Isolation and characterization of the pigment-protein complexes of Rhodopseudomonas sphaeroides by lithium dodecyl sulfate/polyacrylamide gel electrophoresis

Cryo-EM Structure of the Rhodobacter sphaeroides Light-Harvesting 2 Complex at 2.1 Å

Light-harvesting 2 (LH2) antenna complexes augment the collection of solar energy in many phototrophic bacteria. Despite its frequent role as a model for such complexes, there has been no three-dimensional (3D) structure available for the LH2 from the purple phototroph Rhodobacter sphaeroides. We used cryo-electron microscopy (cryo-EM) to determine the 2.1 Å resolution structure of this LH2 antenna, which is a cylindrical assembly of nine αβ heterodimer subunits, each of which binds three bacteriochlorophyll a (BChl) molecules and one carotenoid. The high resolution of this structure reveals all of the interpigment and pigment–protein interactions that promote the assembly and energy-transfer properties of this complex. Near the cytoplasmic face of the complex there is a ring of nine BChls, which absorb maximally at 800 nm and are designated as B800; each B800 is coordinated by the N-terminal carboxymethionine of LH2-α, part of a network of interactions with nearby residues on both LH2-α and LH2-β and with the carotenoid. Nine carotenoids, which are spheroidene in the strain we analyzed, snake through the complex, traversing the membrane and interacting with a ring of 18 BChls situated toward the periplasmic side of the complex. Hydrogen bonds with C-terminal aromatic residues modify the absorption of these pigments, which are red-shifted to 850 nm. Overlaps between the macrocycles of the B850 BChls ensure rapid transfer of excitation energy around this ring of pigments, which act as the donors of energy to neighboring LH2 and reaction center light-harvesting 1 (RC–LH1) complexes.

A previously unrecognized membrane protein in the Rhodobacter sphaeroides LH1-RC photocomplex

Rhodobacter (Rba.) sphaeroides is the most widely used model organism in bacterial photosynthesis. The light-harvesting-reaction center (LH1-RC) core complex of this purple phototroph is characterized by the co-existence of monomeric and dimeric forms, the presence of the protein PufX, and approximately two carotenoids per LH1 αβ-polypeptides. Despite many efforts, structures of the Rba. sphaeroides LH1-RC have not been obtained at high resolutions. Here we report a cryo-EM structure of the monomeric LH1-RC from Rba. sphaeroides strain IL106 at 2.9 Å resolution. The LH1 complex forms a C-shaped structure composed of 14 αβ-polypeptides around the RC with a large ring opening. From the cryo-EM density map, a previously unrecognized integral membrane protein, referred to as protein-U, was identified. Protein-U has a U-shaped conformation near the LH1-ring opening and was annotated as a hypothetical protein in the Rba. sphaeroides genome. Deletion of protein-U resulted in a mutant strain that expressed a much-reduced amount of the dimeric LH1-RC, indicating an important role for protein-U in dimerization of the LH1-RC complex. PufX was located opposite protein-U on the LH1-ring opening, and both its position and conformation differed from that of previous reports of dimeric LH1-RC structures obtained at low-resolution. Twenty-six molecules of the carotenoid spheroidene arranged in two distinct configurations were resolved in the Rba. sphaeroides LH1 and were positioned within the complex to block its channels. Our findings offer an exciting new view of the core photocomplex of Rba. sphaeroides and the connections between structure and function in bacterial photocomplexes in general.

Rhodobacter (Rba.) sphaeroides is a model organism for studying bacterial photosynthesis. Here, the authors present the 2.9 Å cryo-EM structure of the monomeric light-harvesting-reaction center core complex from Rba. sphaeroides strain IL106, which revealed the position and conformation of PufX and the presence of an additional component protein-U, an integral membrane protein.

Cryo-EM structure of the monomeric Rhodobacter sphaeroides RC-LH1 core complex at 2.5 Å

Reaction centre light-harvesting 1 (RC–LH1) complexes are the essential components of bacterial photosynthesis. The membrane-intrinsic LH1 complex absorbs light and the energy migrates to an enclosed RC where a succession of electron and proton transfers conserves the energy as a quinol, which is exported to the cytochrome bc₁ complex. In some RC–LH1 variants quinols can diffuse through small pores in a fully circular, 16-subunit LH1 ring, while in others missing LH1 subunits create a gap for quinol export. We used cryogenic electron microscopy to obtain a 2.5 Å resolution structure of one such RC–LH1, a monomeric complex from Rhodobacter sphaeroides. The structure shows that the RC is partly enclosed by a 14-subunit LH1 ring in which each αβ heterodimer binds two bacteriochlorophylls and, unusually for currently reported complexes, two carotenoids rather than one. Although the extra carotenoids confer an advantage in terms of photoprotection and light harvesting, they could impede passage of quinones through small, transient pores in the LH1 ring, necessitating a mechanism to create a dedicated quinone channel. The structure shows that two transmembrane proteins play a part in stabilising an open ring structure; one of these components, the PufX polypeptide, is augmented by a hitherto undescribed protein subunit we designate as protein-Y, which lies against the transmembrane regions of the thirteenth and fourteenth LH1α polypeptides. Protein-Y prevents LH1 subunits 11–14 adjacent to the RC Q_B site from bending inwards towards the RC and, with PufX preventing complete encirclement of the RC, this pair of polypeptides ensures unhindered quinone diffusion.

Biosynthesis of the modified tetrapyrroles-the pigments of life

Modified tetrapyrroles are large macrocyclic compounds, consisting of diverse conjugation and metal chelation systems and imparting an array of colors to the biological structures that contain them. Tetrapyrroles represent some of the most complex small molecules synthesized by cells and are involved in many essential processes that are fundamental to life on Earth, including photosynthesis, respiration, and catalysis. These molecules are all derived from a common template through a series of enzyme-mediated transformations that alter the oxidation state of the macrocycle and also modify its size, its side-chain composition, and the nature of the centrally chelated metal ion. The different modified tetrapyrroles include chlorophylls, hemes, siroheme, corrins (including vitamin B12), coenzyme F430, heme d1, and bilins. After nearly a century of study, almost all of the more than 90 different enzymes that synthesize this family of compounds are now known, and expression of reconstructed operons in heterologous hosts has confirmed that most pathways are complete. Aside from the highly diverse nature of the chemical reactions catalyzed, an interesting aspect of comparative biochemistry is to see how different enzymes and even entire pathways have evolved to perform alternative chemical reactions to produce the same end products in the presence and absence of oxygen. Although there is still much to learn, our current understanding of tetrapyrrole biogenesis represents a remarkable biochemical milestone that is summarized in this review.

