Native architecture of the photosynthetic membrane from Rhodobacter veldkampii Lu-Ning Liu a , James N. Sturgis b , Simon Scheuring a,⇑ a Institut Curie, U1006 INSERM, UMR168 CNRS, 26 Rue d’Ulm, 75248 Paris, France b LISM CNRS, Aix Marseille Université, 31 Chemin Joseph Aiguier, 13402 Marseille, France article info Article history: Received 1 June 2010 Received in revised form 18 August 2010 Accepted 19 August 2010 Available online 24 August 2010 Keywords: Atomic force microscopy Membrane curvature Membrane protein Photosynthesis PufX Supramolecular organization abstract The photosynthetic membrane in purple bacteria contains several pigment–protein complexes that assure light capture and establishment of the chemiosmotic gradient. The bioenergetic tasks of the pho- tosynthetic membrane require the strong interaction between these various complexes. In the present work, we acquired the first images of the native outer membrane architecture and the supramolecular organization of the photosynthetic apparatus in vesicular chromatophores of Rhodobacter (Rb.) vel- dkampii. Mixed with LH2 (light-harvesting complex 2) rings, the PufX-containing LH1–RC (light-harvest- ing complex 1 – reaction center) core complexes appear as C-shaped monomers, with random orientations in the photosynthetic membrane. Within the LH1 fence surrounding the RC, a remarkable gap that is probably occupied (or partially occupied) by PufX is visualized. Sequence alignment revealed that one specific region in PufX may be essential for PufX-induced core dimerization. In this region of ten amino acids in length all Rhodobacter species had five conserved amino acids, with the exception of Rb. veldkampii. Our findings provide direct evidence that the presence of PufX in Rb. veldkampii does not directly govern the dimerization of LH1–RC core complexes in the native membrane. It is indicated, fur- thermore, that the high membrane curvature of Rb. veldkampii chromatophores (Rb. veldkampii features equally small vesicular chromatophores alike Rb. sphaeroides) is not due to membrane bending induced by dimeric RC–LH1–PufX cores, as it has been proposed in Rb. sphaeroides. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction Photosynthesis is an important biological process performed by plants, algae and some bacteria. In purple bacteria, photosynthesis requires the high connectivity between several different membrane protein complexes: the peripheral light-harvesting complex 2 (LH2), the central light-harvesting complex 1 (LH1), the photochemical reaction center (RC) and a proton translocating cytochrome (cyt) bc1 complex that converts the energy into an electrochemical poten- tial gradient, as well as the ATP synthase which is able to convert the energy into a phosphodiester bond of ATP (Cogdell et al., 2006; Hu et al., 2002). Most of these pigment–protein complexes are housed in specialized intracytoplasmic membranes (ICMs). Much effort has been made to explore the architecture of pho- tosynthetic complexes. In spite of the structural information of individual photosynthetic components, knowledge of the macro- molecular organization of these protein complexes in the native state required for understanding the physiological activities and functional cooperativity of the photosynthetic apparatus awaited the dawn of atomic force microscopy (AFM). AFM with high lateral resolution and high signal-to-noise ratio has evolved into a power- ful tool to directly and precisely visualize biological samples under physiological conditions (Engel and Gaub, 2008; Gonçalves and Scheuring, 2006; Scheuring, 2006). To date, AFM has significantly advanced the elucidation of the native surface views of ICMs from different photosynthetic bacteria, including lamellar discs in Blast- ochloris (Blc.) viridis (Scheuring et al., 2003a), Rhodospirillum (Rsp.) photometricum (Scheuring and Sturgis, 2005; Scheuring et al., 2004a,b) and Rhodopseudomonas (Rps.) palustris (Scheuring et al., 2006); vesicular structures in Phaeospirillum (Phs.) molischianum (Gonçalves et al., 2005a), Rhodobacter (Rb.) sphaeroides (Bahatyrova et al., 2004) and Rb. blasticus (Scheuring et al., 2005). These works unveiled the organization and dense packing of photosynthetic complexes, with a considerable mixing of peripheral LH2 and LH1–RC cores. The localization of the cyt bc 1 complex remains so far enigmatic, though evidence has been presented for the exis- tence of pathways around the photosynthetic core complexes that may favor rapid quinone diffusion to distant cyt bc 1 complexes (Liu et al., 2009). AFM data has also been exploited to build three- dimensional models of the photosynthetic unit (Scheuring et al., 2007a; S ßener et al., 2007), and various strategies that purple bacte- ria have evolved for the harvesting and utilization of light energy have been reviewed (Sturgis and Niederman, 2008). 1047-8477/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jsb.2010.08.010 Abbreviations: AFM, atomic force microscopy; EM, electron microscopy; ICM, intracytoplasmic membrane; LH, light-harvesting; OM, outer membrane; RC, reaction center. ⇑ Corresponding author. Fax: +33 1 40510636. E-mail address: simon.scheuring@curie.fr (S. Scheuring). Journal of Structural Biology 173 (2011) 138–145 Contents lists available at ScienceDirect Journal of Structural Biology journal homepage: www.elsevier.com/locate/yjsbi