Isolation and Characterization of Carotenosomes from a Bacteriochlorophyll c-less Mutant ofChlorobium tepidum

Chlorosomes are the light-harvesting organelles in photosynthetic green bacteria and typically contain large amounts of bacteriochlorophyll (BChl) c in addition to smaller amounts of BChl a, carotenoids, and several protein species. We have isolated vestigial chlorosomes, denoted carotenosomes, from a BChl c-less, bchK mutant of the green sulfur bacterium Chlorobium tepidum. The physical shape of the carotenosomes (86 ± 17 nm × 66 ± 13 nm × 4.3 ± 0.8 nm on average) was reminiscent of a flattened chlorosome. The carotenosomes contained carotenoids, BChl a, and the proteins CsmA and CsmD in ratios to each other comparable to their ratios in wild-type chlorosomes, but all other chlorosome proteins normally found in wild-type chlorosomes were found only in trace amounts or were not detected. Similar to wild-type chlorosomes, the CsmA protein in the carotenosomes formed oligomers at least up to homo-octamers as shown by chemical cross-linking and immunoblotting. The absorption spectrum of BChl a in the carotenosomes was also indistinguishable from that in wild-type chlorosomes. Energy transfer from the bulk carotenoids to BChl a in carotenosomes was poor. The results indicate that the carotenosomes have an intact baseplate made of remarkably stable oligomeric CsmA–BChl a complexes but are flattened in structure due to the absence of BChl c. Carotenosomes thus provide a valuable material for studying the biogenesis, structure, and function of the photosynthetic antennae in green bacteria.     

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.     

Characterization of the csmD and csmE genes from Chlorobium tepidum. The CsmA, CsmC, CsmD, and CsmE proteins are components of the chlorosome envelope

The csmD and csmE genes, encoding two proteins of the chlorosome envelope, have been cloned and sequenced from the green sulfur bacterium Chlorobium tepidum. The csmD gene predicts a hydrophobic protein of 113 amino acids with a molecular mass of 11.1 kDa. The csmE gene was identified immediately upstream from csmD; the csmE gene predicts a protein of 82 amino acids (9.0 kDa) which is 49% identical to CsmA (Chung et al. (1994) Photosynthesis Res 41: 261–275). The CsmE protein is post-translationally processed, most likely in a manner similar to CsmA. The csmE and csmD genes are cotranscribed as a dicistronic mRNA but can also be cotranscribed with an open reading frame upstream from csmE that predicts a protein with sequence similarity to the CheY and SpoOF subclass of regulatory proteins. The CsmA, CsmC, CsmD, and CsmE proteins were overproduced in Escherichia coli, purified, and used to raise polyclonal antibodies in rabbits. Protease susceptibility mapping and agglutination experiments using these antibodies indicate that all four proteins are exposed at the surface of isolated chlorosomes and hence are probably components of the chlorosome envelope. Additionally, antigalactose antibodies were used to confirm that the galactosyl moiety of monogalactosyl diglycerol is exposed at the chlorosome surface; this is consistent with the notion that these lipids are components of the chlorosome envelope.Abbreviations BChl- bacteriochlorophyll - bp- basepair(s) - C.- Chloroflexus - Cb.- Chlorobium - Csm- chlorosome protein - csm gene encoding a chlorosome protein - IPTG isopropyl--d-thiogalatoside - nt nucleotide - ORF open reading frame - PAGE polyacrylamide gel electrophoresis - SDS sodium dodecylsulfate     

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.     

Insertional inactivation studies of the csmA and csmC genes of the green sulfur bacterium Chlorobium vibrioforme 8327: the chlorosome protein CsmA is required for viability but CsmC is dispensable

Targeted mutagenesis was used to investigate the roles of the CsmA and CsmC proteins of the chlorosomes of the green bacteria Chlorobium tepidum and Chlorobium vibrioforme 8327. Under the photoautotrophic growth conditions employed, CsmA is required for the viability of the cells but CsmC is dispensable. The absence of CsmC caused a small red shift in the near-infrared absorption maximum of bacteriochlorophyll d in whole cells and chlorosomes, but chlorosomes were assembled in and could be isolated from the csmC mutant. The doubling time of the csmC mutant was approximately twice that of the wild-type strain. Fluorescence emission measurements suggested that energy transfer from the bulk bacteriochlorophyll d to another pigment, perhaps bacteriochlorophyll a, emitting at 800–804 nm, was less efficient in the csmC mutant cells than in wild-type cells. These studies establish that transformation and homologous recombination can be employed in targeted mutagenesis of Chlorobium sp. and further demonstrate that chlorosome proteins play important roles in the structure and function of these light-harvesting organelles.     