Engineering of B800 bacteriochlorophyll binding site specificity in the Rhodobacter sphaeroides LH2 antenna

The light-harvesting 2 complex (LH2) of the purple phototrophic bacterium Rhodobacter sphaeroides is a highly efficient, light-harvesting antenna that allows growth under a wide-range of light intensities. In order to expand the spectral range of this antenna complex, we first used a series of competition assays to measure the capacity of the non-native pigments 3-acetyl chlorophyll (Chl) a, Chl d, Chl f or bacteriochlorophyll (BChl) b to replace native BChl a in the B800 binding site of LH2. We then adjusted the B800 site and systematically assessed the binding of non-native pigments. We find that Arg_-10 of the LH2 β polypeptide plays a crucial role in binding specificity, by providing a hydrogen-bond to the 3-acetyl group of native and non-native pigments. Reconstituted LH2 complexes harbouring the series of (B)Chls were examined by transient absorption and steady-state fluorescence spectroscopies. Although slowed 10-fold to ~6 ps, energy transfer from Chl a to B850 BChl a remained highly efficient. We measured faster energy-transfer time constants for Chl d (3.5 ps) and Chl f (2.7 ps), which have red-shifted absorption maxima compared to Chl a. BChl b, red-shifted from the native BChl a, gave extremely rapid (≤0.1 ps) transfer. These results show that modified LH2 complexes, combined with engineered (B)Chl biosynthesis pathways in vivo, have potential for retaining high efficiency whilst acquiring increased spectral range.

Selective Removal of B800 Bacteriochlorophyll a from Light-Harvesting Complex 2 of the Purple Photosynthetic Bacterium Phaeospirillum molischianum

The selective removal of B800 bacteriochlorophyll (BChl) a from light-harvesting complex 2 (LH2) in purple photosynthetic bacteria is a clue about elucidation of the mechanism for the transfer of energy from these pigments to B850 BChl a and their roles in the LH2 protein structure. We demonstrated that the kinetics of the removal of B800 BChl a from two representative LH2 proteins derived from Phaeospirillum molischianum and Rhodoblastus acidophilus differed significantly, in contrast to the calculated binding enthalpy. These results may be interpreted as changes in the local structure near B800 BChl a with respect to the geometries of the original crystal structures upon removal of B800 BChl a. Despite the difficulty of removing B800 BChl a from molischianum-LH2, we prepared the molischianum-LH2 protein lacking B800 BChl a by combination of two detergents, n-dodecyl β-d-maltoside and n-octyl β-d-glucoside, under acidic conditions. Spectral and atomic force microscopy analyses indicated that the absence of B800 BChl a had little effect on the local structure in the vicinity of B850 BChl a and the circular arrangement in this protein. These results suggest that the hydrophobic domain near B850 BChl a is rigid and plays a major role in the structural formation of molischianum-LH2.

Engineering of a calcium-ion binding site into the RC-LH1-PufX complex of Rhodobacter sphaeroides to enable ion-dependent spectral red-shifting

The reaction centre-light harvesting 1 (RC-LH1) complex of Thermochromatium (Tch.) tepidum has a unique calcium-ion binding site that enhances thermal stability and red-shifts the absorption of LH1 from 880nm to 915nm in the presence of calcium-ions. The LH1 antenna of mesophilic species of phototrophic bacteria such as Rhodobacter (Rba.) sphaeroides does not possess such properties. We have engineered calcium-ion binding into the LH1 antenna of Rba. sphaeroides by progressively modifying the native LH1 polypeptides with sequences from Tch. tepidum. We show that acquisition of the C-terminal domains from LH1 α and β of Tch. tepidum is sufficient to activate calcium-ion binding and the extent of red-shifting increases with the proportion of Tch. tepidum sequence incorporated. However, full exchange of the LH1 polypeptides with those of Tch. tepidum results in misassembled core complexes. Isolated α and β polypeptides from our most successful mutant were reconstituted in vitro with BChl a to form an LH1-type complex, which was stabilised 3-fold by calcium-ions. Additionally, carotenoid specificity was changed from spheroidene found in Rba. sphaeroides to spirilloxanthin found in Tch. tepidum, with the latter enhancing in vitro formation of LH1. These data show that the C-terminal LH1 α/β domains of Tch. tepidum behave autonomously, and are able to transmit calcium-ion induced conformational changes to BChls bound to the rest of a foreign antenna complex. Thus, elements of foreign antenna complexes, such as calcium-ion binding and blue/red switching of absorption, can be ported into Rhodobacter sphaeroides using careful design processes.

The PufX quinone channel enables the light-harvesting 1 antenna to bind more carotenoids for light collection and photoprotection

Photosynthesis in some phototrophic bacteria requires the PufX component of the reaction centre–light‐harvesting 1–PufX (RC‐LH1‐PufX) complex, which creates a pore for quinone/quinol (Q/QH₂) exchange across the LH1 barrier surrounding the RC. However, photosynthetic bacteria such as Thermochromatium (T.) tepidum do not require PufX because there are fewer carotenoid binding sites, which creates multiple pores in the LH1 ring for Q/QH₂ exchange. We show that an αTrp_‐24→Phe alteration of the Rhodobacter (Rba.) sphaeroides LH1 antenna impairs carotenoid binding and allows photosynthetic growth in the absence of PufX. We propose that acquisition of PufX and confining Q/QH₂ traffic to a pore adjacent to the RC Q_B site is an evolutionary upgrade that allows increased LH1 carotenoid content for enhanced light absorption and photoprotection.

