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J. Biol. Chem., Vol. 279, Issue 3, 2063-2068, January 16, 2004

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Structural Analysis of the Reaction Center Light-harvesting Complex I Photosynthetic Core Complex of Rhodospirillum rubrum Using Atomic Force Microscopy^*

Dimitrios Fotiadis, Pu Qian, Ansgar Philippsen, Per A. Bullough, Andreas Engel¶, and C. Neil Hunter||

From the Krebs Institute, Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield S10 2TN, United Kingdom and M. E. Müller Institute for Structural Biology, Biozentrum, University of Basel, Basel CH-4056, Switzerland

The bacterium Rhodospirillum rubrum contains a simple photosynthetic system, in which the reaction center (RC) receives energy from the light-harvesting (LH1) complex. We have used high-resolution atomic force microscopy (AFM) to image two-dimensional crystals of the RC-LH1 complex of R. rubrum. The AFM topographs show that the RC-LH1 complex is 94 Å in height, the RC-H subunit protrudes from the cytoplasmic face of the membrane by 40 Å, and it sits 21 Å above the highest point of the surrounding LH1 ring. In contrast, the RC on the periplasmic side is at a lower level than LH1, which protrudes from the membrane by 12 Å. The RC-LH1 complex can adopt an irregular shape in regions of uneven packing forces in the crystal; this reflects a likely flexibility in the natural membrane, which might be functionally important by allowing the export of quinol formed as a result of RC photochemistry. Nanodissection of the RC by the AFM tip removes the RC-H subunit and reveals the underlying RC-L and -M subunits. LH1 complexes completely lacking the RC were also found, providing ideal conditions for imaging both rings of LH1 polypeptides for the first time by AFM. In addition, we demonstrate the ellipticity of the LH1 ring at the cytoplasmic and periplasmic sides of the membrane, in both the presence and absence of the RC. These AFM measurements have been reconciled with previous electron microscopy and NMR data to produce a model of the RC-LH1 complex.

Received for publication, September 18, 2003 , and in revised form, October 20, 2003.

^* This work was supported by the Macromolecular Assemblies: Structure and Function Initiative of the Biotechnology and Biological Sciences Research Council and North of England Structural Biology Centre (both United Kingdom). 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.

^¶ Supported by the Swiss National Research Foundation, the M. E. Müller Foundation, the Swiss National Center of Competence in Research (NCCR) "Structural Biology," and the NCCR "Nanoscale Science."

^|| To whom correspondence should be addressed. Tel.: 0114-222-4191; Fax: 0114-222-2711; E-mail: c.n.hunter{at}sheffield.ac.uk.

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