Modeling of oligomeric-state dependent spectral heterogeneity in the B875 light-harvesting complex of Rhodobacter sphaeroides by numerical simulation

A series of detergent-isolated light-harvesting 1 (LH1, B875) complexes from Rhodobacter sphaeroides, estimated to range in size from (RâBChl2)4 to (RâBChl2)13, was used to study the combined effects of spectral disorder and excitonic interactions on oligomeric-state dependent optical properties. Numerical simulations of absorption and fluorescence emission, excitation, and polarization spectra, based on the structure of the related LH2 complex, were compared to spectra measured experimentally at 77 K (Westerhuis and Niederman, in preparation). The aggregation-state dependence of the polarization spectra was found to be particularly sensitive to the choice of parameters, and vibronic components were included to obtain satisfactory simulations. Good agreement with most experimental features, including the oligomeric-state dependence of the absorption and emission maxima, was obtained only when the inter- and intradimer coupling strengths for adjacent BChls were similar (200-260 cm-1), and the width for the inhomogeneous distribution function (300-400 cm-1) was comparable. The relevance of these findings to existing controversies on the physical origin of spectral heterogeneity observed for the LH1 complex is discussed

The effects of protein crowding in bacterial photosynthetic membranes on the flow of quinone redox species between the photochemical reaction center and the ubiquinolcytochrome c2 oxidoreductase

Atomic force microscopy (AFM) of the native architecture of the intracytoplasmic membrane (ICM) of a variety of species of purple photosynthetic bacteria, obtained at submolecular resolution, shows a tightly packed arrangement of light harvesting (LH) and reaction center (RC) complexes. Since there are no unattributed structures or gaps with space sufficient for the cytochrome b

Supplementary Material for: Structural and Functional Proteomics of Intracytoplasmic Membrane Assembly in <b><i>Rhodobacter sphaeroides</i></b>

The results of a detailed structural and functional proteomic analysis of intracytoplasmic membrane (ICM) assembly in the model purple phototrophic bacterium <i>Rhodobacter sphaeroides</i> are reviewed in this report. Proteomics approaches have focused upon identification of membrane proteins temporally expressed during ICM development and spatially localized within the internal cell membranes, together with their structural and functional correlates. For the examination of temporal protein expression, procedures were established for the induction of ICM formation at low oxygen tension and for ICM remodeling in cells adapting to low intensity illumination, which permitted isolation by rate-zone sedimentation of ICM growth initiation sites (CM invaginations) in an upper-pigmented band (UPB), together with more mature ICM vesicles (chromatophores) as the main band. Nondenaturing clear native gel electrophoresis of the chromatophore fraction gave rise to four pigmented bands: the top and bottom bands contained the reaction center-light-harvesting 1 (RC-LH1) core complex and the LH2 peripheral antenna, respectively, while two bands of intermediate migration exhibited distinct associations of LH2 and core complexes. Proteomic analysis of the gel bands revealed developmental changes including increasing levels of LH2 polypeptides relative to those of core complexes as ICM development proceeded, as well as a large array of other associated proteins including high spectral counts for the F₁F_O-ATP synthase subunits, and the cytochrome <i>bc</i>₁ complex. High counts were also observed for RSP6124, a protein of unknown function, that were correlated with increasing LH2 levels. RC-LH1-containing clear native electrophoresis gel bands from the UPB were enriched in cytoplasmic membrane (CM) markers, including electron transfer and transport proteins, as well as general membrane assembly factors (viz., preprotein translocases YidC, YajC and SecY, bacterial type 1 signal peptidase and twin arg translocation subunit TatA), thereby confirming the origin of the UPB from both peripheral respiratory membrane and sites of active CM invagination in which preferential assembly of the RC-LH1 complex occurs. Functional aspects of the photosynthetic unit assembly process were monitored by fluorescence induction/relaxation measurements of the variable fluorescence arising from LH-bacteriochlorophyll <i>a</i>. Slowing of the rate of RC electron transfer turnover (<i>τ</i>_QA), as assessed from the relaxation phase, was correlated with the growth of the functional absorption cross section (σ) and LH2/LH1 molar ratios. This is thought to arise from the imposition of constraints upon free diffusion of ubiquinone (UQ) redox species between the RC and cytochrome <i>bc</i>₁ complex as the ICM bilayer becomes densely packed with LH2 rings. Such LH2 packing was confirmed in a comparison by high-resolution atomic force microscopy of ICM patches from cells grown at high and low light intensity [Adams and Hunter: Biochim Biophys Acta 2012;1817:1616-1627], in which the increasing LH2 levels form densely packed LH2-only domains, representing the light-responsive antenna complement arising under low illumination. In contrast, LH2 is initially dispersed in rows and small cluster-separating linear arrays of largely dimeric RC-LH1 core complexes, which become filled with LH2 during acclimation to reduced light intensity. In phototrophically grown cells that were transferred to oxic conditions in the dark, fluorescence induction/relaxation measurements showed that despite a growth burst independent of photosynthetic pathways, functional photosynthetic units were maintained for up to 24 h after the transition. The <i>τ</i>_QA was accelerated from ∼1 to 0.5 ms by 8 h, reflecting the decrease in LH2 levels, facilitating more rapid UQ redox species diffusion in the membrane bilayer as crowding by LH2 is overcome. Under these circumstances, UPB levels were elevated with significant increases in LH1/LH2 molar ratio. These changes indicate that vesiculation of CM growth initiation sites to form vesicular ICM was arrested under oxic conditions