Solubilization, purification, and characterization of the hexameric form of phosphatidylserine synthase from Candida albicans.

Phosphatidylserine (PS) synthase from Candida albicans, encoded by the CHO1 gene, has been identified as a potential drug target for new antifungals against systemic candidiasis. Rational drug design or small molecule screening are effective ways to identify specific inhibitors of Cho1, but both will be facilitated by protein purification. Due to the transmembrane nature of Cho1, methods were needed to solubilize and purify the native form of Cho1. Here, we used six non-ionic detergents and three styrene maleic acids (SMAs) to solubilize an HA-tagged Cho1 protein from the total microsomal fractions. Blue native PAGE and immunoblot analysis revealed a single band corresponding to Cho1 in all detergent-solubilized fractions, while two bands were present in the SMA2000-solubilized fraction. Our enzymatic assay suggests that digitonin- or DDM-solubilized enzyme has the most PS synthase activity. Pull-downs of HA-tagged Cho1 from the digitonin-solubilized fraction reveal an apparent MW of Cho1 consistent with a hexamer. Furthermore, negative-staining electron microscopy analysis and AlphaFold2 structure prediction modeling suggest the hexamer is composed of a trimer of dimers. We purified Cho1 protein to near-homogeneity as a hexamer using affinity chromatography and TEV protease treatment, and optimized Cho1 enzyme activity for manganese and detergent concentrations, temperature (24 °C), and pH (8.0). The purified Cho1 has a Km for its substrate CDP-diacylglycerol of 72.20 µM with a Vmax of 0.079 nmol/(µg∗min) while exhibiting a sigmoidal kinetic curve for its other substrate serine, indicating cooperative binding. Purified hexameric Cho1 can potentially be used in downstream structure determination and small drug screening.

Unique organization of photosystem II supercomplexes and megacomplexes in Norway spruce.

Photosystem II (PSII) complexes are organized into large supercomplexes with variable amounts of light-harvesting proteins (Lhcb). A typical PSII supercomplex in plants is formed by four trimers of Lhcb proteins (LHCII trimers), which are bound to the PSII core dimer via monomeric antenna proteins. However, the architecture of PSII supercomplexes in Norway spruce[Picea abies (L.) Karst.] is different, most likely due to a lack of two Lhcb proteins, Lhcb6 and Lhcb3. Interestingly, the spruce PSII supercomplex shares similar structural features with its counterpart in the green alga Chlamydomonas reinhardtii [Kouril et al. (2016) New Phytol. 210, 808-814]. Here we present a single-particle electron microscopy study of isolated PSII supercomplexes from Norway spruce that revealed binding of a variable amount of LHCII trimers to the PSII core dimer at positions that have never been observed in any other plant species so far. The largest spruce PSII supercomplex, which was found to bind eight LHCII trimers, is even larger than the current largest known PSII supercomplex from C. reinhardtii. We have also shown that the spruce PSII supercomplexes can form various types of PSII megacomplexes, which were also identified in intact grana membranes. Some of these large PSII supercomplexes and megacomplexes were identified also in Pinus sylvestris, another representative of the Pinaceae family. The structural variability and complexity of LHCII organization in Pinaceae seems to be related to the absence of Lhcb6 and Lhcb3 in this family, and may be beneficial for the optimization of light-harvesting under varying environmental conditions.

Supercomplexes of plant photosystem I with cytochrome b6f, light-harvesting complex II and NDH.

Photosystem I (PSI) is a pigment-protein complex required for the light-dependent reactions of photosynthesis and participates in light-harvesting and redox-driven chloroplast metabolism. Assembly of PSI into supercomplexes with light harvesting complex (LHC) II, cytochrome b6f (Cytb6f) or NAD(P)H dehydrogenase complex (NDH) has been proposed as a means for regulating photosynthesis. However, structural details about the binding positions in plant PSI are lacking. We analyzed large data sets of electron microscopy single particle projections of supercomplexes obtained from the stroma membrane of Arabidopsis thaliana. By single particle analysis, we established the binding position of Cytb6f at the antenna side of PSI. The rectangular-shaped Cytb6f dimer binds at the side where Lhca1 is located. The complex binds with its short side rather than its long side to PSI, which may explain why these supercomplexes are difficult to purify and easily disrupted. Refined analysis of the interaction between PSI and the NDH complex indicates that in total up to 6 copies of PSI can arrange with one NDH complex. Most PSI-NDH supercomplexes appeared to have 1-3 PSI copies associated. Finally, the PSI-LHCII supercomplex was found to bind an additional LHCII trimer at two positions on the LHCI side in Arabidopsis. The organization of PSI, either in a complex with NDH or with Cytb6f, may improve regulation of electron transport by the control of binding partners and distances in small domains.