Selective protein extraction from Chlorobium tepidum chlorosomes using detergents. Evidence that CsmA forms multimers and binds bacteriochlorophyll a

Chlorosomes of the photosynthetic green sulfur bacterium Chlorobium tepidum consist of bacteriochlorophyll (BChl) c aggregates that are surrounded by a lipid-protein monolayer envelope that contains ten different proteins. Chlorosomes also contain a small amount of BChl a, but the organization and location of this BChl a are not yet clearly understood. Chlorosomes were treated with sodium dodecyl sulfate (SDS), Lubrol PX, or Triton X-100, separately or in combination with 1-hexanol, and the extracted components were separated from the residual chlorosomes by ultrafiltration on centrifugal filters. When chlorosomes were treated with low concentrations of SDS, all proteins except CsmA were extracted. However, this treatment did not significantly alter the size and shape of the chlorosomes, did not extract the BChl a, and caused only minor changes in the absorption spectrum of the chlorosomes. Cross-linking studies with SDS-treated chlorosomes revealed the presence of multimers of the major chlorosome protein, CsmA, up to homooctamers. Extraction of chlorosomes with SDS and 1-hexanol solubilized all ten chlorosome envelope proteins as well as BChl a. Although the size and shape of these extracted chlorosomes did not initially differ significantly from untreated chlorosomes, the extracted chlorosomes gradually disintegrated, and rod-shaped BChl c aggregates were sometimes observed. These results strongly suggest that CsmA binds the BChl a in Chlorobium-type chlorosomes and further indicate that none of the nine other chlorosome envelope proteins are absolutely required for maintaining the shape and integrity of chlorosomes. Quantitative estimates suggest that chlorosomes contain approximately equimolar amounts of CsmA and BChl a and that roughly one-third of the surface of the chlorosome is covered by CsmA.     

X-ray scattering and electron cryomicroscopy study on the effect of carotenoid biosynthesis to the structure of Chlorobium tepidum chlorosomes

Chlorosomes, the main antenna complexes of green photosynthetic bacteria, were isolated from null mutants of Chlorobium tepidum, each of which lacked one enzyme involved in the biosynthesis of carotenoids. The effects of the altered carotenoid composition on the structure of the chlorosomes were studied by means of x-ray scattering and electron cryomicroscopy. The chlorosomes from each mutant strain exhibited a lamellar arrangement of the bacteriochlorophyll c aggregates, which are the major constituents of the chlorosome interior. However, the carotenoid content and composition had a pronounced effect on chlorosome biogenesis and structure. The results indicate that carotenoids with a sufficiently long conjugated system are important for the biogenesis of the chlorosome baseplate. Defects in the baseplate structure affected the shape of the chlorosomes and were correlated with differences in the arrangement of lamellae and spacing between the lamellar planes of bacteriochlorophyll aggregates. In addition, comparisons among the various mutants enabled refinement of the assignments of the x-ray scattering peaks. While the main scattering peaks come from the lamellar structure of bacteriochlorophyll c aggregates, some minor peaks may originate from the paracrystalline arrangement of CsmA in the baseplate.     

Characterization of csmB genes,encoding a 7.5-kDa protein of the chlorosome envelope,from the green sulfur bacteria Chlorobium vibrioforme 8327D and Chlorobium tepidum

The csmB gene, encoding the 7.5-kDa “Gerola-Olson” protein of chlorosomes, has been cloned and sequenced from the green sulfur bacteria Chlorobium vibrioforme strain 8327D and Chlorobium tepidum. Two potential start codons were identified, and the csmB gene may be translated into a preprotein with an amino-terminal extension. Two forms of the mature CsmB protein (74 or 75 amino acids in length) were identified that differ by the presence or absence of a methionine residue at the amino terminus. The csmB gene of Chl. tepidum is transcribed as an abundant monocistronic mRNA of approximately 350 nucleotides; primer extension mapping of the 5′ endpoint of the csmB mRNA suggests there is strong similarity between the csmB promoter and the σ⁷⁰ promoters of Escherichia coli. The CsmB protein of Chl. tepidum was overproduced as a histidine-tagged fusion protein in E. coli, purified to homogeneity by Ni²⁺ chelation affinity chromatography, and used to raise polyclonal antibodies in rabbits. Protease susceptibility mapping and agglutination experiments with isolated chlorosomes using anti-CsmB antibodies indicate that the CsmB protein is a component of the chlorosome envelope. Received: 28 May 1996 / Accepted: 17 July 1996     

