Substrate-binding Model of the Chlorophyll Biosynthetic Magnesium Chelatase BchH Subunit

Integrating the multiple functions of CHLH into chloroplast-derived signaling fundamental to plant development and adaptation as well as fruit ripening

Chlorophyll (Chl)-mediated oxygenic photosynthesis sustains life on Earth. Greening leaves play fundamental roles in plant growth and crop yield, correlating with the idea that more Chls lead to better adaptation. However, they face significant challenges from various unfavorable environments. Chl biosynthesis hinges on the first committed step, which involves inserting Mg2+ into protoporphyrin. This step is facilitated by the H subunit of magnesium chelatase (CHLH) and features a conserved mechanism from cyanobacteria to plants. For better adaptation to fluctuating land environments, especially drought, CHLH evolves multiple biological functions, including Chl biosynthesis, retrograde signaling, and abscisic acid (ABA) responses. Additionally, it integrates into various chloroplast-derived signaling pathways, encompassing both retrograde signaling and hormonal signaling. The former comprises ROS (reactive oxygen species), heme, GUN (genomes uncoupled), MEcPP (methylerythritol cyclodiphosphate), β-CC (β-cyclocitral), and PAP (3'-phosphoadenosine-5'-phosphate). The latter involves phytohormones like ABA, ethylene, auxin, cytokinin, gibberellin, strigolactone, brassinolide, salicylic acid, and jasmonic acid. Together, these elements create a coordinated regulatory network tailored to plant development and adaptation. An intriguing example is how drought-mediated improvement of fruit quality provides insights into chloroplast-derived signaling, aiding the shift from vegetative to reproductive growth. In this context, we explore the integration of CHLH's multifaceted roles into chloroplast-derived signaling, which lays the foundation for plant development and adaptation, as well as fruit ripening and quality. In the future, manipulating chloroplast-derived signaling may offer a promising avenue to enhance crop yield and quality through the homeostasis, function, and regulation of Chls.

Exploring the mechanism of blindness physiopathy in Brassica oleracea var italica L. by comprehensive transcriptomics and metabolomics analysis

Blindness is a physiopathy characterized by apical abortion that particularly affects the Brassica family. The occurrence of blindness has been related to exposure to low temperatures during early developmental stages. However, the causes of this selective sensitivity and how they affect the correct development remain unknown. In this study, we investigated the mechanisms involved in the occurrence of blindness in broccoli plants. The analysis of RNAseq, focused on membrane transporters and the synthesis pathways of glucosinolates and phenolics, was related with physiological changes in nutrient and water uptake, gas exchange, and metabolism. Comparative gene expression analysis between control and blindness-affected broccoli plants revealed distinct regulation patterns in roots and shoots, leading to reduced synthesis of glucosinolates and phenolics. Additionally, the expression levels of aquaporins and potassium transporters were found to be associated with mineral and water transport. In this way, our results revealed the causes of blindness by identifying differentially expressed genes, highlighting those related to secondary metabolism, as well as genes involved in water and nutrient uptake and transport as the crucial involved in the physiopathy appearance.

Conformational variability of cyanobacterial ChlI, the AAA+ motor of magnesium chelatase involved in chlorophyll biosynthesis

IMPORTANCE

Photosynthesis is an essential life process that relies on chlorophyll. In photosynthetic organisms, chlorophyll synthesis involves multiple steps and depends on magnesium chelatase. This enzyme complex is responsible for inserting magnesium into the chlorophyll precursor, but the molecular mechanism of this process is not fully understood. By using cryogenic electron microscopy and conducting functional analyses, we have discovered that the motor subunit ChlI of magnesium chelatase undergoes conformational changes in the presence of ATP. Our findings offer new insights into how energy is transferred from ChlI to the other components of magnesium chelatase. This information significantly contributes to our understanding of the initial step in chlorophyll biosynthesis and lays the foundation for future studies on the entire process of chlorophyll production.

