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  • Title: The projection structure of the low temperature K intermediate of the bacteriorhodopsin photocycle determined by electron diffraction.
    Author: Bullough PA, Henderson R.
    Journal: J Mol Biol; 1999 Mar 12; 286(5):1663-71. PubMed ID: 10064722.
    Abstract:
    Bacteriorhodopsin (bR) is an integral membrane protein which absorbs visible light and pumps protons across the cell membrane of Halobacterium salinarium. bR is one of the few membrane-bound pumps whose structure is known at atomic resolution. Changes in the protein structure of bR are a crucial element in the mechanism of proton pumping and can be followed by a variety of spectroscopic, and diffraction methods. A number of intermediates in the photocycle have been identified spectroscopically and a number of laboratories have been successful in reporting the structural changes taking place in the later stages of the photocycle over the millisecond time-scale using diffraction techniques. These studies have revealed significant changes in the protein structure, possibly involving changes in flexibility and/or movement of helices. Earlier intermediates which arise and decay on the picosecond to microsecond time-scale have proven more difficult to trap. Here, we report for the first time the successful trapping and diffraction analysis of bR in a low temperature state resembling the very early intermediate, K. We have calculated a projection difference map to 3.5 A resolution. The map reveals no significant structural changes in the molecule, despite having a very low background noise level. This does not rule out the possibility of movements in a direction perpendicular to the plane of the membrane. However, the data are consistent with other evidence that significant structural changes do not occur in the protein itself.
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