Electron diffraction analysis of structural changes in the photocycle of bacteriorhodopsin.

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Abstract
Structural changes are central to the mechanism of light‐driven proton transport by bacteriorhodopsin, a seven‐helix membrane protein. The main intermediate formed upon light absorption is M, which occurs between the proton release and uptake steps of the photocycle. To investigate the structure of the M intermediate, we have carried out electron diffraction studies with two‐dimensional crystals of wild‐type bacteriorhodopsin and the Asp96‐‐>Gly mutant. The M intermediate was trapped by rapidly freezing the crystals in liquid ethane following illumination with a xenon flash lamp at 5 and 25 degrees C. Here, we present 3.5 A resolution Fourier projection maps of the differences between the M intermediate and the ground state of bacteriorhodopsin. The most prominent structural changes are observed in the vicinity of helices F and G and are localized to the cytoplasmic half of the membrane.