Engineered Protein Model of the ATP synthase H⁺- Channel Shows No Salt Bridge at the Rotor-Stator Interface

ATP synthase is powered by the flow of protons through the molecular turbine composed of two α-helical integral membrane proteins, subunit a, which makes a stator, and a cylindrical rotor assembly made of multiple copies of subunit c. Transient protonation of a universally conserved carboxylate on subunit c (D61 in E. coli) gated by the electrostatic interaction with arginine on subunit a (R210 in E. coli) is believed to be a crucial step in proton transfer across the membrane. We used a fusion protein consisting of subunit a and the adjacent helices of subunit c to test by NMR spectroscopy if cD61 and aR210 are involved in an electrostatic interaction with each other, and found no evidence of such interaction. We have also determined that R140 does not form a salt bridge with either D44 or D124 as was suggested previously by mutation analysis. Our results demonstrate the potential of using arginines as NMR reporter groups for structural and functional studies of challenging membrane proteins.