Cryo-EM structure of a tetrameric cyanobacterial photosystem I complex reveals novel subunit interactions.

Photosystem I (PSI) of the thermophilic cyanobacterium Chroococcidiopsis sp. TS-821 (TS-821) forms tetramers Li et al. (2014). Two-dimensional maps obtained by single particle electron microscopy (EM) clearly show that the tetramer lacks four-fold symmetry and is actually composed of a dimer of dimers with C2 symmetry. The resolution of these negative stain 2D maps did not permit the placement of most of the small PSI subunits, except for PsaL. Therefore cryo-EM was used for 3D reconstruction of the PSI tetramer complex. A 3D model at ~11.5Å resolution was obtained and a 2D map within the membrane plane of ~6.1Å. This data was used to build a model that was compared with the high-resolution structure of the PSI of Thermosynechococcus elongatus (T. elongatus) at 2.5Å. This comparison reveals key differences in which subunits are involved in the two different interfaces, interface type 1 within a dimer and interface type 2 between dimers. The type 1 interface in TS-821 is similar to the monomer interface in the trimeric PSI from T. elongatus, with interactions between subunits PsaA, -B, -I, -L and M. In type 2 the interaction is only between PsaA, -B and -L. Unlike the trimeric PSI, the central cavity of the complex is not filled with the PsaL-derived helical bundle, but instead seems filled with lipids. The physiological or evolutionary advantage of the tetramer is unknown. However, the presence of both dimers and tetramers in the thylakoid membrane suggest a dynamic equilibrium that shifts towards the tetramers in high light.

Characterization and evolution of tetrameric photosystem I from the thermophilic cyanobacterium Chroococcidiopsis sp TS-821.

Photosystem I (PSI) is a reaction center associated with oxygenic photosynthesis. Unlike the monomeric reaction centers in green and purple bacteria, PSI forms trimeric complexes in most cyanobacteria with a 3-fold rotational symmetry that is primarily stabilized via adjacent PsaL subunits; however, in plants/algae, PSI is monomeric. In this study, we discovered a tetrameric form of PSI in the thermophilic cyanobacterium Chroococcidiopsis sp TS-821 (TS-821). In TS-821, PSI forms tetrameric and dimeric species. We investigated these species by Blue Native PAGE, Suc density gradient centrifugation, 77K fluorescence, circular dichroism, and single-particle analysis. Transmission electron microscopy analysis of native membranes confirms the presence of the tetrameric PSI structure prior to detergent solubilization. To investigate why TS-821 forms tetramers instead of trimers, we cloned and analyzed its psaL gene. Interestingly, this gene product contains a short insert between the second and third predicted transmembrane helices. Phylogenetic analysis based on PsaL protein sequences shows that TS-821 is closely related to heterocyst-forming cyanobacteria, some of which also have a tetrameric form of PSI. These results are discussed in light of chloroplast evolution, and we propose that PSI evolved stepwise from a trimeric form to tetrameric oligomer en route to becoming monomeric in plants/algae.

The light-harvesting chlorophyll a/b binding proteins Lhcb1 and Lhcb2 play complementary roles during state transitions in Arabidopsis.

Photosynthetic light harvesting in plants is regulated by phosphorylation-driven state transitions: functional redistributions of the major trimeric light-harvesting complex II (LHCII) to balance the relative excitation of photosystem I and photosystem II. State transitions are driven by reversible LHCII phosphorylation by the STN7 kinase and PPH1/TAP38 phosphatase. LHCII trimers are composed of Lhcb1, Lhcb2, and Lhcb3 proteins in various trimeric configurations. Here, we show that despite their nearly identical amino acid composition, the functional roles of Lhcb1 and Lhcb2 are different but complementary. Arabidopsis thaliana plants lacking only Lhcb2 contain thylakoid protein complexes similar to wild-type plants, where Lhcb2 has been replaced by Lhcb1. However, these do not perform state transitions, so phosphorylation of Lhcb2 seems to be a critical step. In contrast, plants lacking Lhcb1 had a more profound antenna remodeling due to a decrease in the amount of LHCII trimers influencing thylakoid membrane structure and, more indirectly, state transitions. Although state transitions are also found in green algae, the detailed architecture of the extant seed plant light-harvesting antenna can now be dated back to a time after the divergence of the bryophyte and spermatophyte lineages, but before the split of the angiosperm and gymnosperm lineages more than 300 million years ago.

