Monomeric RC-LH1 core complexes retard LH2 assembly and intracytoplasmic membrane formation in PufX-minus mutants of Rhodobacter sphaeroides

Excitation energy transfer in proteoliposomes reconstituted with LH2 and RC-LH1 complexes from Rhodobacter sphaeroides

Light-harvesting 2 (LH2) and reaction-centre light-harvesting 1 (RC-LH1) complexes purified from the photosynthetic bacterium Rhodobacter (Rba.) sphaeroides were reconstituted into proteoliposomes either separately, or together at three different LH2:RC-LH1 ratios, for excitation energy transfer studies. Atomic force microscopy (AFM) was used to investigate the distribution and association of the complexes within the proteoliposome membranes. Absorption and fluorescence emission spectra were similar for LH2 complexes in detergent and liposomes, indicating that reconstitution retains the structural and optical properties of the LH2 complexes. Analysis of fluorescence emission shows that when LH2 forms an extensive series of contacts with other such complexes, fluorescence is quenched by 52.6 ± 1.4%. In mixed proteoliposomes, specific excitation of carotenoids in LH2 donor complexes resulted in emission of fluorescence from acceptor RC-LH1 complexes engineered to assemble with no carotenoids. Extents of energy transfer were measured by fluorescence lifetime microscopy; the 0.72 ± 0.08 ns lifetime in LH2-only membranes decreases to 0.43 ± 0.04 ns with a ratio of 2:1 LH2 to RC-LH1, and to 0.35 ± 0.05 ns for a 1:1 ratio, corresponding to energy transfer efficiencies of 40 ± 14% and 51 ± 18%, respectively. No further improvement is seen with a 0.5:1 LH2 to RC-LH1 ratio. Thus, LH2 and RC-LH1 complexes perform their light harvesting and energy transfer roles when reconstituted into proteoliposomes, providing a way to integrate native, non-native, engineered and de novo designed light-harvesting complexes into functional photosynthetic systems.

Enhancing the spectral range of plant and bacterial light-harvesting pigment-protein complexes with various synthetic chromophores incorporated into lipid vesicles

The Light-Harvesting (LH) pigment-protein complexes found in photosynthetic organisms have the role of absorbing solar energy with high efficiency and transferring it to reaction centre complexes. LH complexes contain a suite of pigments that each absorb light at specific wavelengths, however, the natural combinations of pigments within any one protein complex do not cover the full range of solar radiation. Here, we provide an in-depth comparison of the relative effectiveness of five different organic "dye" molecules (Texas Red, ATTO, Cy7, DiI, DiR) for enhancing the absorption range of two different LH membrane protein complexes (the major LHCII from plants and LH2 from purple phototrophic bacteria). Proteoliposomes were self-assembled from defined mixtures of lipids, proteins and dye molecules and their optical properties were quantified by absorption and fluorescence spectroscopy. Both lipid-linked dyes and alternative lipophilic dyes were found to be effective excitation energy donors to LH protein complexes, without the need for direct chemical or generic modification of the proteins. The Förster theory parameters (e.g., spectral overlap) were compared between each donor-acceptor combination and found to be good predictors of an effective dye-protein combination. At the highest dye-to-protein ratios tested (over 20:1), the effective absorption strength integrated over the full spectral range was increased to ∼180% of its natural level for both LH complexes. Lipophilic dyes could be inserted into pre-formed membranes although their effectiveness was found to depend upon favourable physicochemical interactions. Finally, we demonstrated that these dyes can also be effective at increasing the spectral range of surface-supported models of photosynthetic membranes, using fluorescence microscopy. The results of this work provide insight into the utility of self-assembled lipid membranes and the great flexibility of LH complexes for interacting with different dyes.

Structural basis for the assembly and quinone transport mechanisms of the dimeric photosynthetic RC-LH1 supercomplex

The reaction center (RC) and light-harvesting complex 1 (LH1) form a RC-LH1 core supercomplex that is vital for the primary reactions of photosynthesis in purple phototrophic bacteria. Some species possess the dimeric RC-LH1 complex with a transmembrane polypeptide PufX, representing the largest photosynthetic complex in anoxygenic phototrophs. However, the details of the architecture and assembly mechanism of the RC-LH1 dimer are unclear. Here we report seven cryo-electron microscopy (cryo-EM) structures of RC-LH1 supercomplexes from Rhodobacter sphaeroides. Our structures reveal that two PufX polypeptides are positioned in the center of the S-shaped RC-LH1 dimer, interlocking association between the components and mediating RC-LH1 dimerization. Moreover, we identify another transmembrane peptide, designated PufY, which is located between the RC and LH1 subunits near the LH1 opening. PufY binds a quinone molecule and prevents LH1 subunits from completely encircling the RC, creating a channel for quinone/quinol exchange. Genetic mutagenesis, cryo-EM structures, and computational simulations provide a mechanistic understanding of the assembly and electron transport pathways of the RC-LH1 dimer and elucidate the roles of individual components in ensuring the structural and functional integrity of the photosynthetic supercomplex.

