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J. Biol. Chem., Vol. 279, Issue 20, 21327-21333, May 14, 2004
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From the
Biophysical Techniques Group, Department of Science and Technology, University of Twente, 7500 AE Enschede, The Netherlands and ¶Robert Hill Institute for Photosynthesis Research, Krebs Institute for Biomolecular Research, Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield S10 2TN, United Kingdom
Previous electron microscopic studies of bacterial RCLH1 complexes demonstrated both circular and elliptical conformations of the LH1 ring, and this implied flexibility has been suggested to allow passage of quinol from the QB site of the RC to the quinone pool prior to reduction of the cytochrome bc1 complex. We have used atomic force microscopy to demonstrate that these are just two of many conformations for the LH1 ring, which displays large molecule-to-molecule variations, in terms of both shape and size. This atomic force microscope study has used a mutant lacking the reaction center complex, which normally sits within the LH1 ring providing a barrier to substantial changes in shape. This approach has revealed the inherent flexibility and lack of structural coherence of this complex in a reconstituted lipid bilayer at room temperature. Circular, elliptical, and even polygonal ring shapes as well as arcs and open rings have been observed for LH1; in contrast, no such variations in structure were observed for the LH2 complex under the same conditions. The basis for these differences between LH1 and LH2 is suggested to be the H-bonding patterns that stabilize binding of the bacteriochlorophylls to the LH polypeptides. The existence of open rings and arcs provides a direct visualization of the consequences of the relatively weak associations that govern the aggregation of the protomers (
1
1Bchl2) comprising the LH1 complex. The demonstration that the linkage between adjacent protomer units is flexible and can even be uncoupled at room temperature in a detergent-free membrane bilayer provides a rationale for the dynamic separation of individual protomers, and we may now envisage experiments that seek to prove this active opening process.
Received for publication, December 1, 2003 , and in revised form, February 2, 2004.
* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
The on-line version of this article (available at http://www.jbc.org) contains one figure and two tables.
Supported by the Netherlands Organization for Scientific Research.
|| Funded by the Biotechnology and Biological Sciences Research Council.
** Funded by the Biotechnology and Biological Sciences Research Council. To whom correspondence should be addressed. Tel.: 440-114-222-4240; Fax: 440-114-222-2711; E-mail: j.olsen{at}sheffield.ac.uk.
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