Attachment of phycobilisomes in an antenna-photosystem I supercomplex of cyanobacteria.

Oxygenic photosynthesis is driven by photosystems I and II (PSI and PSII, respectively). Both have specific antenna complexes and the phycobilisome (PBS) is the major antenna protein complex in cyanobacteria, typically consisting of a core from which several rod-like subcomplexes protrude. PBS preferentially transfers light energy to PSII, whereas a PSI-specific antenna has not been identified. The cyanobacterium Anabaena sp. PCC 7120 has rod-core linker genes (cpcG1-cpcG2-cpcG3-cpcG4). Their products, except CpcG3, have been detected in the conventional PBS. Here we report the isolation of a supercomplex that comprises a PSI tetramer and a second, unique type of a PBS, specific to PSI. This rod-shaped PBS includes phycocyanin (PC) and CpcG3 (hereafter renamed "CpcL"), but no allophycocyanin or CpcGs. Fluorescence excitation showed efficient energy transfer from PBS to PSI. The supercomplex was analyzed by electron microscopy and single-particle averaging. In the supercomplex, one to three rod-shaped CpcL-PBSs associate to a tetrameric PSI complex. They are mostly composed of two hexameric PC units and bind at the periphery of PSI, at the interfaces of two monomers. Structural modeling indicates, based on 2D projection maps, how the PsaI, PsaL, and PsaM subunits link PSI monomers into dimers and into a rhombically shaped tetramer or "pseudotetramer." The 3D model further shows where PBSs associate with the large subunits PsaA and PsaB of PSI. It is proposed that the alternative form of CpcL-PBS is functional in harvesting energy in a wide number of cyanobacteria, partially to facilitate the involvement of PSI in nitrogen fixation.

Peptidoglycan-binding protein TsaP functions in surface assembly of type IV pili.

Type IV pili (T4P) are ubiquitous and versatile bacterial cell surface structures involved in adhesion to host cells, biofilm formation, motility, and DNA uptake. In Gram-negative bacteria, T4P pass the outer membrane (OM) through the large, oligomeric, ring-shaped secretin complex. In the ß-proteobacterium Neisseria gonorrhoeae, the native PilQ secretin ring embedded in OM sheets is surrounded by an additional peripheral structure, consisting of a peripheral ring and seven extending spikes. To unravel proteins important for formation of this additional structure, we identified proteins that are present with PilQ in the OM. One such protein, which we name T4P secretin-associated protein (TsaP), was identified as a phylogenetically widely conserved component of the secretin complex that co-occurs with genes for T4P in Gram-negative bacteria. TsaP contains an N-terminal carbohydrate-binding lysin motif (LysM) domain and a C-terminal domain of unknown function. In N. gonorrhoeae, lack of TsaP results in the formation of membrane protrusions containing multiple T4P, concomitant with reduced formation of surface-exposed T4P. Lack of TsaP did not affect the oligomeric state of PilQ, but resulted in loss of the peripheral structure around the PilQ secretin. TsaP binds peptidoglycan and associates strongly with the OM in a PilQ-dependent manner. In the δ-proteobacterium Myxococcus xanthus, TsaP is also important for surface assembly of T4P, and it accumulates and localizes in a PilQ-dependent manner to the cell poles. Our results show that TsaP is a novel protein associated with T4P function and suggest that TsaP functions to anchor the secretin complex to the peptidoglycan.

Functional Implications of Photosystem II Crystal Formation in Photosynthetic Membranes.

