Recent crosslinking studies indicated the localization of the coupling ion binding site in the Na + -translocating F1F0 ATP synthase ofIlyobacter tartaricus within the hydrophobic part of the , a membrane embedded H + -binding site is accepted for the H + -trans-locating F1F0 ATP synthase ofEscherichia a more definite analysis, we performed parallax analysis of fluorescence quenching with ATP synthases from both I. tartaricus and E. coli.