Architecture of the native photosynthetic apparatus of Phaeospirillum molischianum

Gonçalves, Rui Pedro; Bernadac, Alain; Sturgis, James N.; Scheuring, Simon

doi:10.1016/j.jsb.2005.10.002

Cited by 76 publications

(105 citation statements)

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“…4 we illustrate one of the consequences on the organization of LH1 (Larger Rings) and LH2 complexes (Smaller Rings) within a photosynthetic membrane of Ph. molischianum, as has been observed by atomic force microscopy (AFM) (6). Within these membranes the role of the LH2 complexes is to absorb incident radiation and transfer the energy to the reaction center situated at the center of the LH1 complexes.…”

Section: Resultsmentioning

confidence: 93%

“…Both types of light-harvesting complex, LH1 and LH2, are constructed from oligomers of a similar basic subunit containing 2 polypeptides (α and β) with associated bacteriochlorophyll and carotenoid pigments. Their structures are known from x-ray crystallography (2-4), electron crystallography (5), and more recently from atomic force microscopy (6)(7)(8)(9)(10)(11).…”

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Antenna mixing in photosynthetic membranes from Phaeospirillum molischianum

Mascle-Allemand¹,

Duquesne²,

Lebrun

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

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We have investigated the adaptation of the light-harvesting system of the photosynthetic bacterium Phaeospirillum molischianum (DSM120) to very low light conditions. This strain is able to respond to changing light conditions by differentially modulating the expression of a family of puc operons that encode for peripheral light-harvesting complex (LH2) polypeptides. This modulation can result in a complete shift between the production of LH2 complexes absorbing maximally near 850 nm to those absorbing near 820 nm. In contradiction to prevailing wisdom, analysis of the LH2 rings found in the photosynthetic membranes during light adaptation are shown to have intermediate spectral and electrostatic properties. By chemical cross-linking and mass-spectrometry we show that individual LH2 rings and subunits can contain a mixture of polypeptides derived from the different operons. These observations show that polypeptide synthesis and insertion into the membrane are not strongly coupled to LH2 assembly. We show that the light-harvesting complexes resulting from this mixing could be important in maintaining photosynthetic efficiency during adaptation.chromatic adaptation | membrane protein | photosynthetic bacteria | Rhodospirillum molischianum P hotosynthetic bacteria are able to efficiently convert incident light into chemical potential energy via a light-driven cyclic electron transfer pathway (1). This process depends on the efficient absorption of incident light energy and the transfer of this energy to the reaction center. Measurements of the quantum efficiency of the light-harvesting apparatus indicate that very little energy is lost during the migration of the exciton from the point of initial absorption to the reaction center (1). The light-harvesting system of many bacteria contains two different pigment-protein complexes. The core complex contains one or two reaction centers in close association with a light-harvesting system (LH1) and a smaller peripheral complex (LH2) that transfers the absorbed excitation energy to the core complex. Both types of light-harvesting complex, LH1 and LH2, are constructed from oligomers of a similar basic subunit containing 2 polypeptides (α and β) with associated bacteriochlorophyll and carotenoid pigments. Their structures are known from x-ray crystallography (2-4), electron crystallography (5), and more recently from atomic force microscopy (6-11).The LH2 complexes are circular oligomers of typically 9 αβ subunits (3), in which 18 long wavelength-absorbing bacteriochlorophyll molecules are sandwiched between the inner ring of α-polypeptides and an outer-ring of β-polypeptides. A second series of 9 bacteriochlorohpyll molecules, closer to the cytoplasmic membrane surface, occupies gaps between the β-polypeptides. This general architecture is slightly variable, with in particular the number of monomeric units forming the circular architecture depending on the species and environmental conditions (2,7,12,13). In the species studied in this work, Phaeospirillum (Ph.) molischianum...

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Section: Resultsmentioning

confidence: 93%

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confidence: 99%

Antenna mixing in photosynthetic membranes from Phaeospirillum molischianum

Mascle-Allemand¹,

Duquesne²,

Lebrun

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

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“…acidophila LH2 (16,30), as the large majority of LH2 observed in Rsp. photometricum are nonameric (11,19,32). The sequence identities of the α-and β-polypeptides of Rsp.…”

