Chlorosome proteins studied by MALDI-TOF-MS: topology of CsmA in Chlorobium tepidum

In situ high-resolution structure of the baseplate antenna complex in Chlorobaculum tepidum

Photosynthetic antenna systems enable organisms harvesting light and transfer the energy to the photosynthetic reaction centre, where the conversion to chemical energy takes place. One of the most complex antenna systems, the chlorosome, found in the photosynthetic green sulfur bacterium Chlorobaculum (Cba.) tepidum contains a baseplate, which is a scaffolding super-structure, formed by the protein CsmA and bacteriochlorophyll a. Here we present the first high-resolution structure of the CsmA baseplate using intact fully functional, light-harvesting organelles from Cba. tepidum, following a hybrid approach combining five complementary methods: solid-state NMR spectroscopy, cryo-electron microscopy, isotropic and anisotropic circular dichroism and linear dichroism. The structure calculation was facilitated through development of new software, GASyCS for efficient geometry optimization of highly symmetric oligomeric structures. We show that the baseplate is composed of rods of repeated dimers of the strongly amphipathic CsmA with pigments sandwiched within the dimer at the hydrophobic side of the helix.

The chlorosome of the photosynthetic bacterium C. tepidum harvests light and transfers the energy to the photosynthetic reaction centre. Here the authors determine the structure of the baseplate, a scaffolding super-structure, to show that the baseplate consists of rods of repeated CsmA dimers containing pigment molecules.

Computational determination of the pigment binding motif in the chlorosome protein a of green sulfur bacteria

We present a molecular-scale model of Bacteriochlorophyll a (BChl a) binding to the chlorosome protein A (CsmA) of Chlorobaculum tepidum, and the aggregated pigment–protein dimer, as determined from protein–ligand docking and quantum chemistry calculations. Our calculations provide strong evidence that the BChl a molecule is coordinated to the His25 residue of CsmA, with the magnesium center of the bacteriochlorin ring situated\3 A° from the imidazole nitrogen atom of the histidine sidechain, and the phytyl tail aligned along the nonpolar residues of the a-helix of CsmA. We also confirm that the Qy band in the absorption spectra of BChl a experiences a large (?16 to ?43 nm) redshift when aggregated with another BChl a molecule in the CsmA dimer, compared to the BChl a in solvent; this redshift has been previously established by experimental researchers. We propose that our model of the BChl a–CsmA binding motif, where the dimer contains parallel aligned N-terminal regions, serves as the smallest repeating unit in a larger model of the para-crystalline chlorosome baseplate protein.

A novel and mild isolation procedure of chlorosomes from the green sulfur bacterium Chlorobaculum tepidum

In this article, we developed a new and mild procedure for the isolation of chlorosomes from a green sulfur bacterium Chlorobaculum tepidum. In this procedure, Fenna-Matthews-Olson (FMO) protein was released by long cold treatment (6°C) of cells under the presence of a chaotrope (2 M NaSCN) and 0.6 M sucrose. Chlorosomes were released by an osmotic shock of the cold-treated cells after the formation of spheroplasts without mechanical disruption. Chlorosomes were finally purified by a sucrose step-wise density gradient centrifugation. We obtained two samples with different density (20 and 23% sucrose band, respectively) and compared them by SDS-PAGE, absorption spectroscopy at 80 K, fluorescence and CD spectroscopy at room temperature. Cells whose absorption maximum was longer than 750 nm yielded higher amount of the 20% sucrose fraction than those having an absorption maximum shorter than 750 nm.

A model of the protein-pigment baseplate complex in chlorosomes of photosynthetic green bacteria

In contrast to photosynthetic reaction centers, which share the same structural architecture, more variety is found in the light-harvesting antenna systems of phototrophic organisms. The largest antenna system described, so far, is the chlorosome found in anoxygenic green bacteria, as well as in a recently discovered aerobic phototroph. Chlorosomes are the only antenna system, in which the major light-harvesting pigments are organized in self-assembled supramolecular aggregates rather than on protein scaffolds. This unique feature is believed to explain why some green bacteria are able to carry out photosynthesis at very low light intensities. Encasing the chlorosome pigments is a protein-lipid monolayer including an additional antenna complex: the baseplate, a two-dimensional paracrystalline structure containing the chlorosome protein CsmA and bacteriochlorophyll a (BChl a). In this article, we review current knowledge of the baseplate antenna complex, which physically and functionally connects the chlorosome pigments to the reaction centers via the Fenna-Matthews-Olson protein, with special emphasis on the well-studied green sulfur bacterium Chlorobaculum tepidum (previously Chlorobium tepidum). A possible role for the baseplate in the biogenesis of chlorosomes is discussed. In the final part, we present a structural model of the baseplate through combination of a recent NMR structure of CsmA and simulation of circular dichroism and optical spectra for the CsmA-BChl a complex.

