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In the photosynthetic purple bacterium Rhodobacter (Rba.) sphaeroides, light is absorbed by membrane-bound light-harvesting (LH) proteins LH1 and LH2. LH1 directly surrounds the reaction center (RC) and, together with PufX, forms a dimeric (RC-LH1-PufX)2 protein complex. In LH2-deficient Rba. sphaeroides mutants, RC-LH1-PufX dimers aggregate into tubular vesicles with a radius of approximately 250-550 A, making RC-LH1-PufX one of the few integral membrane proteins known to actively induce membrane curvature. Recently, a three-dimensional electron microscopy density map showed that the Rba. sphaeroides RC-LH1-PufX dimer exhibits a prominent bend at its dimerizing interface. To investigate the curvature properties of this highly bent protein, we employed molecular dynamics simulations to fit an all-atom structural model of the RC-LH1-PufX dimer within the electron microscopy density map. The simulations reveal how the dimer produces a membrane with high local curvature, even though the location of PufX cannot yet be determined uniquely. The resulting membrane curvature agrees well with the size of RC-LH1-PufX tubular vesicles, and demonstrates how the local curvature properties of the RC-LH1-PufX dimer propagate to form the observed long-range organization of the Rba. sphaeroides tubular vesicles.

We present a molecular model of a chromatophore vesicle from Rhodobacter sphaeroides. These vesicles are ideal benchmark systems for molecular and systemic simulations, because they have been well studied, they are small, and they are naturally separated from their cellular environment. To set up a photosynthetic chain working under steady-state conditions, we compiled from the experimental literature the specific activities and geometries that have been determined for their constituents. This data then allowed defining the stoichiometries for all membrane proteins. This article contains the kinetic part of the reconstructed model, while the spatial reconstruction is presented in a companion article. By considering the transport properties of the Cytochrome c(2) and ubiquinone pools, we show that their size and oxidation states allow for an efficient buffering of the statistical fluctuations that arise from the small size of the vesicles. Stoichiometric and kinetic considerations indicate that a typical chromatophore vesicle of Rb. sphaeroides with a diameter of 45 nm should contain approximately five bc(1) monomers.

The photosynthetic apparatus of purple bacteria is generally considered a well-studied and understood system. However, recent atomic force microscopy images of flattened chromatophore vesicles from Rhodobacter sphaeroides restarted a debate about the stoichiometry and positions of the membrane proteins, with the interpretations of the observed images only partly being in agreement with earlier models. The most puzzling observation from the recent images is that the Cytochrome bc(1) complex, which is a central part of the photosynthetic apparatus, seems to be missing on the chromatophore vesicles, even when these were extracted from photosynthetically grown bacteria. From the available information on the geometry of the vesicle and of the proteins we reconstructed here a three-dimensional model vesicle at molecular resolution. Its central feature, also determining its diameter of approximately 45 nm, is an equatorial array of LH1 dimers, lined by a region of LH2 rings. This naturally puts the Cytochrome bc(1) complexes and the ATPase at the vesicle's poles. This spatial model may explain why the vesicle's endcaps with the bc(1) complexes are lost during the preparatory steps of the imaging process together with the ATPase and are therefore absent from the available images.