The structural organization of proteins in biological membranes can affect their function. Photosynthetic thylakoid membranes in chloroplasts have the remarkable ability to change their supramolecular organization between disordered and semicrystalline states. Although the change to the semicrystalline state is known to be triggered by abiotic factors, the functional significance of this protein organization has not yet been understood. Taking advantage of an Arabidopsis thaliana fatty acid desaturase mutant (fad5) that constitutively forms semicrystalline arrays, we systematically test the functional implications of protein crystals in photosynthetic membranes. Here, we show that the change into an ordered state facilitates molecular diffusion of photosynthetic components in crowded thylakoid membranes. The increased mobility of small lipophilic molecules like plastoquinone and xanthophylls has implications for diffusion-dependent electron transport and photoprotective energy-dependent quenching. The mobility of the large photosystem II supercomplexes, however, is impaired, leading to retarded repair of damaged proteins. Our results demonstrate that supramolecular changes into more ordered states have differing impacts on photosynthesis that favor either diffusion-dependent electron transport and photoprotection or protein repair processes, thus fine-tuning the photosynthetic energy conversion.

The native structure and composition of the cruciferin complex in Brassica napus.

Globulins are an important group of seed storage proteins in dicotyledonous plants. They are synthesized during seed development, assembled into very compact protein complexes, and finally stored in protein storage vacuoles (PSVs). Here, we report a proteomic investigation on the native composition and structure of cruciferin, the 12 S globulin of Brassica napus. PSVs were directly purified from mature seeds by differential centrifugations. Upon analyses by blue native (BN) PAGE, two major types of cruciferin complexes of ∼ 300-390 kDa and of ∼470 kDa are resolved. Analyses by two-dimensional BN/SDS-PAGE revealed that both types of complexes are composed of several copies of the cruciferin α and ß polypeptide chains, which are present in various isoforms. Protein analyses by two-dimensional isoelectric focusing (IEF)/SDS-PAGE not only revealed different α and ß isoforms but also several further versions of the two polypeptide chains that most likely differ with respect to posttranslational modifications. Overall, more than 30 distinct forms of cruciferin were identified by mass spectrometry. To obtain insights into the structure of the cruciferin holocomplex, a native PSV fraction was analyzed by single particle electron microscopy. More than 20,000 images were collected, classified, and used for the calculation of detailed projection maps of the complex. In contrast to previous reports on globulin structure in other plant species, the cruciferin complex of Brassica napus has an octameric barrel-like structure, which represents a very compact building block optimized for maximal storage of amino acids within minimal space.

Delineating organizational principles of the endogenous L-A virus by cryo-EM and computational analysis of native cell extracts.

The high abundance of most viruses in infected host cells benefits their structural characterization. However, endogenous viruses are present in low copy numbers and are therefore challenging to investigate. Here, we retrieve cell extracts enriched with an endogenous virus, the yeast L-A virus. The determined cryo-EM structure discloses capsid-stabilizing cation-π stacking, widespread across viruses and within the Totiviridae, and an interplay of non-covalent interactions from ten distinct capsomere interfaces. The capsid-embedded mRNA decapping active site trench is supported by a constricting movement of two flexible opposite-facing loops. tRNA-loaded polysomes and other biomacromolecules, presumably mRNA, are found in virus proximity within the cell extract. Mature viruses participate in larger viral communities resembling their rare in-cell equivalents in terms of size, composition, and inter-virus distances. Our results collectively describe a 3D-architecture of a viral milieu, opening the door to cell-extract-based high-resolution structural virology.

Cryo-EM of a heterogeneous biochemical fraction elucidates multiple protein complexes from a multicellular thermophilic eukaryote.

Biomolecular complexes and their interactions govern cellular structure and function. Understanding their architecture is a prerequisite for dissecting the cell's inner workings, but their higher-order assembly is often transient and challenging for structural analysis. Here, we performed cryo-EM on a single, highly heterogeneous biochemical fraction derived from Chaetomium thermophilum cell extracts to visualize the biomolecular content of the multicellular eukaryote. After cryo-EM single-particle image processing, results showed that a simultaneous three-dimensional structural characterization of multiple chemically diverse biomacromolecules is feasible. Namely, the thermophilic, eukaryotic complexes of (a) ATP citrate-lyase, (b) Hsp90, (c) 20S proteasome, (d) Hsp60 and (e) UDP-glucose pyrophosphorylase were characterized. In total, all five complexes have been structurally dissected in a thermophilic eukaryote in a total imaged sample area of 190.64 µm2, and two, in particular, 20S proteasome and Hsp60, exhibit side-chain resolution features. The C. thermophilum Hsp60 near-atomic model was resolved at 3.46 Å (FSC = 0.143) and shows a hinge-like conformational change of its equatorial domain, highly similar to the one previously shown for its bacterial orthologue, GroEL. This work demonstrates that cryo-EM of cell extracts will greatly accelerate the structural analysis of cellular complexes and provide unprecedented opportunities to annotate architectures of biomolecules in a holistic approach.