Development and dynamics of the photosynthetic apparatus in purple phototrophic bacteria

The purple bacterium Rhodobacter sphaeroides provides a useful model system for studies of the assembly and dynamics of bacterial photosynthetic membranes. For the nascent developing membrane, proteomic analyses showed an ~2-fold enrichment in general membrane assembly factors, compared to chromatophores. When the protonophore carbonyl-cyanide m-chlorophenyl-hydrazone (CCCP) was added to an ICM inducing culture, an ~2-fold elevation in spectral counts vs. the control was seen for the SecA translocation ATPase, the preprotein translocase SecY, SecD and SecF insertion components, and chaperonins DnaJ and DnaK, which act early in the assembly process. It is suggested that these factors accumulated with their nascent polypeptides, as putative assembly intermediates in a functionally arrested state. Since in Synechocystis PCC 6803, a link has been established between Chl delivery involving the high-light HilD protein and the SecY/YidC-requiring cotranslational insertion of nascent polypeptides, such a connection between BChl biosynthesis and insertion and folding of nascent Rba. sphaeroides BChl binding proteins is likely to also occur. AFM imaging studies of the formation of the reaction center (RC)-light harvesting 1 (LH1) complex suggested a cooperative assembly mechanism in which, following the association between the RC template and the initial LH1 unit, addition of successive LH1 units to the RC drives the assembly process to completion. Alterations in membrane dynamics as the developing membrane becomes filled with LH2-rings were assessed by fluorescence induction/relaxation kinetics, which showed a slowing in RC electron transfer rate thought to mainly reflect alterations in donor side electron transfer. This was attributed to an increased distance for electron flow in cytochrome c2 between the RC and cytochrome bc1 complexes, as suggested in the current structural models. This article is part of a Special Issue entitled Organization and dynamics of bioenergetic systems in bacteria, edited by Prof Conrad Mullineaux.

Assembly of functional photosystem complexes in Rhodobacter sphaeroides incorporating carotenoids from the spirilloxanthin pathway

Carotenoids protect the photosynthetic apparatus against harmful radicals arising from the presence of both light and oxygen. They also act as accessory pigments for harvesting solar energy, and are required for stable assembly of many light-harvesting complexes. In the phototrophic bacterium Rhodobacter (Rba.) sphaeroides phytoene desaturase (CrtI) catalyses three sequential desaturations of the colourless carotenoid phytoene, extending the number of conjugated carbon–carbon double bonds, N, from three to nine and producing the yellow carotenoid neurosporene; subsequent modifications produce the yellow/red carotenoids spheroidene/spheroidenone (N = 10/11). Genomic crtI replacements were used to swap the native three-step Rba. sphaeroides CrtI for the four-step Pantoea agglomerans enzyme, which re-routed carotenoid biosynthesis and culminated in the production of 2,2′-diketo-spirilloxanthin under semi-aerobic conditions. The new carotenoid pathway was elucidated using a combination of HPLC and mass spectrometry. Premature termination of this new pathway by inactivating crtC or crtD produced strains with lycopene or rhodopin as major carotenoids. All of the spirilloxanthin series carotenoids are accepted by the assembly pathways for LH2 and RC–LH1–PufX complexes. The efficiency of carotenoid-to-bacteriochlorophyll energy transfer for 2,2′-diketo-spirilloxanthin (15 conjugated C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]>C bonds; N = 15) in LH2 complexes is low, at 35%. High energy transfer efficiencies were obtained for neurosporene (N = 9; 94%), spheroidene (N = 10; 96%) and spheroidenone (N = 11; 95%), whereas intermediate values were measured for lycopene (N = 11; 64%), rhodopin (N = 11; 62%) and spirilloxanthin (N = 13; 39%). The variety and stability of these novel Rba. sphaeroides antenna complexes make them useful experimental models for investigating the energy transfer dynamics of carotenoids in bacterial photosynthesis.

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The spirilloxanthin biosynthetic pathway has been constructed in Rba. sphaeroides.

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The new carotenoids are accepted by the photosystem assembly pathways.

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These pigments are efficiently integrated into LH2 and RC–LH1–PufX complexes.

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Carotenoid–BChl energy transfer drops with the number of conjugated CC bonds (N).

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The lowest efficiency, 35%, is for the N = 15 carotenoid 2,2′ diketospirilloxanthin.

Carotenoid charge transfer states and their role in energy transfer processes in LH1-RC complexes from aerobic anoxygenic phototrophs

Light-harvesting complexes ensure necessary flow of excitation energy into photosynthetic reaction centers. In the present work, transient absorption measurements were performed on LH1-RC complexes isolated from two aerobic anoxygenic phototrophs (AAPs), Roseobacter sp. COL2P containing the carotenoid spheroidenone, and Erythrobacter sp. NAP1 which contains the carotenoids zeaxanthin and bacteriorubixanthinal. We show that the spectroscopic data from the LH1-RC complex of Roseobacter sp. COL2P are very similar to those previously reported for Rhodobacter sphaeroides, including the transient absorption spectrum originating from the intramolecular charge-transfer (ICT) state of spheroidenone. Although the ICT state is also populated in LH1-RC complexes of Erythrobacter sp. NAP1, its appearance is probably related to the polarity of the bacteriorubixanthinal environment rather than to the specific configuration of the carotenoid, which we hypothesize is responsible for populating the ICT state of spheroidenone in LH1-RC of Roseobacter sp. COL2P. The population of the ICT state enables efficient S1/ICT-to-bacteriochlorophyll (BChl) energy transfer which would otherwise be largely inhibited for spheroidenone and bacteriorubixanthinal due to their low energy S1 states. In addition, the triplet states of these carotenoids appear well-tuned for efficient quenching of singlet oxygen or BChl-a triplets, which is of vital importance for oxygen-dependent organisms such as AAPs.