Cryo-EM structure of the dimeric Rhodobacter sphaeroides RC-LH1 core complex at 2.9 Å: the structural basis for dimerisation

The dimeric reaction centre light-harvesting 1 (RC-LH1) core complex of Rhodobacter sphaeroides converts absorbed light energy to a charge separation, and then it reduces a quinone electron and proton acceptor to a quinol. The angle between the two monomers imposes a bent configuration on the dimer complex, which exerts a major influence on the curvature of the membrane vesicles, known as chromatophores, where the light-driven photosynthetic reactions take place. To investigate the dimerisation interface between two RC-LH1 monomers, we determined the cryogenic electron microscopy structure of the dimeric complex at 2.9 Å resolution. The structure shows that each monomer consists of a central RC partly enclosed by a 14-subunit LH1 ring held in an open state by PufX and protein-Y polypeptides, thus enabling quinones to enter and leave the complex. Two monomers are brought together through N-terminal interactions between PufX polypeptides on the cytoplasmic side of the complex, augmented by two novel transmembrane polypeptides, designated protein-Z, that bind to the outer faces of the two central LH1 β polypeptides. The precise fit at the dimer interface, enabled by PufX and protein-Z, by C-terminal interactions between opposing LH1 αβ subunits, and by a series of interactions with a bound sulfoquinovosyl diacylglycerol lipid, bring together each monomer creating an S-shaped array of 28 bacteriochlorophylls. The seamless join between the two sets of LH1 bacteriochlorophylls provides a path for excitation energy absorbed by one half of the complex to migrate across the dimer interface to the other half.

The effectiveness of styrene-maleic acid (SMA) copolymers for solubilisation of integral membrane proteins from SMA-accessible and SMA-resistant membranes

Solubilisation of biological lipid bilayer membranes for analysis of their protein complement has traditionally been carried out using detergents, but there is increasing interest in the use of amphiphilic copolymers such as styrene maleic acid (SMA) for the solubilisation, purification and characterisation of integral membrane proteins in the form of protein/lipid nanodiscs. Here we survey the effectiveness of various commercially-available formulations of the SMA copolymer in solubilising Rhodobacter sphaeroides reaction centres (RCs) from photosynthetic membranes. We find that formulations of SMA with a 2:1 or 3:1 ratio of styrene to maleic acid are almost as effective as detergent in solubilising RCs, with the best solubilisation by short chain variants (<30kDa weight average molecular weight). The effectiveness of 10kDa 2:1 and 3:1 formulations of SMA to solubilise RCs gradually declined when genetically-encoded coiled-coil bundles were used to artificially tether normally monomeric RCs into dimeric, trimeric and tetrameric multimers. The ability of SMA to solubilise reaction centre-light harvesting 1 (RC-LH1) complexes from densely packed and highly ordered photosynthetic membranes was uniformly low, but could be increased through a variety of treatments to increase the lipid:protein ratio. However, proteins isolated from such membranes comprised clusters of complexes in small membrane patches rather than individual proteins. We conclude that short-chain 2:1 and 3:1 formulations of SMA are the most effective in solubilising integral membrane proteins, but that solubilisation efficiencies are strongly influenced by the size of the target protein and the density of packing of proteins in the membrane.