Absolute quantification of cellular levels of photosynthesis-related proteins in Synechocystis sp. PCC 6803

Quantifying cellular components is a basic and important step for understanding how a cell works, how it responds to environmental changes, and for re-engineering cells to produce valuable metabolites and increased biomass. We quantified proteins in the model cyanobacterium Synechocystis sp. PCC 6803 given the general importance of cyanobacteria for global photosynthesis, for synthetic biology and biotechnology research, and their ancestral relationship to the chloroplasts of plants. Four mass spectrometry methods were used to quantify cellular components involved in the biosynthesis of chlorophyll, carotenoid and bilin pigments, membrane assembly, the light reactions of photosynthesis, fixation of carbon dioxide and nitrogen, and hydrogen and sulfur metabolism. Components of biosynthetic pathways, such as those for chlorophyll or for photosystem II assembly, range between 1000 and 10,000 copies per cell, but can be tenfold higher for CO₂ fixation enzymes. The most abundant subunits are those for photosystem I, with around 100,000 copies per cell, approximately 2 to fivefold higher than for photosystem II and ATP synthase, and 5-20 fold more than for the cytochrome b₆f complex. Disparities between numbers of pathway enzymes, between components of electron transfer chains, and between subunits within complexes indicate possible control points for biosynthetic processes, bioenergetic reactions and for the assembly of multisubunit complexes.

A mutation in SlCHLH encoding a magnesium chelatase H subunit is involved in the formation of yellow stigma in tomato (Solanum lycopersicum L.)

Chlorophylls are ubiquitous pigments responsible for the green color in plants. Changes in the chlorophyll content have a significant impact on photosynthesis, plant growth and development. In this study, we used a yellow stigma mutant (ys) generated from a green stigma tomato WT by using ethylmethylsulfone (EMS)-induced mutagenesis. Compared with WT, the stigma of ys shows low chlorophyll content and impaired chloroplast ultrastructure. Through map-based cloning, the ys gene is localized to a 100 kb region on chromosome 4 between dCAPS596 and dCAPS606. Gene expression analysis and nonsynonymous SNP determination identified the Solyc04g015750, as the potential candidate gene, which encodes a magnesium chelatase H subunit (CHLH). In ys mutant, a single base C to T substitution in the SlCHLH gene results in the conversion of Serine into Leucine (Ser92Leu) at the N-terminal region. The functional complementation test shows that the SlCHLH from WT can rescue the green stigma phenotype of ys. In contrast, knockdown of SlCHLH in green stigma tomato AC, observed the yellow stigma phenotype at the stigma development stage. Overexpression of the mutant gene Slys in green stigma tomato AC results in the light green stigma. These results indicate that the mutation of the N-terminal S92 to Leu in SlCHLH is the main reason for the formation of the yellow stigma phenotype. Characterization of the ys mutant enriches the current knowledge of the tomato chlorophyll mutant library and provides a novel and effective tool for understanding the function of CHLH in tomato.

Heterologous Expression of the Barley (Hordeum vulgare L.) Xantha-f, -g and -h Genes that Encode Magnesium Chelatase Subunits

Biosynthesis of chlorophyll involves several enzymatic reactions of which many are shared with the heme biosynthesis pathway. Magnesium chelatase is the first specific enzyme in the chlorophyll pathway. It catalyzes the formation of Mg-protoporphyrin IX from the insertion of Mg²⁺ into protoporphyrin IX. The enzyme consists of three subunits encoded by three genes. The three genes are named Xantha-h, Xantha-g and Xantha-f in barley (Hordeum vulgare L.). The products of the genes have a molecular weight of 38, 78 and 148 kDa, respectively, as mature proteins in the chloroplast. Most studies on magnesium chelatase enzymes have been performed using recombinant proteins of Rhodobacter capsulatus, Synechocystis sp. PCC6803 and Thermosynechococcus elongatus, which are photosynthetic bacteria. In the present study we established a recombinant expression system for barley magnesium chelatase with the long-term goal to obtain structural information of this enigmatic enzyme complex from a higher plant. The genes Xantha-h, -g and -f were cloned in plasmid pET15b, which allowed the production of the three subunits as His-tagged proteins in Escherichia coli BL21(DE3)pLysS. The purified subunits stimulated magnesium chelatase activity of barley plastid extracts and produced activity in assays with only recombinant proteins. In preparation for future structural analyses of the barley magnesium chelatase, stability tests were performed on the subunits and activity assays were screened to find an optimal buffer system and pH.

Structure of sirohydrochlorin ferrochelatase SirB: the last of the structures of the class II chelatase family

The crystal structure of Bacillus subtilis SirB, which catalyses the insertion of Fe2+ into the substrate sirohydrochlorin (SHC) in siroheme biosynthesis, is reported herein as the last of the structures of class II chelatases. The structure of SirB with Co2+ showed that the active site of SirB is located at the N-terminal domain with metal-binding amino acid residues His10, Glu43, and His76, which was also predicted for CbiX, but is distinct from the C-terminal active sites of CbiK and HemH. The biosynthetic model reactions using SirB, Co2+ and uroporphyrin I or protoporphyrin IX as a SHC analogue revealed that SirB showed chelatase activity for uroporphyrin I, but not for protoporphyrin IX. Simulations of tetrapyrroles docking to SirB provided an insight into its tetrapyrrole substrate recognition: SHC and uroporphyrin I were suitably bound beside the Co2+ ion-binding site at the active site cavity; protoporphyrin IX was also docked to the active site but its orientation was different from those of the other two tetrapyrroles. Summarizing the present data, it was proposed that the key structural features for substrate recognition of SirB could be the hydrophobic area at the active site as well as the substituents of the tetrapyrroles.

