Cation binding sites on the projected structure of bacteriorhodopsin

Divalent cations are involved in the function of bacteriorhodopsin (bR) as a light-driven proton pump. If cations are removed from purple membranes they become blue. Divalent cations such as Ca2+ or Pb2+ or trivalent ions, can be stoichiometrically titrated back on to these deionized membranes. The color transitions as a function of ion concentration for Ca2+ or Pb2+ are precisely comparable and indicate that approximately three stoichiometric equivalents of cations are required to effect the color transition (Kimura et al., 1984). We found four main partially occupied binding sites for cations at a stoichiometric ratio of 3 Pb2+/bR. We localized the binding sites for Pb2+ using x-ray diffraction of membranes reconstituted with 1, 2, and 3 equivalents of Pb2+ per bR. The site of highest affinity is located on helix 7. At 2 Pb2+/bR, sites on helix 6 and between helix 2 and 3 are occupied. At 3 Pb2+/bR a fourth site above helix 3 is occupied.