The C-terminus of PufX plays a key role in dimerisation and assembly of the reaction center light-harvesting 1 complex from Rhodobacter sphaeroides

In bacterial photosynthesis reaction center-light-harvesting 1 (RC-LH1) complexes trap absorbed solar energy by generating a charge separated state. Subsequent electron and proton transfers form a quinol, destined to diffuse to the cytochrome bc₁ complex. In bacteria such as Rhodobacter (Rba.) sphaeroides and Rba. capsulatus the PufX polypeptide creates a channel for quinone/quinol traffic across the LH1 complex that surrounds the RC, and it is therefore essential for photosynthetic growth. PufX also plays a key role in dimerization of the RC-LH1-PufX core complex, and the structure of the Rba. sphaeroides complex shows that the PufX C-terminus, particularly the region from X49-X53, likely mediates association of core monomers. To investigate this putative interaction we analysed mutations PufX R49L, PufX R53L, PufX R49/53L and PufX G52L by measuring photosynthetic growth, fractionation of detergent-solubilised membranes, formation of 2-D crystals and electron microscopy. We show that these mutations do not affect assembly of PufX within the core or photosynthetic growth but they do prevent dimerization, consistent with predictions from the RC-LH1-PufX structure. We obtained low resolution structures of monomeric core complexes with and without PufX, using electron microscopy of negatively stained single particles and 3D reconstruction; the monomeric complex with PufX corresponds to one half of the dimer structure whereas LH1 completely encloses the RC if the gene encoding PufX is deleted. On the basis of the insights gained from these mutagenesis and structural analyses we propose a sequence for assembly of the dimeric RC-LH1-PufX complex.

Direct Imaging of Protein Organization in an Intact Bacterial Organelle Using High-Resolution Atomic Force Microscopy

The function of bioenergetic membranes is strongly influenced by the spatial arrangement of their constituent membrane proteins. Atomic force microscopy (AFM) can be used to probe protein organization at high resolution, allowing individual proteins to be identified. However, previous AFM studies of biological membranes have typically required that curved membranes are ruptured and flattened during sample preparation, with the possibility of disruption of the native protein arrangement or loss of proteins. Imaging native, curved membranes requires minimal tip-sample interaction in both lateral and vertical directions. Here, long-range tip-sample interactions are reduced by optimizing the imaging buffer. Tapping mode AFM with high-resonance-frequency small and soft cantilevers, in combination with a high-speed AFM, reduces the forces due to feedback error and enables application of an average imaging force of tens of piconewtons. Using this approach, we have imaged the membrane organization of intact vesicular bacterial photosynthetic "organelles", chromatophores. Despite the highly curved nature of the chromatophore membrane and lack of direct support, the resolution was sufficient to identify the photosystem complexes and quantify their arrangement in the native state. Successive imaging showed the proteins remain surprisingly static, with minimal rotation or translation over several-minute time scales. High-order assemblies of RC-LH1-PufX complexes are observed, and intact ATPases are successfully imaged. The methods developed here are likely to be applicable to a broad range of protein-rich vesicles or curved membrane systems, which are an almost ubiquitous feature of native organelles.

PucC and LhaA direct efficient assembly of the light-harvesting complexes in Rhodobacter sphaeroides

The mature architecture of the photosynthetic membrane of the purple phototroph Rhodobacter sphaeroides has been characterised to a level where an atomic-level membrane model is available, but the roles of the putative assembly proteins LhaA and PucC in establishing this architecture are unknown. Here we investigate the assembly of light-harvesting LH2 and reaction centre-light-harvesting1-PufX (RC-LH1-PufX) photosystem complexes using spectroscopy, pull-downs, native gel electrophoresis, quantitative mass spectrometry and fluorescence lifetime microscopy to characterise a series of lhaA and pucC mutants. LhaA and PucC are important for specific assembly of LH1 or LH2 complexes, respectively, but they are not essential; the few LH1 subunits found in ΔlhaA mutants assemble to form normal RC-LH1-PufX core complexes showing that, once initiated, LH1 assembly round the RC is cooperative and proceeds to completion. LhaA and PucC form oligomers at sites of initiation of membrane invagination; LhaA associates with RCs, bacteriochlorophyll synthase (BchG), the protein translocase subunit YajC and the YidC membrane protein insertase. These associations within membrane nanodomains likely maximise interactions between pigments newly arriving from BchG and nascent proteins within the SecYEG-SecDF-YajC-YidC assembly machinery, thereby co-ordinating pigment delivery, the co-translational insertion of LH polypeptides and their folding and assembly to form photosynthetic complexes.

