SURFACE OF BACTERIORHODOPSIN REVEALED BY HIGH-RESOLUTION ELECTRON CRYSTALLOGRAPHY

Bacteriorhodopsin is a transmembrane protein that uses light energy, a bsorbed by its chromophore retinal, to pump protons from the cytoplasm of bacteria such as Halobacterium salinarium into the extracellular s pace(1,2.) It is made up of seven alpha-helices, and in the bacterium forms natural, two-dimensional crystals called purple membranes. We ha ve analysed these crystals by electron cryo-microscopy to obtain image s of bacteriorhodopsin at 3.0 Angstrom resolution. The structure cover s nearly all 248 amino acids, including loops outside the membrane, an d reveals the distribution of charged residues on both sides of the me mbrane surface. In addition, analysis of the electron-potential map pr oduced by this method allows the determination of the charge status of these residues. On the extracellular side, four glutamate residues su rround the entrance to the proton channel, whereas on the cytoplasmic side, four aspartic acids occur in a plane at the boundary of the hydr ophobic-hydrophilic interface. The negative charges produced by these aspartate residues is encircled by areas of positive charge that may f acilitate accumulation and lateral movement of protons on this surface .