Journal
EMBO JOURNAL
Volume 19, Issue 10, Pages 2152-2160Publisher
WILEY
DOI: 10.1093/emboj/19.10.2152
Keywords
bacteriorhodopsin; conformational change; electron crystallography; proton pump; trapped intermediates
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Bacteriorhodopsin is a light-driven proton pump in halobacteria that forms crystalline patches in the cell membrane. Isomerization of the bound retinal initiates a photocycle resulting in the extrusion of a proton. An electron crystallographic analysis of the N intermediate from the mutant F219L gives a three-dimensional view of the large conformational change that occurs on the cytoplasmic side after deprotonation of the retinal Schiff base. Helix F, together with helix E, tilts away from the center of the molecule, causing a shift of similar to 3 Angstrom at the EF loop. The top of helix G moves slightly toward the ground state location of helix F, These movements open a water-accessible channel in the protein, enabling the transfer of a proton from an aspartate residue to the Schiff base. The movement of helix F toward neighbors in the crystal lattice is so large that it would not allow all molecules to change conformation simultaneously, limiting the occupancy of this state in the membrane to 33%, This explains photocooperative phenomena in the purple membrane.
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