Exploiting lipopolysaccharide-induced deformation of lipid bilayers to modify membrane composition and generate two-dimensional geometric membrane array patterns

Supported lipid bilayers have proven effective as model membranes for investigating biophysical processes and in development of sensor and array technologies. The ability to modify lipid bilayers after their formation and in situ could greatly advance membrane technologies, but is difficult via current state-of-the-art technologies. Here we demonstrate a novel method that allows the controlled post-formation processing and modification of complex supported lipid bilayer arrangements, under aqueous conditions. We exploit the destabilization effect of lipopolysaccharide, an amphiphilic biomolecule, interacting with lipid bilayers to generate voids that can be backfilled to introduce desired membrane components. We further demonstrate that when used in combination with a single, traditional soft lithography process, it is possible to generate hierarchically-organized membrane domains and microscale 2-D array patterns of domains. Significantly, this technique can be used to repeatedly modify membranes allowing iterative control over membrane composition. This approach expands our toolkit for functional membrane design, with potential applications for enhanced materials templating, biosensing and investigating lipid-membrane processes.

Diblock copolymer micelles and supported films with noncovalently incorporated chromophores: a modular platform for efficient energy transfer

We report generation of modular, artificial light-harvesting assemblies where an amphiphilic diblock copolymer, poly(ethylene oxide)-block-poly(butadiene), serves as the framework for noncovalent organization of BODIPY-based energy donor and bacteriochlorin-based energy acceptor chromophores. The assemblies are adaptive and form well-defined micelles in aqueous solution and high-quality monolayer and bilayer films on solid supports, with the latter showing greater than 90% energy transfer efficiency. This study lays the groundwork for further development of modular, polymer-based materials for light harvesting and other photonic applications.

Aberrant assembly complexes of the reaction center light-harvesting 1 PufX (RC-LH1-PufX) core complex of Rhodobacter sphaeroides imaged by atomic force microscopy

In the purple phototrophic bacterium Rhodobacter sphaeroides, many protein complexes congregate within the membrane to form operational photosynthetic units consisting of arrays of light-harvesting LH2 complexes and monomeric and dimeric reaction center (RC)-light-harvesting 1 (LH1)-PufX “core” complexes. Each half of a dimer complex consists of a RC surrounded by 14 LH1 αβ subunits, with two bacteriochlorophylls (Bchls) sandwiched between each αβ pair of transmembrane helices. We used atomic force microscopy (AFM) to investigate the assembly of single molecules of the RC-LH1-PufX complex using membranes prepared from LH2-minus mutants. When the RC and PufX components were also absent, AFM revealed a series of LH1 variants where the repeating α₁β₁(Bchl)₂ units had formed rings of variable size, ellipses, and spirals and also arcs that could be assembly products. The spiral complexes occur when the LH1 ring has failed to close, and short arcs are suggestive of prematurely terminated LH1 complex assembly. In the absence of RCs, we occasionally observed captive proteins enclosed by the LH1 ring. When production of LH1 units was restricted by lowering the relative levels of the cognate pufBA transcript, we imaged a mixture of complete RC-LH1 core complexes, empty LH1 rings, and isolated RCs, leading us to conclude that once a RC associates with the first α₁β₁(Bchl)₂ subunit, cooperative associations between subsequent subunits and the RC tend to drive LH1 ring assembly to completion.

Organization in photosynthetic membranes of purple bacteria in vivo: the role of carotenoids

Photosynthesis in purple bacteria is performed by pigment-protein complexes that are closely packed within specialized intracytoplasmic membranes. Here we report on the influence of carotenoid composition on the organization of RC-LH1 pigment-protein complexes in intact membranes and cells of Rhodobacter sphaeroides. Mostly dimeric RC-LH1 complexes could be isolated from strains expressing native brown carotenoids when grown under illuminated/anaerobic conditions, or from strains expressing green carotenoids when grown under either illuminated/anaerobic or dark/semiaerobic conditions. However, mostly monomeric RC-LH1 complexes were isolated from strains expressing the native photoprotective red carotenoid spheroidenone, which is synthesized during phototrophic growth in the presence of oxygen. Despite this marked difference, linear dichroism (LD) and light-minus-dark LD spectra of oriented intact intracytoplasmic membranes indicated that RC-LH1 complexes are always assembled in ordered arrays, irrespective of variations in the relative amounts of isolated dimeric and monomeric RC-LH1 complexes. We propose that part of the photoprotective response to the presence of oxygen mediated by synthesis of spheroidenone may be a switch of the structure of the RC-LH1 complex from dimers to monomers, but that these monomers are still organized into the photosynthetic membrane in ordered arrays. When levels of the dimeric RC-LH1 complex were very high, and in the absence of LH2, LD and ∆LD spectra from intact cells indicated an ordered arrangement of RC-LH1 complexes. Such a degree of ordering implies the presence of highly elongated, tubular membranes with dimensions requiring orientation along the length of the cell and in a proportion larger than previously observed.