Differential assembly of polypeptides of the light-harvesting 2 complex encoded by distinct operons during acclimation of Rhodobacter sphaeroides to low light intensity

In order to obtain an improved understanding of the assembly of the bacterial photosynthetic apparatus, we have conducted a proteomic analysis of pigment-protein complexes isolated from the purple bacterium Rhodobacter sphaeroides undergoing acclimation to reduced incident light intensity. Photoheterotrophically growing cells were shifted from 1,100 to 100 W/m(2) and intracytoplasmic membrane (ICM) vesicles isolated over 24-h were subjected to clear native polyacrylamide gel electrophoresis. Bands containing the LH2 and reaction center (RC)-LH1 complexes were excised and subjected to in-gel trypsin digestion followed by liquid chromatography (LC)-mass spectroscopy (MS)/MS. The results revealed that the LH2 band contained distinct levels of the LH2-α and -β polypeptides encoded by the two puc operons. Polypeptide subunits encoded by the puc2AB operon predominated under high light and in the early stages of acclimation to low light, while after 24 h, the puc1BAC components were most abundant. Surprisingly, the Puc2A polypeptide containing a 251 residue C-terminal extension not present in Puc1A, was a protein of major abundance. A predominance of Puc2A components in the LH2 complex formed at high light intensity is followed by a >2.5-fold enrichment in Puc1B levels between 3 and 24 h of acclimation, accompanied by a nearly twofold decrease in Puc2A levels. This indicates that the puc1BAC operon is under more stringent light control, thought to reflect differences in the puc1 upstream regulatory region. In contrast, elevated levels of Puc2 polypeptides were seen 48 h after the gratuitous induction of ICM formation at low aeration in the dark, while after 24 h of acclimation to low light, an absence of alterations in Puc polypeptide distributions was observed in the upper LH2-enriched gel band, despite an approximate twofold increase in overall LH2 levels. This is consistent with the origin of this band from a pool of LH2 laid down early in development that is distinct from subsequently assembled LH2-only domains, forming the LH2 gel band.

Carotenoids are essential for normal levels of dimerisation of the RC-LH1-PufX core complex of Rhodobacter sphaeroides: characterisation of R-26 as a crtB (phytoene synthase) mutant

Carotenoids play important roles in photosynthesis where they are involved in light-harvesting, in photo-protection and in the assembly and structural stability of light-harvesting and reaction centre complexes. In order to examine the effects of carotenoids on the oligomeric state of the reaction centre-light-harvesting 1 -PufX (RC-LH1-PufX) core complex of Rhodobacter sphaeroides two carotenoid-less mutants, TC70 and R-26, were studied. Detergent fractionation showed that in the absence of carotenoids LH2 complexes do not assemble, as expected, but also that core complexes are predominantly found as monomers, although levels of the PufX polypeptide appeared to be unaffected. Analysis of R-26 membranes by electron microscopy and atomic force microscopy reveals arrays of hexagonally packed monomeric RC-LH1-PufX complexes. Transfer of the crtB gene encoding phytoene synthase to TC70 and R-26 restores the normal synthesis of carotenoids demonstrating that the R-26 mutant of Rba. sphaeroides harbours a mutation in crtB, among its other defects. The transconjugant TC70 and R-26 strains containing crtB had regained their ability to assemble wild-type levels of dimeric RC-LH1-PufX core complexes and normal energy transfer pathways were restored, demonstrating that carotenoids are essential for the normal assembly and function of both the LH2 and RC-LH1-PufX complexes in this bacterial photosystem.

Differential assembly of polypeptides of the light-harvesting 2 complex encoded by distinct operons during acclimation of Rhodobacter sphaeroides to low light intensity

In order to obtain an improved understanding of the assembly of the bacterial photosynthetic apparatus, we have conducted a proteomic analysis of pigment-protein complexes isolated from the purple bacterium Rhodobacter sphaeroides undergoing acclimation to reduced incident light intensity. Photoheterotrophically growing cells were shifted from 1,100 to 100 W/m(2) and intracytoplasmic membrane (ICM) vesicles isolated over 24-h were subjected to clear native polyacrylamide gel electrophoresis. Bands containing the LH2 and reaction center (RC)-LH1 complexes were excised and subjected to in-gel trypsin digestion followed by liquid chromatography (LC)-mass spectroscopy (MS)/MS. The results revealed that the LH2 band contained distinct levels of the LH2-α and -β polypeptides encoded by the two puc operons. Polypeptide subunits encoded by the puc2AB operon predominated under high light and in the early stages of acclimation to low light, while after 24 h, the puc1BAC components were most abundant. Surprisingly, the Puc2A polypeptide containing a 251 residue C-terminal extension not present in Puc1A, was a protein of major abundance. A predominance of Puc2A components in the LH2 complex formed at high light intensity is followed by a >2.5-fold enrichment in Puc1B levels between 3 and 24 h of acclimation, accompanied by a nearly twofold decrease in Puc2A levels. This indicates that the puc1BAC operon is under more stringent light control, thought to reflect differences in the puc1 upstream regulatory region. In contrast, elevated levels of Puc2 polypeptides were seen 48 h after the gratuitous induction of ICM formation at low aeration in the dark, while after 24 h of acclimation to low light, an absence of alterations in Puc polypeptide distributions was observed in the upper LH2-enriched gel band, despite an approximate twofold increase in overall LH2 levels. This is consistent with the origin of this band from a pool of LH2 laid down early in development that is distinct from subsequently assembled LH2-only domains, forming the LH2 gel band.

Proteomic analysis of the developing intracytoplasmic membrane in Rhodobacter sphaeroides during adaptation to low light intensity

Although the primary photochemical events in the facultative photoheterotrophic purple bacterium Rhodobacter sphaeroides are now well understood, comparatively little is known about how their photosynthetic apparatus is assembled. Here we present a proteomic analysis of the intracytoplasmic membrane (ICM) assembly process during adaptation to lowered light intensity, in which the size of the photosynthetic units is greatly expanded by addition of the light-harvesting 2 (LH2) peripheral antenna complex. When the isolated ICM-derived chromatophore vesicles were subjected to clear native gel electrophoresis (CNE), four pigmented bands appeared; the top and bottom bands contained the reaction center - light-harvesting 1 (RC-LH1) core complex and LH2 peripheral antenna, respectively, while the two bands of intermediate migration contained associations of the LH2 and core complexes. Proteomic analysis revealed a large array of other proteins associated with the CNE gel bands - in particular, several F(1)F(O)-ATP synthase subunits gave unexpectedly high spectral counts, given the inability to detect this coupling factor, as well as the more abundant cytochrome bc (1) complex, by atomic force microscopy. Significant levels of general membrane assembly factors were also found, as well as numerous proteins of unknown function including high counts for RSP6124 that were correlated with LH2 levels. When combined with further AFM and spectroscopic studies, these proteomic approaches are expected to provide a much-improved understanding of the overall assembly process.