Section: Resultsmentioning

confidence: 99%

Forces guiding assembly of light-harvesting complex 2 in native membranes

Liu

Duquesne

Oesterhelt

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

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Interaction forces of membrane protein subunits are of importance in their structure, assembly, membrane insertion, and function. In biological membranes, and in the photosynthetic apparatus as a paradigm, membrane proteins fulfill their function by ensemble actions integrating a tight assembly of several proteins. In the bacterial photosynthetic apparatus light-harvesting complexes 2 (LH2) transfer light energy to neighboring tightly associated core complexes, constituted of light-harvesting complexes 1 (LH1) and reaction centers (RC). While the architecture of the photosynthetic unit has been described, the forces and energies assuring the structural and functional integrity of LH2, the assembly of LH2 complexes, and how LH2 interact with the other proteins in the supramolecular architecture are still unknown. Here we investigate the molecular forces of the bacterial LH2 within the native photosynthetic membrane using atomic force microscopy single-molecule imaging and force measurement in combination. The binding between LH2 subunits is fairly weak, of the order of k B T , indicating the importance of LH2 ring architecture. In contrast LH2 subunits are solid with a free energy difference of 90 k B T between folded and unfolded states. Subunit α-helices unfold either in one-step, α-and β-polypeptides unfold together, or sequentially. The unfolding force of transmembrane helices is approximately 150 pN. In the two-step unfolding process, the β-polypeptide is stabilized by the molecular environment in the membrane. Hence, intermolecular forces influence the structural and functional integrity of LH2.atomic force microscopy | photosynthesis | protein unfolding | membrane protein assembly M embrane protein function is intimately linked and influenced by its structural integrity and molecular environment (1-3). Photosynthesis is a paradigm of such orchestrated molecular action, for which the implicated proteins have evolved to form supramolecular assemblies (4). Knowledge of supramolecular architecture and interaction forces and energies are indispensable for a complete understanding of a membrane protein structure and function.The atomic force microscope (AFM) (5) can be used as a high-resolution imaging (6) and as a force-measurement tool (7). Most elegantly the two applications were combined allowing the attribution of structural changes to force events (8). More recently, the AFM has proven the unique tool for studying the supramolecular assembly of the bacterial photosynthetic unit (9-12) and has provided a solid basis for the understanding of the ensemble function of the photosynthetic proteins (4, 13-15).The light-harvesting process in photosynthetic bacteria starts with photon capture at the peripheral light-harvesting complex 2 (LH2) that transfers the absorbed excitation energy to the light-harvesting complex 1 (LH1) associated with the reaction center (RC). The LH2 structure is well described by X-ray crystallography (16, 17) and AFM in native membranes (12,(18)(19)(20). Typically, the LH2 complexes appe...

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“…We have demonstrated that the comparatively disordered intracytoplasmic membrane from the model photosynthetic bacterium Rhodobacter sphaeroides can be successfully imaged by AFM (9) to reveal the hidden architecture that nature has developed to harvest, transfer, and finally utilize light energy with great efficiency. Scheuring and co-workers have also produced high resolution images of the photosynthetic membranes of Blastochloris viridis (10), Rhodospirillum photometricum (11), Rhodobacter blasticus (12), and Phaeospirillum molischianum (13), allowing a comparison of the differing strategies that purple bacteria have evolved for the harvesting and utilization of light energy (14).…”

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The Organization of LH2 Complexes in Membranes from Rhodobacter sphaeroides

Olsen

Tucker

Timney

et al. 2008

Journal of Biological Chemistry

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The mapping of the photosynthetic membrane of Rhodobacter sphaeroides by atomic force microscopy (AFM) revealed a unique organization of arrays of dimeric reaction center-light harvesting I-PufX (RC-LH1-PufX) core complexes surrounded and interconnected by light-harvesting LH2 complexes (Bahatyrova, S., Frese, R. N., Siebert, C. A., Olsen, J. D., van der Werf, K. O., van Grondelle, R., Niederman, R. A., Bullough, P. A., Otto, C., and Hunter, C. N. (2004) Nature 430, 1058 -1062). However, membrane regions consisting solely of LH2 complexes were under-represented in these images because these small, highly curved areas of membrane rendered them difficult to image even using gentle tapping mode AFM and impossible with contact mode AFM. We report AFM imaging of membranes prepared from a mutant of R. sphaeroides, DPF2G, that synthesizes only the LH2 complexes, which assembles spherical intracytoplasmic membrane vesicles of ϳ53 nm diameter in vivo. By opening these vesicles and adsorbing them onto mica to form small, <120 nm, largely flat sheets we have been able to visualize the organization of these LH2-only membranes for the first time. The transition from highly curved vesicle to the planar sheet is accompanied by a change in the packing of the LH2 complexes such that approximately half of the complexes are raised off the mica surface by ϳ1 nm relative to the rest. This vertical displacement produces a very regular corrugated appearance of the planar membrane sheets. Analysis of the topographs was used to measure the distances and angles between the complexes. These data are used to model the organization of LH2 complexes in the original, curved membrane. The implications of this architecture for the light harvesting function and diffusion of quinones in native membranes of R. sphaeroides are discussed.The biological membrane at its simplest is a lipid bilayer that divides cellular contents from the exterior medium. Yet the bilayer performs more than a simple barrier function as it plays host to a wide variety of integral membrane proteins that perform transport, sensing, motility, biosynthesis, energy generation (in the form of ATP), as well as energy harvesting in the case of phototrophic organisms. We have structural information about the membrane proteins involved in photosynthesis, e.g. the bacterial reaction center (1), the peripheral light-harvesting complex (2), and the photosystem I (3) and photosystem II (4) supercomplexes and in transport (5), sensing (6), and electron transport (7). However, there is a need now to gain information upon the organization of these proteins within the membrane in vivo. The use of AFM 2 to directly visualize membrane proteins in situ allows us to make precise measurements of the disposition of any protein within the membrane (8) under nearly native conditions. We have demonstrated that the comparatively disordered intracytoplasmic membrane from the model photosynthetic bacterium Rhodobacter sphaeroides can be successfully imaged by AFM (9) to reveal the hidden architecture ...

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Architecture of the native photosynthetic apparatus of Phaeospirillum molischianum

Cited by 76 publications

References 36 publications

Antenna mixing in photosynthetic membranes from Phaeospirillum molischianum

Antenna mixing in photosynthetic membranes from Phaeospirillum molischianum

Forces guiding assembly of light-harvesting complex 2 in native membranes

The Organization of LH2 Complexes in Membranes from Rhodobacter sphaeroides

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