The role of charge in lipid selectivity for the nicotinic acetylcholine receptor

The effect of salt and pH titration on the selectivity of spin-labeled analogues of phosphatidic acid, phosphatidylserine, phosphatidylcholine, and stearic acid for the nicotinic acetylcholine receptor (nAcChoR) reconstituted into dioleoylphosphatidylcholine was examined at 0 degrees C using electron spin resonance spectroscopy. The order of selectivity at pH 7.4 and 0 mM NaCl was phosphatidylserine > stearic acid > phosphatidic acid > phosphatidylcholine. The addition up to 2 M NaCl or titration of pH from 5.0 to > 9.0 did not alter the selectivity of the phospholipids for the nAcChoR. For stearic acid, conversely, titration of pH from 5.0 to 9.0 at 0 mM NaCl and titration of NaCl from 0 to 2 M at pH 9.0 both increased selectivity for the nAcChoR. It is concluded that electrostatic interactions do not account for the selectivity of the negatively charged phospholipids, phosphatidylserine, and phosphatidic acid for the nAcChoR. This is consistent with the known orientation of the transmembrane sequences M1 and M4, which predicts a balance in the number of negative and positive charges in the lipid-protein interface and suggests that the two positive charges on each M3 helix are not exposed to the lipid-protein interface.