Triazine herbicide resistance in the photosynthetic bacterium Rhodopseudomonas sphaeroides

The photoaffinity herbicide azidoatrazine (2-azido-4-ethylamino-6-isopropylamino-s-triazine) selectively labels the L subunit of the reaction center of the photosynthetic bacterium Rhodopseudomonas sphaeroides. Herbicide-resistant mutants retain the L subunit and have altered binding properties for methylthio- and chloro-substituted triazines as well as altered equilibrium constants for electron transfer between primary and secondary electron acceptors. We suggest that a subtle alteration in the L subunit is responsible for herbicide resistance and that the L subunit is the functional analog of the 32-kDa Q(B) protein of chloroplast membranes.

B850 pigment-protein complex of Rhodopseudomonas sphaeroides: Extinction coefficients, circular dichroism, and the reversible binding of bacteriochlorophyll

Chromatophores of Rhodopseudomonas sphaeroides yield the antenna complex B850 in either of two states, depending on the method of isolation. Methods using dodecyl (= lauryl) dimethylamine oxide yield B850 with an absorption spectrum like that in vivo: the bands at 800 and 850 nm, due to the bacteriochlorophyll (Bchl) components Bchl-800 and Bchl-850, are in ratio A(800)/A(850) = 0.65 +/- 0.05. When B850 is isolated by methods using dodecyl sulfate, the Bchl-800 is attenuated or absent. Bchl assays of these materials and of the isolated antenna complex B875 yielded the following extinction coefficients, +/-SD, on the basis of the molarity of Bchl: For B875, epsilon(875) = 126 +/- 8 mM(-1) cm(-1). For B850 in the normal (high-Bchl-800) state, epsilon(850) = 132 +/- 10 mM(-1) cm(-1). For the individual components of Bchl in B850, epsilon(850) of Bchl-850 = 184 +/- 13 mM(-1) cm(-1) and epsilon(800) of Bchl-800 = 213 +/- 28 mM(-1) cm(-1). With these coefficients the molecular ratio of Bchl-850 to Bchl-800 equals 1.8 +/- 0.4 for B850 in the high-Bchl-800 state. Starting with B850 depleted of Bchl-800, the addition of dodecyldimethylamine oxide restored the 800-nm absorption band. The 850-nm band became shifted toward the blue, narrowed, and slightly attenuated, and its associated circular dichroism became 20% more intense. Free Bchl added with dodecyldimethylamine oxide accelerated the restoration of Bchl-800 and retarded the attenuation of Bchl-850. We conclude that free Bchl can interact reversibly with a binding site for Bchl-800 in the B850 complex, with dodecyl sulfate favoring dissociation and dodecyldimethylamine oxide promoting association. Thus the reversible dissociation of a native chlorophyll-protein complex has now been demonstrated.

Antenna mixing in photosynthetic membranes from Phaeospirillum molischianum

We have investigated the adaptation of the light-harvesting system of the photosynthetic bacterium Phaeospirillum molischianum (DSM120) to very low light conditions. This strain is able to respond to changing light conditions by differentially modulating the expression of a family of puc operons that encode for peripheral light-harvesting complex (LH2) polypeptides. This modulation can result in a complete shift between the production of LH2 complexes absorbing maximally near 850 nm to those absorbing near 820 nm. In contradiction to prevailing wisdom, analysis of the LH2 rings found in the photosynthetic membranes during light adaptation are shown to have intermediate spectral and electrostatic properties. By chemical cross-linking and mass-spectrometry we show that individual LH2 rings and subunits can contain a mixture of polypeptides derived from the different operons. These observations show that polypeptide synthesis and insertion into the membrane are not strongly coupled to LH2 assembly. We show that the light-harvesting complexes resulting from this mixing could be important in maintaining photosynthetic efficiency during adaptation.

Atomic force microscopy of the bacterial photosynthetic apparatus: plain pictures of an elaborate machinery

Photosynthesis both in the past and present provides the vast majority of the energy used on the planet. The purple photosynthetic bacteria are a group of organisms that are able to perform photosynthesis using a particularly simple system that has been much studied. The main molecular constituents required for photosynthesis in these organisms are a small number of transmembrane pigment-protein complexes. These are able to function together with a high quantum efficiency (about 95%) to convert light energy into chemical potential energy. While the structure of the various proteins have been solved for several years, direct studies of the supramolecular assembly of these complexes in native membranes needed maturity of the atomic force microscope (AFM). Here, we review the novel findings and the direct conclusions that could be drawn from high-resolution AFM analysis of photosynthetic membranes. These conclusions rely on the possibility that the AFM brings of obtaining molecular resolution images of large membrane areas and thereby bridging the resolution gap between atomic structures and cellular ultrastructure.

Interaction of actin and the chloroplast protein import apparatus

Actin filaments are major components of the cytoskeleton and play numerous essential roles, including chloroplast positioning and plastid stromule movement, in plant cells. Actin is present in pea chloroplast envelope membrane preparations and is localized at the surface of the chloroplasts, as shown by agglutination of intact isolated chloroplasts by antibodies to actin. To identify chloroplast envelope proteins involved in actin binding, we have carried out actin co-immunoprecipitation and co-sedimentation experiments on detergent-solubilized pea chloroplast envelope membranes. Proteins co-immunoprecipitated with actin were identified by mass spectrometry and by Western blotting and included the Toc159, Toc75, Toc34, and Tic110 components of the TOC-TIC protein import apparatus. A direct interaction of actin with Escherichia coli-expressed Toc159, but not Toc33, was shown by co-sedimentation experiments, suggesting that Toc159 is the component of the TOC complex that interacts with actin on the cytosolic side of the outer envelope membrane. The physiological significance of this interaction is unknown, but it may play a role in the import of nuclear-encoded photosynthesis proteins.