Lipopolysaccharide-induced dynamic lipid membrane reorganization: tubules, perforations, and stacks

Lipopolysaccharide (LPS) is a unique lipoglycan, with two major physiological roles: 1), as a major structural component of the outer membrane of Gram-negative bacteria and 2), as a highly potent mammalian toxin when released from cells into solution (endotoxin). LPS is an amphiphile that spontaneously inserts into the outer leaflet of lipid bilayers to bury its hydrophobic lipidic domain, leaving the hydrophilic polysaccharide chain exposed to the exterior polar solvent. Divalent cations have long been known to neutralize and stabilize LPS in the outer membrane, whereas LPS in the presence of monovalent cations forms highly mobile negatively-charged aggregates. Yet, much of our understanding of LPS and its interactions with the cell membrane does not take into account its amphiphilic biochemistry and charge polarization. Herein, we report fluorescence microscopy and atomic force microscopy analysis of the interaction between LPS and fluid-phase supported lipid bilayer assemblies (sLBAs), as model membranes. Depending on cation availability, LPS induces three remarkably different effects on simple sLBAs. Net-negative LPS-Na(+) leads to the formation of 100-μm-long flexible lipid tubules from surface-associated lipid vesicles and the destabilization of the sLBA resulting in micron-size hole formation. Neutral LPS-Ca(2+) gives rise to 100-μm-wide single- or multilamellar planar sheets of lipid and LPS formed from surface-associated lipid vesicles. Our findings have important implications about the physical interactions between LPS and lipids and demonstrate that sLBAs can be useful platforms to study the interactions of amphiphilic virulence factors with cell membranes. Additionally, our study supports the general phenomenon that lipids with highly charged or bulky headgroups can promote highly curved membrane architectures due to electrostatic and/or steric repulsions.

Variation in supramolecular organisation of the photosynthetic membrane of Rhodobacter sphaeroides induced by alteration of PufX

In purple bacteria of the genus Rhodobacter (Rba.), an LH1 antenna complex surrounds the photochemical reaction centre (RC) with a PufX protein preventing the LH1 complex from completely encircling the RC. In membranes of Rba. sphaeroides, RC-LH1 complexes associate as dimers which in turn assemble into longer range ordered arrays. The present work uses linear dichroism (LD) and dark-minus-light difference LD (ΔLD) to probe the organisation of genetically altered RC-LH1 complexes in intact membranes. The data support previous proposals that Rba. capsulatus, and Rba. sphaeroides heterologously expressing the PufX protein from Rba. capsulatus, produce monomeric core complexes in membranes that lack long-range order. Similarly, Rba. sphaeroides with a point mutation in the Gly 51 residue of PufX, which is located on the membrane-periplasm interface, assembles mainly non-ordered RC-LH1 complexes that are most likely monomeric. All the Rba. sphaeroides membranes in their ΔLD spectra exhibited a spectral fingerprint of small degree of organisation implying the possibility of ordering influence of LH1, and leading to an important conclusion that PufX itself has no influence on ordering RC-LH1 complexes, as long-range order appears to be induced only through its role of configuring RC-LH1 complexes into dimers.

Investigation of photosynthetic membrane structure using atomic force microscopy

Photosynthetic processes, including light capture, electron transfer, and energy conversion, are not only ensured by the activities of individual photosynthetic complexes but also substantially determined and regulated by the composition and assembly of the overall photosynthetic apparatus at the supramolecular level. In recent years, atomic force microscopy (AFM) has matured as a unique and powerful tool for directly assessing the supramolecular assembly of integral membrane protein complexes in their native membrane environment at submolecular resolution. This review highlights the major contributions and advances of AFM studies to our understanding of the structure of the bacterial photosynthetic machinery and its regulatory arrangement during chromatic adaptation. AFM topographs of other biological membrane systems and potential future applications of AFM are also discussed.