The ChlD subunit links the motor and porphyrin binding subunits of magnesium chelatase

Magnesium chelatase initiates chlorophyll biosynthesis, catalysing the MgATP²⁻-dependent insertion of a Mg²⁺ ion into protoporphyrin IX. The catalytic core of this large enzyme complex consists of three subunits: Bch/ChlI, Bch/ChlD and Bch/ChlH (in bacteriochlorophyll and chlorophyll producing species, respectively). The D and I subunits are members of the AAA⁺ (ATPases associated with various cellular activities) superfamily of enzymes, and they form a complex that binds to H, the site of metal ion insertion. In order to investigate the physical coupling between ChlID and ChlH in vivo and in vitro, ChlD was FLAG-tagged in the cyanobacterium Synechocystis sp. PCC 6803 and co-immunoprecipitation experiments showed interactions with both ChlI and ChlH. Co-production of recombinant ChlD and ChlH in Escherichia coli yielded a ChlDH complex. Quantitative analysis using microscale thermophoresis showed magnesium-dependent binding (K_d 331 ± 58 nM) between ChlD and H. The physical basis for a ChlD–H interaction was investigated using chemical cross-linking coupled with mass spectrometry (XL–MS), together with modifications that either truncate ChlD or modify single residues. We found that the C-terminal integrin I domain of ChlD governs association with ChlH, the Mg²⁺ dependence of which also mediates the cooperative response of the Synechocystis chelatase to magnesium. The interaction site between the AAA⁺ motor and the chelatase domain of magnesium chelatase will be essential for understanding how free energy from the hydrolysis of ATP on the AAA⁺ ChlI subunit is transmitted via the bridging subunit ChlD to the active site on ChlH.

Factors controlling the reactivity of divalent metal ions towards pheophytin a

In this study, we evaluate the factors which determine the reactivity of divalent metal ions in the spontaneous formation of metallochlorophylls, using experimental and computational approaches. Kinetic studies were carried out using pheophytin a in reactions with various divalent metal ions combined with non- or weakly-coordinative counter ions in a series of organic solvents. To obtain detailed insights into the solvent effect, the metalations with the whole set of cations were investigated in three solvents and with Zn²⁺ in seven solvents. The reactions were monitored using electronic absorption spectroscopy and the stopped-flow technique. DFT calculations were employed to shed light on the role of solvent in activating the metal ions towards porphyrinoids. This experimental and computational analysis gives detailed information regarding how the solvent and the counter ion assist/hinder the metalation reaction as activators/inhibitors. The metalation course is dictated to a large extent by the reaction medium, via either the activation or deactivation of the incoming metal ion. The solvent may affect the metalation in several ways, mainly via H-bonding with pyrrolenine nitrogens and the activation/deactivation of the incoming cation. It also seems to affect the activation enthalpy by causing slight conformational changes in the macrocyclic ligand. These new mechanistic insights contribute to a better understanding of the “metal–counterion–solvent” interplay in the metalation of porphyrinoids. In addition, they are highly relevant to the mechanisms of metalation reactions catalyzed by chelatases and explain the differences between the insertion of Mg²⁺ and other divalent cations.

CHLH/GUN5 Function in Tetrapyrrole Metabolism Is Correlated with Plastid Signaling but not ABA Responses in Guard Cells

Expression of Photosynthesis-Associated Nuclear Genes (PhANGs) is controlled by environmental stimuli and plastid-derived signals ("plastid signals") transmitting the developmental and functional status of plastids to the nucleus. Arabidopsis genomes uncoupled (gun) mutants exhibit defects in plastid signaling, leading to ectopic expression of PhANGs in the absence of chloroplast development. GUN5 encodes the plastid-localized Mg-chelatase enzyme subunit (CHLH), and recent studies suggest that CHLH is a multifunctional protein involved in tetrapyrrole biosynthesis, plastid signaling and ABA responses in guard cells. To understand the basis of CHLH multifunctionality, we investigated 15 gun5 missense mutant alleles and transgenic lines expressing a series of truncated CHLH proteins in a severe gun5 allele (cch) background (tCHLHs, 10 different versions). Here, we show that Mg-chelatase function and plastid signaling are generally correlated; in contrast, based on the analysis of the gun5 missense mutant alleles, ABA-regulated stomatal control is distinct from these two other functions. We found that none of the tCHLHs could restore plastid-signaling or Mg-chelatase functions. Additionally, we found that both the C-terminal half and N-terminal half of CHLH function in ABA-induced stomatal movement.

