PARENT SESSION

Symposium S8B Supermolecular organization of the photosynthetic apparatus
Friday September 3rd, 2004 8:30 AM-10:30 AM Room 210A
Chair: Jan Dekker
Co-Chair: André Verméglio

Structural organization of the dimeric core-complex of Rhodobacter sphaeroides. Daniel Levy^*,1, Simon Scheuring¹, Johan Busselez¹, Francesco Francia², Bruno Melandri², Jean-Louis Rigaud¹, ¹ Institut Curie UMR 168 CNRS, Paris, France² University of Bologna Dept of Biology, Bologna, Italy

ABSTRACT- In Rb. sphaeroides, a small subunit, named PufX present in the core-complex formed by the light harvesting complex 1 (LH1) and reaction center (RC), has been shown to be involved in the fast exchange of quinones between the RC and the cytochrome bc1, as well as in the supramolecular organization of the core-complexes in the membranes. In order to analyse the role of PufX, we have performed a structural study on PufX containing core-complexes purified from membranes of wild-type Rhodobacter sphaeroides. Two-dimensional crystals have been produced by detergent removal in the presence of lipids, and further analysed by cryo-electron microscopy and atomic force microscopy. A projection map calculated to 26 Angstrom resolution reveals that core-complexes assemble in an elongated S-shaped dimeric core-complex. The core-complex is composed of two RC, twenty-four LH1 / heterodimers, and two PufX proteins. The LH1 assemblies are open with a gap of density of around 30 Angstrom, and surround specifically oriented RCs. Two PufX proteins are located at the dimer junction and induce dimerization of monomeric core-complexes upon reconstitution into a lipid bilayer. This location of PufX in the core-complex indicates that PufX is the structural key for the complex formation rather than a channel-forming protein for the exchange of ubiquinone/ubiquinol between the reaction center and the cytochrome bc1 complex 1. 1. Scheuring S. et al., (2004) J. Biol. Chem. 279, 3620-3626.

KEY WORDS: pufX, bacterial core-complex, supercomplex, 2D crystal