Adaptation of intracytoplasmic membranes to altered light intensity in Rhodobacter sphaeroides

The model photosynthetic bacterium Rhodobacter sphaeroides uses a network of bacteriochlorophyll (BChl)-protein complexes embedded in spherical intracytoplasmic membranes (ICM) to collect and utilise solar energy. We studied the effects of high- and low-light growth conditions, where BChl levels increased approximately four-fold from 1.6×10(6) to 6.5×10(6) molecules per cell. Most of this extra pigment is accommodated in the proliferating ICM system, which increases from approximately 274 to 1468 vesicles per cell. Thus, 16×10(6)nm(2) of specialised membrane surface area is made available for harvesting and utilising solar energy compared to 3×10(6)nm(2) under high-light conditions. Membrane mapping using atomic force microscopy revealed closely packed dimeric and monomeric reaction centre-light harvesting 1-PufX (RC-LH1-PufX) complexes in high-light ICM with room only for small clusters of LH2, whereas extensive LH2-only domains form during adaptation to low light, with the LH2/RC ratio increasing three-fold. The number of upper pigmented band (UPB) sites where membrane invagination is initiated hardly varied; 704 (5.8×10(5) BChls/cell) and 829 (4.9×10(5) BChls/cell) UPB sites per cell were estimated under high- and low-light conditions, respectively. Thus, the lower ICM content in high-light cells is a consequence of fewer ICM invaginations reaching maturity. Taking into account the relatively poor LH2-to-LH1 energy transfer in UPB membranes it is likely that high-light cells are relatively inefficient at energy trapping, but can grow well enough without the need to fully develop their photosynthetic membranes from the relatively inefficient UPB to highly efficient mature ICM.

Quantitative proteomic analysis of intracytoplasmic membrane development in Rhodobacter sphaeroides

The purple phototrophic bacteria elaborate a specialized intracytoplasmic membrane (ICM) system for the conversion of solar energy to ATP. Previous radiolabelling and ultrastructural experiments have shown that ICM assembly in Rhodobacter sphaeroides is initiated at indentations of the cytoplasmic membrane, termed UPB. Here, we report proteomic analyses of precursor (UPB) and mature (ICM) fractions. Qualitative data identified 387 proteins, only 43 of which were found in the ICM, reflecting its specialized role within the cell, the conversion of light into chemical energy; 236 proteins were found in the significantly more complex UPB proteome. Metabolic labelling was used to quantify the relative distribution of 173 proteins between the UPB and ICM fractions. Quantification reveals new information on assembly of the RC-LH1-PufX, ATP synthase and NAD(P)H transhydrogenase complexes, as well as showing that the UPB is enriched in enzymes for lipid, carbohydrate and amino acid metabolism, tetrapyrrole biosynthesis and proteins representing a wide range of other metabolic and biosynthetic functions. Proteins involved in light harvesting, photochemistry, electron transport and ATP synthesis are all enriched in ICM, consistent with the spatial proximity of energy capturing and transducing functions. These data provide further support to the developmental precursor-product relationship between UPB and ICM.

Cross-species investigation of the functions of the Rhodobacter PufX polypeptide and the composition of the RC-LH1 core complex

In well-characterised species of the Rhodobacter (Rba.) genus of purple photosynthetic bacteria it is known that the photochemical reaction centre (RC) is intimately-associated with an encircling LH1 antenna pigment protein, and this LH1 antenna is prevented from completely surrounding the RC by a single copy of the PufX protein. In Rba. veldkampii only monomeric RC-LH1 complexes are assembled in the photosynthetic membrane, whereas in Rba. sphaeroides and Rba. blasticus a dimeric form is also assembled in which two RCs are surrounded by an S-shaped LH1 antenna. The present work established that dimeric RC-LH1 complexes can also be isolated from Rba. azotoformans and Rba. changlensis, but not from Rba. capsulatus or Rba. vinaykumarii. The compositions of the monomers and dimers isolated from these four species of Rhodobacter were similar to those of the well-characterised RC-LH1 complexes present in Rba. sphaeroides. Pigment proteins were also isolated from strains of Rba. sphaeroides expressing chimeric RC-LH1 complexes. Replacement of either the Rba. sphaeroides LH1 antenna or PufX with its counterpart from Rba. capsulatus led to a loss of the dimeric form of the RC-LH1 complex, but the monomeric form had a largely unaltered composition, even in strains in which the expression level of LH1 relative to the RC was reduced. The chimeric RC-LH1 complexes were also functional, supporting bacterial growth under photosynthetic conditions. The findings help to tease apart the different functions of PufX in different species of Rhodobacter, and a specific protein structural arrangement that allows PufX to fulfil these three functions is proposed.