Biosynthesis of chlorosome proteins is not inhibited in acetylene-treated cultures of Chlorobium vibrioforme

The composition, abundance and apparent molecular masses of chlorosome polypeptides from Chlorobium tepidum and Chlorobium vibrioforme 8327 were compared. The most abundant, low-molecular-mass chlorosome polypeptides of both strains had similar electrophoretic mobilities and abundances, but several of the larger proteins were different in both apparent mass and abundance. Polyclonal antisera raised against recombinant chlorosome proteins of Cb. tepidum recognized the homologous proteins in Cb. vibrioforme, and a one-to-one correspondence between the chlorosome proteins of the two species was confirmed. As previously shown [Ormerod et al. (1990) J Bacteriol 172: 1352–1360], acetylene strongly suppressed the synthesis of bacteriochlorophyll c in Cb. vibrioforme strain 8327. No correlation was found between the bacteriochlorophyll c content of cells and the cellular content of chlorosome proteins. Nine of ten chlorosome proteins were detected in acetylene-treated cultures, and the chlorosome proteins were generally present in similar amounts in control and acetylene-treated cells. These results suggest that the synthesis of chlorosome proteins and the assembly of the chlorosome envelope is constitutive. It remains possible that the synthesis of bacteriochlorophyll c and its insertion into chlorosomes might be regulated by environmental parameters such as light intensity.This revised version was published online in October 2005 with corrections to the Cover Date.     

Initial characterization of the photosynthetic apparatus of "Candidatus Chlorothrix halophila," a filamentous, anoxygenic photoautotroph

“Candidatus Chlorothrix halophila” is a recently described halophilic, filamentous, anoxygenic photoautotroph (J. A. Klappenbach and B. K. Pierson, Arch. Microbiol. 181:17-25, 2004) that was enriched from the hypersaline microbial mats at Guerrero Negro, Mexico. Analysis of the photosynthetic apparatus by negative staining, spectroscopy, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that the photosynthetic apparatus in this organism has similarities to the photosynthetic apparatus in both the Chloroflexi and Chlorobi phyla of green photosynthetic bacteria. The chlorosomes were found to be ellipsoidal and of various sizes, characteristics that are comparable to characteristics of chlorosomes in other species of green photosynthetic bacteria. The absorption spectrum of whole cells was dominated by the chlorosome bacteriochlorophyll c (BChl c) peak at 759 nm, with fluorescence emission at 760 nm. A second fluorescence emission band was observed at 870 nm and was tentatively attributed to a membrane-bound antenna complex. Fluorescence emission spectra obtained at 77 K revealed another complex that fluoresced at 820 nm, which probably resulted from the chlorosome baseplate complex. All of these results suggest that BChl c is present in the chlorosomes of “Ca. Chlorothrix halophila,” that BChl a is present in the baseplate, and that there is a membrane-bound antenna complex. Analysis of the proteins in the chlorosomes revealed an ~6-kDa band, which was found to be related to the BChl c binding protein CsmA found in other green bacteria. Overall, the absorbance and fluorescence spectra of “Ca. Chlorothrix halophila” revealed an interesting mixture of photosynthetic characteristics that seemed to have properties similar to properties of both phyla of green bacteria when they were compared to the photosynthetic characteristics of Chlorobium tepidum and Chloroflexus aurantiacus.     