Identifying components of the hair-cell interactome involved in cochlear amplification

BACKGROUND

Although outer hair cells (OHCs) play a key role in cochlear amplification, it is not fully understood how they amplify sound signals by more than 100 fold. Two competing or possibly complementary mechanisms, stereocilia-based and somatic electromotility-based amplification, have been considered. Lacking knowledge about the exceptionally rich protein networks in the OHC plasma membrane, as well as related protein-protein interactions, limits our understanding of cochlear function. Therefore, we focused on finding protein partners for two important membrane proteins: Cadherin 23 (cdh23) and prestin. Cdh23 is one of the tip-link proteins involved in transducer function, a key component of mechanoelectrical transduction and stereocilia-based amplification. Prestin is a basolateral membrane protein responsible for OHC somatic electromotility.

RESULTS

Using the membrane-based yeast two-hybrid system to screen a newly built cDNA library made predominantly from OHCs, we identified two completely different groups of potential protein partners using prestin and cdh23 as bait. These include both membrane bound and cytoplasmic proteins with 12 being de novo gene products with unknown function(s). In addition, some of these genes are closely associated with deafness loci, implying a potentially important role in hearing. The most abundant prey for prestin (38%) is composed of a group of proteins involved in electron transport, which may play a role in OHC survival. The most abundant group of cdh23 prey (55%) contains calcium-binding domains. Since calcium performs an important role in hair cell mechanoelectrical transduction and amplification, understanding the interactions between cdh23 and calcium-binding proteins should increase our knowledge of hair cell function at the molecular level.

CONCLUSION

The results of this study shed light on some protein networks in cochlear hair cells. Not only was a group of de novo genes closely associated with known deafness loci identified, but the data also indicate that the hair cell tip link interacts directly with calcium binding proteins. The OHC motor protein, prestin, also appears to be associated with electron transport proteins. These unanticipated results open potentially fruitful lines of investigation into the molecular basis of cochlear amplification.

Analysis of the oligomeric structure of the motor protein prestin

Prestin, a member of the solute carrier family 26, is expressed in the basolateral membrane of outer hair cells. This protein provides the molecular basis for outer hair cell somatic electromotility, which is crucial for the frequency selectivity and sensitivity of mammalian hearing. It has long been known that there are abundantly expressed approximately 11-nM protein particles present in the basolateral membrane. These particles were hypothesized to be the motor proteins that drive electromotility. Because the calculated size of a prestin monomer is too small to form an approximately 11-nM particle, the possibility of prestin oligomerization was examined. We investigated possible quaternary structures of prestin by lithium dodecyl sulfate-PAGE, perfluoro-octanoate-PAGE, a membrane-based yeast two-hybrid system, and chemical cross-linking experiments. Prestin, obtained from different host or native cells, is resistant to dissociation by lithium dodecyl sulfate and behaves as a stable oligomer on lithium dodecyl sulfate-PAGE. In the membrane-based yeast two-hybrid system, homo-oligomeric interactions between prestin-bait/prestin-prey suggest that prestin molecules can associate with each other. Chemical cross-linking experiments, perfluoro-octanoate-PAGE/Western blot, and affinity purification experiments all indicate that prestin exists as a higher order oligomer, such as a tetramer, in prestin-expressing yeast, mammalian cell lines and native outer hair cells. Our data from experiments using hydrophobic and hydrophilic reducing reagents suggest that the prestin dimer is connected by a disulfide bond embedded in the prestin hydrophobic core. This stable dimer may act as the building block for producing the higher order oligomers that form the approximately 11-nM particles in the outer hair cell's basolateral membrane.

Characterization of a highly purified, fully active, crystallizable RC-LH1-PufX core complex from Rhodobacter sphaeroides

Photosynthetic complexes in bacteria absorb light and undergo photochemistry with high quantum efficiency. We describe the isolation of a highly purified, active, reaction center-light-harvesting 1-PufX complex (RC-LH1-PufX core complex) from a strain of the photosynthetic bacterium, Rhodobacter sphaeroides, which lacks the light-harvesting 2 (LH2) and contains a 6 histidine tag on the H subunit of the RC. The complex was solubilized with diheptanoyl-sn-glycero-3-phosphocholine (DHPC), and purified by Ni-affinity, size-exclusion and ion-exchange chromatography in dodecyl maltoside. SDS-PAGE analysis shows the complex to be highly purified. The quantum efficiency was determined by measuring the charge separation (DQA --> D+QA -) in the RC as a function of light intensity. The RC-LH1-PufX complex had a quantum efficiency of 0.95 +/- 0.05, indicating full activity. The stoichiometry of LH1 subunits per RC was determined by two independent methods: (i) solvent extraction and absorbance spectroscopy of bacteriochlorophyll, and (ii) density scanning of the SDS-PAGE bands. The average stoichiometry from the two measurements was 13.3 +/- 0.9 LH1/RC. The presence of PufX was observed in SDS-PAGE gels at a stoichiometry of 1.1 +/- 0.1/RC. Crystals of the core complex have been obtained which diffract X-rays to 12 A. A preliminary analysis of the space group and unit cell analysis indicated a P1 space group with unit cell dimensions of a = 76.3 A, b = 137.2 A, c = 137.5 A; alpha = 60.0 degrees , beta = 89.95 degrees , gamma =90.02 degrees .