Membrane orientation of the FMO antenna protein from Chlorobaculum tepidum as determined by mass spectrometry-based footprinting

The high excitation energy-transfer efficiency demanded in photosynthetic organisms relies on the optimal pigment-protein binding orientation in the individual protein complexes and also on the overall architecture of the photosystem. In green sulfur bacteria, the membrane-attached Fenna-Matthews-Olson (FMO) antenna protein functions as a "wire" to connect the large peripheral chlorosome antenna complex with the reaction center (RC), which is embedded in the cytoplasmic membrane (CM). Energy collected by the chlorosome is funneled through the FMO to the RC. Although there has been considerable effort to understand the relationships between structure and function of the individual isolated complexes, the specific architecture for in vivo interactions of the FMO protein, the CM, and the chlorosome, ensuring highly efficient energy transfer, is still not established experimentally. Here, we describe a mass spectrometry-based method that probes solvent-exposed surfaces of the FMO by labeling solvent-exposed aspartic and glutamic acid residues. The locations and extents of labeling of FMO on the native membrane in comparison with it alone and on a chlorosome-depleted membrane reveal the orientation. The large differences in the modification of certain peptides show that the Bchl a #3 side of the FMO trimer interacts with the CM, which is consistent with recent theoretical predictions. Moreover, the results also provide direct experimental evidence to confirm the overall architecture of the photosystem from Chlorobaculum tepidum (C. tepidum) and give information on the packing of the FMO protein in its native environment.

Role of the AcsF protein in Chloroflexus aurantiacus

The green phototrophic bacteria contain a unique complement of chlorophyll pigments, which self-assemble efficiently into antenna structures known as chlorosomes with little involvement of protein. The few proteins found in chlorosomes have previously been thought to have a primarily structural function. The biosynthetic pathway of the chlorosome pigments, bacteriochlorophylls c, d, and e, is not well understood. In this report, we used spectroscopic, proteomic, and gene expression approaches to investigate the chlorosome proteins of the green filamentous anoxygenic phototrophic bacterium Chloroflexus aurantiacus. Surprisingly, Mg-protoporphyrin IX monomethyl ester (oxidative) cyclase, AcsF, was identified under anaerobic growth conditions. The AcsF protein was found in the isolated chlorosome fractions, and the proteomics analysis suggested that significant portions of the AcsF proteins are not accessible to protease digestion. Additionally, quantitative real-time PCR studies showed that the transcript level of the acsF gene is not lower in anaerobic growth than in semiaerobic growth. Since the proposed enzymatic activity of AcsF requires molecular oxygen, our studies suggest that the roles of AcsF in C. aurantiacus need to be investigated further.

Chlorosome lipids from Chlorobium tepidum: characterization and quantification of polar lipids and wax esters

We have extracted polar lipids and waxes from isolated chlorosomes from the green sulfur bacterium Chlorobium tepidum and determined the fatty acid composition of each lipid class. Polar lipids amounted to 4.8 mol per 100 mol bacteriochlorophyll in the chlorosomes, while non-polar lipids (waxes) were present at a ratio of 5.9 mol per 100 mol bacteriochlorophyll. Glycolipids constitute 60 % of the polar lipids while phosphatidylglycerol, diphosphatidylglycerol, phosphatidylethanolamine, and an aminoglycosphingolipid make up respectively 15, 3, 8 and 12 %. A novel glycolipid was identified as a rhamnose derivative of monogalactosyldiacylglycerol, while the other major glycolipid was monogalactosyldiacylglycerol. Tetradecanoic acid was the major fatty acid in the aminoglycosphingolipid, while the other polar lipids contained predominantly hexandecanoic acid. The chlorosome waxes are esters of unbranched fatty acids and fatty alcohols with 14 or 16 carbon atoms, joined to form molecules with between 28 and 32 carbon atoms. The stoichiometry between lipids and bacteriochlorophyll suggests that much of the chlorosome surface is covered by protein.

The light-harvesting antenna of Chlorobium tepidum: interactions between the FMO protein and the major chlorosome protein CsmA studied by surface plasmon resonance

Green sulfur bacteria possess two external light-harvesting antenna systems, the chlorosome and the FMO protein, which participate in a sequential energy transfer to the reaction centers embedded in the cytoplasmic membrane. However, little is known about the physical interaction between these two antenna systems. We have studied the interaction between the major chlorosome protein, CsmA, and the FMO protein in Chlorobium tepidum using surface plasmon resonance (SPR). Our results show an interaction between the FMO protein and an immobilized synthetic peptide corresponding to 17 amino acids at the C terminal of CsmA. This interaction is dependent on the presence of a motif comprising six amino acids that are highly conserved in all the currently available CsmA protein sequences.