Nine mutants of Chlorobium tepidum each unable to synthesize a different chlorosome protein still assemble functional chlorosomes

Chlorosomes of the green sulfur bacterium Chlorobium tepidum comprise mostly bacteriochlorophyll c (BChl c), small amounts of BChl a, carotenoids, and quinones surrounded by a lipid-protein envelope. These structures contain 10 different protein species (CsmA, CsmB, CsmC, CsmD, CsmE, CsmF, CsmH, CsmI, CsmJ, and CsmX) but contain relatively little total protein compared to other photosynthetic antenna complexes. Except for CsmA, which has been suggested to bind BChl a, the functions of the chlorosome proteins are not known. Nine mutants in which a single csm gene was inactivated were created; these mutants included genes encoding all chlorosome proteins except CsmA. All mutants had BChl c contents similar to that of the wild-type strain and had growth rates indistinguishable from or within approximately 90% (CsmC(-) and CsmJ(-)) of those of the wild-type strain. Chlorosomes isolated from the mutants lacked only the protein whose gene had been inactivated and were generally similar to those from the wild-type strain with respect to size, shape, and BChl c, BChl a, and carotenoid contents. However, chlorosomes from the csmC mutant were about 25% shorter than those from the wild-type strain, and the BChl c absorbance maximum was blue-shifted about 8 nm, indicating that the structure of the BChl c aggregates in these chlorosomes is altered. The results of the present study establish that, except with CsmA, when the known chlorosome proteins are eliminated individually, none of them are essential for the biogenesis, light harvesting, or structural organization of BChl c and BChl a within the chlorosome. These results demonstrate that chlorosomes are remarkably robust structures that can tolerate considerable changes in protein composition.     

Quinones in chlorosomes of green sulfur bacteria and their role in the redox-dependent fluorescence studied in chlorosome-like bacteriochlorophyll c aggregates

The light-harvesting chlorosome antennae of anaerobic, photosynthetic green sulfur bacteria exhibit a highly redox-dependent fluorescence such that the fluorescence intensity decreases under oxidizing conditions. We found that chlorosomes from Chlorobium tepidum contain three isoprenoid quinone species (chlorobiumquinone, menaquinone-7, and an unidentified quinone that probably is a chlorobiumquinone derivative) at a total concentration of approximately 0.1 mol per mol bacteriochlorophyll c. Most of the cellular chlorobiumquinone was found in the chlorosomes and constituted about 70% of the total chlorosome quinone pool. When the quinones were added to artificial, chlorosome-like bacteriochlorophyll c aggregates in an aqueous solution, a high redox dependency of the fluorescence was observed. Chlorobiumquinones were most effective in this respect. A lesser redox dependency of the fluorescence was still observed in the absence of quinones, probably due to another unidentified redox-active component. These results suggest that quinones play a significant, but not exclusive role in controlling the fluorescence and in inhibiting energy transfer in chlorosomes under oxic conditions. Chlorosomes from Chloroflexus aurantiacus contained menaquinone in an amount similar to that of total quinone in Chlorobium tepdium chlorosomes, but did not contain chlorobiumquinones. This may explain the much lower redox-dependent fluorescence observed in Chloroflexus chlorosomes. Received: 4 November 1996 / Accepted: 18 February 1997     

Subcellular localization of chlorosome proteins in Chlorobium tepidum and characterization of three new chlorosome proteins: CsmF, CsmH, and CsmX

Chlorosomes are unique light-harvesting structures found in two families of photosynthetic bacteria. In this study, three chlorosome proteins (CsmF, CsmH, and CsmX) of the green sulfur bacterium Chlorobium tepidum were characterized by cloning and sequencing the genes which encode them, by overproducing the respective proteins in Escherichia coli, and by raising polyclonal antisera to the purified proteins. Three other proteins (AtpF, CT1970, and CT2144) which were identified in chlorosome fractions have similarly been characterized. The antisera were used to establish the distribution of each protein in various cellular fractions. Ten chlorosome proteins (CsmA, CsmB, CsmC, CsmD, CsmE, CsmF, CsmH, CsmI, CsmJ, and CsmX) copurified in a constant proportion together with bacteriochlorophyll c, and none of these 10 proteins was found in substantial amounts in other subcellular fractions. An antiserum to CsmH was highly effective in agglutinating chlorosomes, and antisera to CsmI, CsmJ, CsmX, and CsmA also immunoprecipitated chlorosomes to varying extents. However, an antiserum to CsmF did not agglutinate chlorosomes. The sequences of chlorosome proteins generally are not significantly similar to the sequences of other proteins in the databases. However, the N-terminal domains of three chlorosome proteins, CsmI, CsmJ, and CsmX, are related to adrenodoxin-type ferredoxins that ligate [2Fe-2S] clusters [Vassilieva, E. V., Antonkine, M. L., Zybailov, B. L., Yang, F., Jakobs, C. U., Golbeck, J. H., and Bryant, D. A. (2001) Biochemistry 40, 464-473]. The sequences of the C-terminal domains of these three proteins appear to be distantly related to CsmA and CsmE. The remaining chlorosome proteins can be divided into two additional structural families, CsmB/F and CsmC/D. CsmH is recovered in water-soluble form after overproduction in E. coli. Interestingly, this protein contains an N-terminal domain that is similar to CsmB/D, while its C-terminal domain is related to CsmC/D. The sequence relationships indicate that, although the protein composition of Chlorobium-type chlorosomes is superficially more complex than that of the chlorosomes of Chloroflexus aurantiacus, this heterogeneity is mostly produced by gene duplication and divergence among a small number of protein types.     