Cryo-EM structure of a plant photosystem II supercomplex with light-harvesting protein Lhcb8 and α-tocopherol.

The heart of oxygenic photosynthesis is the water-splitting photosystem II (PSII), which forms supercomplexes with a variable amount of peripheral trimeric light-harvesting complexes (LHCII). Our knowledge of the structure of green plant PSII supercomplex is based on findings obtained from several representatives of green algae and flowering plants; however, data from a non-flowering plant are currently missing. Here we report a cryo-electron microscopy structure of PSII supercomplex from spruce, a representative of non-flowering land plants, at 2.8 Å resolution. Compared with flowering plants, PSII supercomplex in spruce contains an additional Ycf12 subunit, Lhcb4 protein is replaced by Lhcb8, and trimeric LHCII is present as a homotrimer of Lhcb1. Unexpectedly, we have found α-tocopherol (α-Toc)/α-tocopherolquinone (α-TQ) at the boundary between the LHCII trimer and the inner antenna CP43. The molecule of α-Toc/α-TQ is located close to chlorophyll a614 of one of the Lhcb1 proteins and its chromanol/quinone head is exposed to the thylakoid lumen. The position of α-Toc in PSII supercomplex makes it an ideal candidate for the sensor of excessive light, as α-Toc can be oxidized to α-TQ by high-light-induced singlet oxygen at low lumenal pH. The molecule of α-TQ appears to shift slightly into the PSII supercomplex, which could trigger important structure-functional modifications in PSII supercomplex. Inspection of the previously reported cryo-electron microscopy maps of PSII supercomplexes indicates that α-Toc/α-TQ can be present at the same site also in PSII supercomplexes from flowering plants, but its identification in the previous studies has been hindered by insufficient resolution.

Cryo-EM structure of a tetrameric photosystem I from Chroococcidiopsis TS-821, a thermophilic, unicellular, non-heterocyst-forming cyanobacterium.

Photosystem I (PSI) is one of two photosystems involved in oxygenic photosynthesis. PSI of cyanobacteria exists in monomeric, trimeric, and tetrameric forms, in contrast to the strictly monomeric form of PSI in plants and algae. The tetrameric organization raises questions about its structural, physiological, and evolutionary significance. Here we report the ∼3.72 Å resolution cryo-electron microscopy structure of tetrameric PSI from the thermophilic, unicellular cyanobacterium Chroococcidiopsis sp. TS-821. The structure resolves 44 subunits and 448 cofactor molecules. We conclude that the tetramer is arranged via two different interfaces resulting from a dimer-of-dimers organization. The localization of chlorophyll molecules permits an excitation energy pathway within and between adjacent monomers. Bioinformatics analysis reveals conserved regions in the PsaL subunit that correlate with the oligomeric state. Tetrameric PSI may function as a key evolutionary step between the trimeric and monomeric forms of PSI organization in photosynthetic organisms.

Integrative structure of a 10-megadalton eukaryotic pyruvate dehydrogenase complex from native cell extracts.

The pyruvate dehydrogenase complex (PDHc) is a giant enzymatic assembly involved in pyruvate oxidation. PDHc components have been characterized in isolation, but the complex's quaternary structure has remained elusive due to sheer size, heterogeneity, and plasticity. Here, we identify fully assembled Chaetomium thermophilum α-keto acid dehydrogenase complexes in native cell extracts and characterize their domain arrangements utilizing mass spectrometry, activity assays, crosslinking, electron microscopy (EM), and computational modeling. We report the cryo-EM structure of the PDHc core and observe unique features of the previously unknown native state. The asymmetric reconstruction of the 10-MDa PDHc resolves spatial proximity of its components, agrees with stoichiometric data (60 E2p:12 E3BP:∼20 E1p: ≤ 12 E3), and proposes a minimum reaction path among component enzymes. The PDHc shows the presence of a dynamic pyruvate oxidation compartment, organized by core and peripheral protein species. Our data provide a framework for further understanding PDHc and α-keto acid dehydrogenase complex structure and function.

Solubilization of artificial mitochondrial membranes by amphiphilic copolymers of different charge.