The assembly and organisation of photosynthetic membranes in Rhodobacter sphaeroides

Recent AFM data demonstrate that mature photosynthetic membranes of R. sphaeroides are composed of rows of dimeric RC-LH1-PufX complexes with some LH2 complexes 'sandwiched' between these rows of core complexes, and others in discrete LH2-only domains which might form the light-responsive complement of the LH2 antenna. The present work applies membrane fractionation, radiolabelling and LDS-PAGE techniques to investigate the response of R. sphaeroides to lowered light intensity. The kinetics underlying this adaptation to low light conditions were revealed by radiolabelling with the bacteriochlorophyll (bchl) biosynthetic precursor, delta-aminolevulinate, which allowed us to measure only the bchls synthesised after the light intensity shift. We show that (1) the increase in LH2 antenna size is mainly restricted to the mature ICM membrane fraction, and the antenna composition of the precursor upper pigmented band (UPB) membrane remains constant, (2) the precursor UPB membrane is enriched in bchl synthase, the terminal enzyme of the bchl biosynthetic pathway, and (3) the LH2 and the complexes of intermediate migration in LDS-PAGE exhibit completely different labelling kinetics. Thus, new photosynthetic complexes, mainly LH2, are synthesised and assembled at the membrane initiation UPB sites, where the LH2 rings pack between the rows of dimeric cores fostering new LH2-LH1 interactions. Mature membranes also assemble new LH2 rings, but in this case the 'sandwich' regions between the rows of core dimers are already fully occupied and the bulk antenna pool is the favoured location for these new LH2 complexes.

Phosphate deprivation induces transfer of DGDG galactolipid from chloroplast to mitochondria

In many soils plants have to grow in a shortage of phosphate, leading to development of phosphate-saving mechanisms. At the cellular level, these mechanisms include conversion of phospholipids into glycolipids, mainly digalactosyldiacylglycerol (DGDG). The lipid changes are not restricted to plastid membranes where DGDG is synthesized and resides under normal conditions. In plant cells deprived of phosphate, mitochondria contain a high concentration of DGDG, whereas mitochondria have no glycolipids in control cells. Mitochondria do not synthesize this pool of DGDG, which structure is shown to be characteristic of a DGD type enzyme present in plastid envelope. The transfer of DGDG between plastid and mitochondria is investigated and detected between mitochondria-closely associated envelope vesicles and mitochondria. This transfer does not apparently involve the endomembrane system and would rather be dependent upon contacts between plastids and mitochondria. Contacts sites are favored at early stages of phosphate deprivation when DGDG cell content is just starting to respond to phosphate deprivation.

Purple Bacterial Light-harvesting Complexes: From Dreams to Structures

This paper describes the main stages involved in the research efforts designed to try and understand the structure and function of purple bacterial antenna complexes. Wherever possible the work has been illustrated by pictures of the major people who carried it out.

Isolation, size estimates, and spectral heterogeneity of an oligomeric series of light-harvesting 1 complexes from Rhodobacter sphaeroides

A series of light-harvesting 1 (LH1) complexes was isolated by lithium dodecyl sulfate-polyacrylamide gel electrophoresis at 4 degrees C from Rhodobacter sphaeroides M21, which lacks the peripheral light-harvesting 2 (LH2) complex. This ladder of LH1 bands was also demonstrated in the wild type, partially superimposed upon a smaller number of LH2 complexes. An assessment of electrophoretic mobility vs acrylamide concentration, in which the reaction center LM particle and annular LH1 and LH2 complexes were used as standards of known structure, indicated that the LH1 gel bands 2 to 10 represent regular oligomers of an alpha beta heterodimeric unit, that vary in size from (alpha beta)(2-3) to (alpha beta)(10-11). The isolated LH1 complexes exhibited oligomeric state dependent optical properties, characterized by red shifts in near-IR absorption and emission maxima at 77 K of approximately 6 nm as aggregate sizes increased from approximately 3 to 7-8 alpha beta-heterodimers, accompanied by shifts in highly polarized fluorescence from the blue to the red side of the absorption band. This has been explained by the oligomerization of heterodimers to form a curvilinear array of excitonically coupled chromophores, with the anisotropic long-wavelength component, designated originally as B896, corresponding to low energy excitonic transitions arising from interactions within inhomogeneous BChl clusters [Westerhuis et al. (1999) J. Phys. Chem. B 103, 7733-7742]. Differences in electrophoretic profiles of LH1 bands between strains M21 and M2192, an LH1-only strain that also lacks PufX, further suggested that the more rapidly migrating bands represent arced fragments of the curvilinear array of LH1 complexes thought to exist as a large closed circular structure only in the latter strain. The electrophoretic banding pattern also indicated that the LH1 complex may be located at the peripheries of dimeric intramembrane particle arrays seen in freeze-fracture replicas of tubular M21 membranes; the possible role for the PufX protein in the assembly of these structures is discussed.

Structure of the Rhodobacter sphaeroides light-harvesting 1 beta subunit in detergent micelles

The light harvesting 1 antenna (LH1) complex from Rhodobacter sphaeroides funnels excitation energy to the photosynthetic reaction center. Our ultimate goal is to build up the structure of LH1 from structures of its individual subunits, much as the antenna can self-assemble from its components in membrane-mimicking detergent micelles. The beta subunit adopts a nativelike conformation in Zwittergent 3:12 micelles as demonstrated by its ability to take the first step of assembly, binding BChl a. Multidimensional NMR spectroscopy shows that the beta subunit folds as a helix((L12-S25))-hinge((G26-W28))-helix((L29-W44)) structure with the helical regions for the 10 lowest-energy structures having backbone rmsds of 0.26 and 0.24 A, respectively. Mn(2+) relaxation data and the protein-detergent NOE pattern show the C-terminal helix embedded in the micelle and the N-terminal helix lying along the detergent micelle surface with a 60 degrees angle between their long axes. (15)N relaxation data for residues L12-W44 are typical of a well-ordered protein with a correlation time of 8.25 +/- 2.1 ns. The presence of the hinge region placing the N-terminal helix along the membrane surface may be the structural feature responsible for the functional differences observed between the LH1 and LH2 beta subunits.