Crystal structure of the catalytic subunit of magnesium chelatase

Tetrapyrroles, including haem and chlorophyll, play vital roles for various biological processes, such as respiration and photosynthesis, and their biosynthesis is critical for virtually all organisms. In photosynthetic organisms, magnesium chelatase (MgCh) catalyses insertion of magnesium into the centre of protoporphyrin IX, the branch-point precursor for both haem and chlorophyll, leading tetrapyrrole biosynthesis into the magnesium branch(1,2). This reaction needs a cooperated action of the three subunits of MgCh: the catalytic subunit ChlH and two AAA(+) subunits, ChlI and ChlD (refs 3-5). To date, the mechanism of MgCh awaits further elucidation due to a lack of high-resolution structures, especially for the ∼150 kDa catalytic subunit. Here we report the crystal structure of ChlH from the photosynthetic cyanobacterium Synechocystis PCC 6803, solved at 2.5 Å resolution. The active site is buried deeply inside the protein interior, and the surrounding residues are conserved throughout evolution. This structure helps to explain the loss of function reported for the cch and gun5 mutations of the ChlH subunit, and to provide the molecular basis of substrate channelling during the magnesium-chelating process. The structure advances our understanding of the holoenzyme of MgCh, a metal chelating enzyme other than ferrochelatase.

Structural and functional consequences of removing the N-terminal domain from the magnesium chelatase ChlH subunit of Thermosynechococcus elongatus

Magnesium chelatase (MgCH) initiates chlorophyll biosynthesis by catalysing the ATP-dependent insertion of Mg²⁺ into protoporphyrin. This large enzyme complex comprises ChlH, I and D subunits, with I and D involved in ATP hydrolysis, and H the protein that handles the substrate and product. The 148 kDa ChlH subunit has a globular N-terminal domain attached by a narrow linker to a hollow cage-like structure. Following deletion of this ~18 kDa domain from the Thermosynechoccus elongatus ChlH, we used single particle reconstruction to show that the apo- and porphyrin-bound forms of the mutant subunit consist of a hollow globular protein with three connected lobes; superposition of the mutant and native ChlH structures shows that, despite the clear absence of the N-terminal ‘head’ region, the rest of the protein appears to be correctly folded. Analyses of dissociation constants shows that the ΔN159ChlH mutant retains the ability to bind protoporphyrin and the Gun4 enhancer protein, although the addition of I and D subunits yields an extremely impaired active enzyme complex. Addition of the Gun4 enhancer protein, which stimulates MgCH activity significantly especially at low Mg²⁺ concentrations, partially reactivates the ΔN159ChlH–I–D mutant enzyme complex, suggesting that the binding site or sites for Gun4 on H do not wholly depend on the N-terminal domain.

The N-terminal domain of the 148 kDa ChlH is essential for normal activity of the ChlH–I–D magnesium chelatase complex. Deleting this 18 kDa domain retains the hollow cage-like structure and porphyrin binding. Chelatase activity is partially restored by the Gun4 protein.

Structural insights into the catalytic mechanism of Synechocystis magnesium protoporphyrin IX O-methyltransferase (ChlM)

Magnesium protoporphyrin IX O-methyltransferase (ChlM) catalyzes transfer of the methyl group from S-adenosylmethionine to the carboxyl group of the C13 propionate side chain of magnesium protoporphyrin IX. This reaction is the second committed step in chlorophyll biosynthesis from protoporphyrin IX. Here we report the crystal structures of ChlM from the cyanobacterium Synechocystis sp. PCC 6803 in complex with S-adenosylmethionine and S-adenosylhomocysteine at resolutions of 1.6 and 1.7 Å, respectively. The structures illustrate the molecular basis for cofactor and substrate binding and suggest that conformational changes of the two "arm" regions may modulate binding and release of substrates/products to and from the active site. Tyr-28 and His-139 were identified to play essential roles for methyl transfer reaction but are not indispensable for cofactor/substrate binding. Based on these structural and functional findings, a catalytic model is proposed.