Certain amphiphilic copolymers form lipid-bilayer nanodiscs from artificial and natural membranes, thereby rendering incorporated membrane proteins optimal for structural analysis. Recent studies have shown that the amphiphilicity of a copolymer strongly determines its solubilization efficiency. This is especially true for highly negatively charged membranes, which experience pronounced Coulombic repulsion with polyanionic polymers. Here, we present a systematic study on the solubilization of artificial multicomponent lipid vesicles that mimic inner mitochondrial membranes, which harbor essential membrane-protein complexes. In particular, we compared the lipid-solubilization efficiencies of established anionic with less densely charged or zwitterionic and even cationic copolymers in low- and high-salt concentrations. The nanodiscs formed under these conditions were characterized by dynamic light scattering and negative-stain electron microscopy, pointing to a bimodal distribution of nanodisc diameters with a considerable fraction of nanodiscs engaging in side-by-side interactions through their polymer rims. Overall, our results show that some recent, zwitterionic copolymers are best suited to solubilize negatively charged membranes at high ionic strengths even at low polymer/lipid ratios.

2.7 Å cryo-EM structure of vitrified M. musculus H-chain apoferritin from a compact 200 keV cryo-microscope.

Here we present the structure of mouse H-chain apoferritin at 2.7 Å (FSC = 0.143) solved by single particle cryogenic electron microscopy (cryo-EM) using a 200 kV device, the Thermo Fisher Glacios®. This is a compact, two-lens illumination system with a constant power objective lens, without any energy filters or aberration correctors, often thought of as a "screening cryo-microscope". Coulomb potential maps reveal clear densities for main chain carbonyl oxygens, residue side chains (including alternative conformations) and bound solvent molecules. We used a quasi-crystallographic reciprocal space approach to fit model coordinates to the experimental cryo-EM map. We argue that the advantages offered by (a) the high electronic and mechanical stability of the microscope, (b) the high emission stability and low beam energy spread of the high brightness Field Emission Gun (X-FEG), (c) direct electron detection technology and (d) particle-based Contrast Transfer Function (CTF) refinement have contributed to achieving high resolution. Overall, we show that basic electron optical settings for automated cryo-electron microscopy imaging can be used to determine structures approaching atomic resolution.

Physiological and evolutionary implications of tetrameric photosystem I in cyanobacteria.

Photosystem I (PSI) is present as trimeric complexes in most characterized cyanobacteria and as monomers in plants and algae. Recent reports of tetrameric PSI have raised questions regarding its structural basis, physiological role, phylogenetic distribution and evolutionary significance. Here, we examined PSI in 61 cyanobacteria, showing that tetrameric PSI, which correlates with the psaL gene and a distinct genomic structure, is widespread among heterocyst-forming cyanobacteria and their close relatives. Physiological studies revealed that expression of tetrameric PSI is favoured under high light, with an increased content of novel PSI-bound carotenoids (myxoxanthophyll, canthaxanthan and echinenone). In sum, this work suggests that tetrameric PSI is an adaptation to high light intensity, and that change in PsaL leads to monomerization of trimeric PSI, supporting the hypothesis of tetrameric PSI being the evolutionary intermediate in the transition from cyanobacterial trimeric PSI to monomeric PSI in plants and algae.

Structure of the dimeric RC-LH1-PufX complex from Rhodobaca bogoriensis investigated by electron microscopy.

Electron microscopy and single-particle averaging were performed on isolated reaction centre (RC)-antenna complexes (RC-LH1-PufX complexes) of Rhodobaca bogoriensis strain LBB1, with the aim of establishing the LH1 antenna conformation, and, in particular, the structural role of the PufX protein. Projection maps of dimeric complexes were obtained at 13 Å resolution and show the positions of the 2 × 14 LH1 α- and ß-subunits. This new dimeric complex displays two open, C-shaped LH1 aggregates of 13 αß polypeptides partially surrounding the RCs plus two LH1 units forming the dimer interface in the centre. Between the interface and the two half rings are two openings on each side. Next to the openings, there are four additional densities present per dimer, considered to be occupied by four copies of PufX. The position of the RC in our model was verified by comparison with RC-LH1-PufX complexes in membranes. Our model differs from previously proposed configurations for Rhodobacter species in which the LH1 ribbon is continuous in the shape of an S, and the stoichiometry is of one PufX per RC.