Internal dynamics of the nicotinic acetylcholine receptor in reconstituted membranes

The structure and H-1/H-2 exchange kinetics of affinity-purified nAChR reconstituted into egg phosphatidylcholine membranes with increasing levels of either dioleoylphosphatidic acid (DOPA) or cholesterol (Chol) have been examined using infrared spectroscopy. All spectra of the reconstituted nAChR membranes recorded after 72 h in (H2O)-H-2 exhibit comparable amide I band shapes, suggesting a similar secondary structure for the nAChR in each lipid environment. Increasing levels of either DOPA or Chol, however, lead to an increasing intensify of the amide II band, indicating a decreasing proportion of nAChR peptide hydrogens that have exchanged for deuterium. Spectra recorded as a function of time after exposure of the nAChR to (H2O)-H-2 show that the presence of either lipid slows down the H-1/H-2 exchange of those peptide hydrogens that normally exchange on the minutes to hours time scale. The slowing of peptide H-1/H-2 exchange correlates with both an increasing ability of the nAChR to undergo agonist-induced conformational change [Baenziger, J. E., Morris, M.-L., Darsaut, T. E., and Ryan, S. E. (1999) in preparation] and possibly a decreasing membrane fluidity. Our data suggest that lipid composition dependent changes in nAChR peptide H-1/H-2 exchange kinetics reflect altered internal dynamics of the nAChR. Lipids may influence protein function by changing the internal dynamics of integral membrane proteins.