Membrane biogenesis in anoxygenic photosynthetic prokaryotes

Following the discovery of photosynthetic bacteria in the nineteenth century, technical developments of the 1950s led to their use in membrane biogenesis studies. These investigations had their origins in the isolation of subcellular particles designated as 'chromatophores' by Roger Stanier and colleagues, which were shown to be photosynthetically competent by Albert Frenkel, and to originate from the intracytoplasmic membrane (ICM) continuum observed in electron micrographs. These ultrastrucutral studies by the G. Drews group, Germaine Cohen-Bazire and others also suggested that the ICM originates by invagination of the cytoplasmic membrane, as later established in the biochemical and biophysical work of the R. Niederman and Drews groups. Through a combination of genetic approaches, first introduced in the early 1980s by Barry Marrs, and the atomic resolution structures determined for light-harvesting antennae and reaction centers, a detailed understanding is emerging of mechanisms regulating their levels in the membrane and the roles played by specific protein domains and additional factors in their assembly and supramolecular organization. Prospects for additional progress during the twenty-first century include further elucidation of molecular aspects of the assembly process and the application of newer spectroscopic probes to photosynthetic unit formation.

Generation of triplet and cation-radical bacteriochlorophyll a in carotenoidless LH1 and LH2 antenna complexes from Rhodobacter sphaeroides

The LH1 antenna complex and a native form of the LH2 complex were isolated from the carotenoidless R26 and R26.1 mutants of Rhodobacter sphaeroides by the use of a new detergent, sucrose monocholate. One-color, pump-and-probe transient Raman spectroscopy of these complexes using 351 nm, approximately 50 ps pulses showed the generation of the triplet state of bacteriochlorophyll a (BChl a), whereas measurements using 355 nm, approximately 12 ns pulses showed the generation of BChl a cation radical. Subpicosecond to nanosecond time-resolved absorption spectroscopy using 388 nm, 200 fs pulses for excitation showed rapid (<1 ps) generation of the triplet state and fast decay (<10 ps) of the singlet state of BChl a. Microsecond absorption spectroscopy confirmed the generation of BChl a cation radical. EPR spectroscopy using 532 nm, approximately 5 ns pulses for excitation established the generation of BChl a cation radical. The EPR line width suggested that the unpaired electron is shared by two BChl a molecules. In LH1, the yield of BChl a cation radical per complex was estimated to be about 80% of that in the reaction center, and in LH2 about 50%. Thus, rapid generation of the triplet state, and its subsequent transformation into the cation-radical state of BChl a have been shown to be intrinsic properties of B870 and B850 BChl a assembly in the carotenoidless LH1 and LH2 antenna complexes. In the case of the carotenoid-containing LH2 complex, the triplet states of BChl a and carotenoid (spheroidene) were generated immediately after excitation, but the triplet-state BChl a was quenched efficiently by the carotenoid so that no BChl a cation radical was generated. Thus, the photoprotective function of the carotenoid in this antenna complex is shown.

The LH1-RC core complex of Rhodobacter sphaeroides: interaction between components, time-dependent assembly, and topology of the PufX protein

Mutant strains of the photosynthetic bacterium Rhodobacter sphaeroides, lacking either LH1, the RC or PufX, were analysed by mild detergent fractionation of the cores. This reveals a hierarchy of binding of PufX in the order RC:LH1 > LH1 > RC. The assembly of photosynthetic membranes was studied by switching highly aerated cells to conditions of low aeration in the dark. The RC-H subunit appears before other components, followed by the pufBALMX then pufBA transcripts. Synthesis of the PufX polypeptide precedes that of LH1alpha and beta, which suggests that PufX associates with a limited amount of LH1alpha, beta and the RC, and prior to the encirclement of the RC by the rest of the LH1 complex. The topology of PufX within the intracytoplasmic membrane was determined by proteolytic treatment of membrane vesicles followed by protein sequencing; PufX is N-terminally exposed on the cytoplasmic surface of the photosynthetic membrane.

Reconstitution of core light-harvesting complexes of photosynthetic bacteria using chemically synthesized polypeptides. 1. Minimal requirements for subunit formation

Described are the chemical synthesis, isolation and characterization of each of three polypeptides whose amino acid sequences reproduce portions of the amino acid sequence of the beta-polypeptides of the core light-harvesting complex (LH1) of Rhodobacter sphaeroides or Rhodospirillum rubrum. The native beta-polypeptides of LH1 of these organisms contain 48 and 54 amino acids, respectively. The smallest synthetic polypeptide had an amino acid sequence identical to that of the last 16 amino acids of the beta-polypeptide of Rb. sphaeroides (sph beta 16) but failed to form either a subunit- or LH1-type complex under reconstitution conditions. Also, this polypeptide, lengthened on the N terminus by adding the sequence Lys-Ile-Ser-Lys to enhance solubility, failed to form a subunit- or LH1-type complex. In contrast, polypeptides containing either the 31 amino acids at the C terminus of the beta-polypeptide of Rb. sphaeroides (sph beta 31) or the equivalent 31 amino acids of the beta-polypeptide of Rs. rubrum (rr beta 31) were fully competent in forming a subunit-type complex and exhibited association constants for complex formation comparable to or exceeding those of the native beta-polypeptides. The absorption and CD spectra of these subunit-type complexes were nearly identical to those of subunit complexes formed with native beta-polypeptides. It may be concluded that all structural features required to make the subunit complex are present in the well-defined, chemically synthesized polypeptides. Neither polypeptide appeared to interact with the native alpha-polypeptides to form a LH1-type complex. However, sph beta 31 formed a LH1-type complex absorbing at 849 nm without an alpha-polypeptide. Although chemical syntheses of polypeptides of this size are common, the purification of membrane-spanning segments is much more challenging because the polypeptides lack solubility in water. The chemical syntheses reported here represent the first such syntheses of membrane-spanning polypeptides which display native activity upon reconstitution.

Purification of an LHI-RC-complex of Rhodospirillum rubrum by solubilization of chromatophores with a short-chain lecithin

Chromatophores from Rhodospirillum rubrum were solubilized using the detergent 1,2-diheptanoyl-sn-phosphatidylcholine (DHPC). The solubilization curves are sigmoidal reaching a plateau at a detergent/protein ratio of 2-3 μmol/mg corresponding to 75-90% solubilized protein. The BChl-binding proteins are stable over a large range of DHPC/protein ratios. A complex of BChl-binding proteins containing both LHI- and RC-polypeptides (LHI-RC-complex) was purified using a two step procedure. RC photochemical activity as well as absorption and near-IR CD spectra showed the complex to be active and stable after purification in presence of DHPC.