STUDIES

The nicotinic acetylcholine receptor is an integral membrane protein a nd a ligand-gated cation channel. It has stoichiometry alpha(2) beta g amma delta, the subunits arranged symmetrically around an approximate five-fold axis. Five M2 helices, one from each subunit, form a paralle l helix bundle surrounding a central pore. Simulated annealing via res trained molecular dynamics (SA/MD) has been employed to generate ensem bles of isolated M2 transmembrane helices. Four ensembles of two diffe rent M2 helix sequences, M2 delta and M2 gamma, have been generated by SA/MD. The ensembles differed in their treatment of electrostatic int eractions. Analysis of the simulated structures showed that intra-heli cal H-bonds were more strongly conserved in the C-terminal (and more h ydrophobic) segment of M2 helices. Conformations of polar sidechains h ave been analyzed, placing particular emphasis on EK (and QK) pairs at the N-termini of M2 delta (and M2 gamma) helices. Conformations of EK sidechain pairs were obtained for the high resolution structures in t he protein database in order to guide our analysis of simulated struct ures. Serine and threonine sidechain conformations in the M2 models al so have been determined. Implications of studies of isolated M2 helice s for models of the intact pore region of the nicotinic receptor are d iscussed.