Pigment interactions in chlorosomes of various green bacteria

Resonance Raman experiments were performed on different green bacteria. With blue excitation, i.e. under Soret resonance or preresonance conditions, resonance Raman contributions were essentially arising from the chlorosome pigments. By comparing these spectra and those of isolated chlorosomes, it is possible to evaluate how the latter retain their native structure during the isolation procedures. The structure of bacteriochlorophyll oligomers in chlorosomes was interspecifically compared, in bacteriochlorophyllc- and bacteriochlorophylle- synthesising bacteria. It appears that interactions assumed by the 9-keto carbonyl group are identical inChlorobium limicola, Chlorobium tepidum, andChlorobium phaeobacteroides. In the latter strain, the 3-formyl carbonyl group of bacteriochlorophylle is kept free from intermolecular interactions. By contrast, resonance Raman spectra unambiguously indicate that the structure of bacteriochlorophyll oligomers is slightly different in chlorosomes fromChloroflexus auranticus, either isolated or in the whole bacteria.     

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.     

Pigments and proteins in green bacterial chlorosomes studied by matrix-assisted laser desorption ionization mass spectrometry.

We have used matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) for mass determination of pigments and proteins in chlorosomes, the light-harvesting organelles from the photosynthetic green sulfur bacterium Chlorobium tepidum. By applying a small volume (1 microL) of a concentrated suspension of isolated chlorosomes directly to the target of the mass spectrometer we have been able to detect bacteriochlorophyll a and all the major homologs of bacteriochlorophyll c. The peak heights of the different bacteriochlorophyll c homologs in the MALDI spectra were proportional to peak areas obtained from HPLC analysis of the same sample. The same result was also obtained when whole cells of Chl. tepidum were applied to the target, indicating that MALDI-MS can provide a rapid method for obtaining a semiquantitative determination or finger-print of the bacteriochlorophyll homologs in a small amount of green bacterial cells. In addition to information on pigments, the MALDI spectra also contained peaks from chlorosome proteins. Thus we have been able with high precision to confirm the molecular masses of the chlorosome proteins CsmA and CsmE which have been previously determined by conventional biochemical and genetic methods, and demonstrate the presence of truncated versions of CsmA and CsmB.     

Characterization of the chlorosome antenna of the filamentous anoxygenic phototrophic bacterium<Emphasis Type="Italic"> Chloronema</Emphasis> sp. strain UdG9001

The absorption and fluorescence properties of chlorosomes of the filamentous anoxygenic phototrophic bacterium Chloronema sp. strain UdG9001 were analyzed. The chlorosome antenna of Chloronema consists of bacteriochlorophyll (BChl) d and BChl c together with -carotene as the main carotenoid. HPLC analysis combined with APCI LC-MS/MS showed that the chlorosomal BChls comprise a highly diverse array of homologues that differ in both the degree of alkylation of the macrocycle at C-8 and/or C-12 and the alcohol moiety esterified to the propionic acid group at C-17. BChl c and BChl d from Chloronema were mainly esterified with geranylgeraniol (33% of the total), heptadecanol (24%), octadecenol (19%), octadecanol (14%), and hexadecenol (9%). Despite this pigment heterogeneity, fluorescence emission of the chlorosomes showed a single peak centered at 765 nm upon excitation at wavelengths ranging from 710 to 740 nm. This single emission, assigned to BChl c, indicates an energy transfer from BChl d to BChl c within the same chlorosome. Likewise, incubation of chlorosomes under reducing conditions caused a weak increase in fluorescence emission, which indicates a small redox-dependent fluorescence. Finally, protein analysis of Chloronema chlorosomes using SDS-PAGE and MALDI-TOF-MS revealed the presence of a chlorosomal polypeptide with a molecular mass of 5.7 kDa, resembling the CsmA protein found in Chloroflexus aurantiacus and Chlorobium tepidum chlorosomes. Several minor polypeptides were also detected but not identified. These results indicate that, compared with other members of filamentous anoxygenic phototrophic bacteria and green sulfur bacteria, Chloronema possesses an antenna system with novel features that may be of interest for further investigations.Abbreviations APCI LC-MS/MS Atmospheric pressure chemical ionization liquid chromatography mass spectrometry - BChl Bacteriochlorophyll - Chl. Chlorobium - Cfl. Chloroflexus - MALDI-TOF-MS Matrix assisted laser desorption/ionization time-of-flight mass spectrometry - [Et] Ethyl - [i-Bu] Isobutyl - [Me] Methyl - [neo-Pent] Neopentyl - [n-Pr] Propyl - t _R Retention time     

Molecular contacts for chlorosome envelope proteins revealed by cross-linking studies with chlorosomes from Chlorobium tepidum

Chlorosomes are unique light-harvesting antennae found in two phyla of green bacteria: Chlorobi and Chloroflexi. In the green sulfur bacterium Chlorobium tepidum, 10 proteins (CsmA, CsmB, CsmC, CsmD, CsmE, CsmF, CsmH, CsmI, CsmJ, and CsmX) exist in the chlorosome envelope. Chlorosomes from the wild type and mutants lacking a single chlorosome protein were cross-linked with the zero-length cross-linker 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDC) and analyzed by gel electrophoresis. Similar cross-linking products were observed when the time and temperature were varied or when EDC was replaced with glutaraldehyde. Specific interactions between chlorosome proteins in cross-linked products were identified by immunoblotting with polyclonal antibodies raised against recombinant chlorosome proteins. We confirmed these interactions by demonstrating that these products were missing in appropriate mutants. Confirming the location of CsmA in the paracrystalline baseplate, cross-linking showed that CsmA forms dimers, trimers, and homomultimers as large as dodecamers and that CsmA directly interacts with the Fenna-Matthews-Olson protein. Cross-linking further suggests that the precursor form of CsmA is inserted near the edges of the baseplate, where CsmA and pre-CsmA interact with CsmB and CsmF. Several chlorosome proteins, including CsmA, CsmC, CsmD, CsmH, CsmI, CsmJ, and CsmX, were shown to exist as homomultimers in the chlorosome envelope. On the basis of the structural information obtained from these cross-linking experiments, a model for the locations and interactions of the proteins of the chlorosome envelope is proposed.     

A reconstituted light-harvesting complex from the green sulfur bacterium Chlorobium tepidum containing CsmA and bacteriochlorophyll a

Green sulfur bacteria possess two light-harvesting antenna systems, the chlorosome and the Fenna-Matthews-Olson (FMO) protein. In addition to self-aggregated bacteriochlorophyll (BChl) c, chlorosomes of Chlorobium tepidum contain a small amount of BChl a (ratio 100:1). The chlorosomal BChl a is associated with CsmA, a 6.2 kDa protein that accounts for more than 50% of the protein content of chlorosomes. This CsmA-BChl a complex is located in the chlorosome baseplate with the hydrophilic C-terminal part of CsmA in contact with the FMO protein. CsmA was purified from Chl. tepidum. Isolated chlorosomes were lyophilized and extracted with chloroform/methanol (1:1, v/v). The extract was further purified using gel filtration and reverse-phase HPLC and the purity of the preparation confirmed by SDS-PAGE. Mass spectrometric analysis showed an m/z of 6154.8, in agreement with the calculated mass of the csmA gene product after C-terminal processing. CD spectroscopy of the isolated protein showed that the main structural motif was an alpha-helix. We have reconstituted the isolated CsmA protein with BChl a in micelles of n-octyl beta-d-glucopyranoside. The resulting preparation reproduced the spectral characteristics of the CsmA-BChl a complex present in the chlorosome baseplate.