Inducing the oxidative stress response in Escherichia coli improves the quality of a recombinant protein: magnesium chelatase ChlH

The ∼150kDa ChlH subunit of magnesium chelatase from Oryza sativa, Hordeum vulgare and Chlamydomonas reinhardtii have been heterologously expressed in Escherichiacoli. The active soluble protein is found as both a multimeric and a monomeric form. The multimeric ChlH appears to be oxidatively damaged but monomer production is favoured in growth conditions that are known to cause an oxidative stress response in E.coli. Inducing an oxidative stress response may be of general utility to improve the quality of proteins expressed in E. coli. The similar responses of ChlH's from the three different species suggest that oligomerization of oxidatively damaged ChlH may have a functional role in the chloroplast, possibly as a signal of oxidative stress or damage.

Characterization of the magnesium chelatase from Thermosynechococcus elongatus

The first committed step in chlorophyll biosynthesis is catalysed by magnesium chelatase (E.C. 6.6.1.1), which uses the free energy of ATP hydrolysis to insert an Mg(2+) ion into the ring of protoporphyrin IX. We have characterized magnesium chelatase from the thermophilic cyanobacterium Thermosynechococcus elongatus. This chelatase is thermostable, with subunit melting temperatures between 55 and 63°C and optimal activity at 50°C. The T. elongatus chelatase (kcat of 0.16 μM/min) shows a Michaelis-Menten-type response to both Mg(2+) (Km of 2.3 mM) and MgATP(2-) (Km of 0.8 mM). The response to porphyrin is more complex; porphyrin inhibits at high concentrations of ChlH, but when the concentration of ChlH is comparable with the other two subunits the response is of a Michaelis-Menten type (at 0.4 μM ChlH, Km is 0.2 μM). Hybrid magnesium chelatases containing a mixture of subunits from the mesophilic Synechocystis and Thermosynechococcus enzymes are active. We generated all six possible hybrid magnesium chelatases; the hybrid chelatase containing Thermosynechococcus ChlD and Synechocystis ChlI and ChlH is not co-operative towards Mg(2+), in contrast with the Synechocystis magnesium chelatase. This loss of co-operativity reveals the significant regulatory role of Synechocystis ChlD.

Catalytic turnover triggers exchange of subunits of the magnesium chelatase AAA+ motor unit

The ATP-dependent insertion of Mg(2+) into protoporphyrin IX is the first committed step in the chlorophyll biosynthetic pathway. The reaction is catalyzed by magnesium chelatase, which consists of three gene products: BchI, BchD, and BchH. The BchI and BchD subunits belong to the family of AAA+ proteins (ATPases associated with various cellular activities) and form a two-ring complex with six BchI subunits in one layer and six BchD subunits in the other layer. This BchID complex is a two-layered trimer of dimers with the ATP binding site located at the interface between two neighboring BchI subunits. ATP hydrolysis by the BchID motor unit fuels the insertion of Mg(2+) into the porphyrin by the BchH subunit. In the present study, we explored mutations that were originally identified in semidominant barley (Hordeum vulgare L.) mutants. The resulting recombinant BchI proteins have marginal ATPase activity and cannot contribute to magnesium chelatase activity although they apparently form structurally correct complexes with BchD. Mixing experiments with modified and wild-type BchI in various combinations showed that an exchange of BchI subunits in magnesium chelatase occurs during the catalytic cycle, which indicates that dissociation of the complex may be part of the reaction mechanism related to product release. Mixing experiments also showed that more than three functional interfaces in the BchI ring structure are required for magnesium chelatase activity.

Interplay between Heat Shock Protein 90 and HY5 controls PhANG expression in response to the GUN5 plastid signal

The presence of genes encoding organellar proteins in different cellular compartments necessitates a tight coordination of expression by the different genomes of the eukaryotic cell. This coordination of gene expression is achieved by organelle-to-nucleus or retrograde communication. Stress-induced perturbations of the tetrapyrrole pathway trigger large changes in nuclear gene expression in plants. Recently, we identified HSP90 proteins as ligands of the putative plastid signal Mg-ProtoIX. In order to investigate whether the interaction between HSP90 and Mg-ProtoIX is biologically relevant, we produced transgenic lines with reduced levels of cytosolic HSP90 in wild-type and gun5 backgrounds. Our work reveals that HSP90 proteins respond to the tetrapyrrole-mediated plastid signal to control expression of photosynthesis-associated nuclear genes (PhANG) during the response to oxidative stress. We also show that the hy5 mutant is insensitive to tetrapyrrole accumulation and that Mg-ProtoIX, cytosolic HSP90, and HY5 are all part of the same signaling pathway. These findings suggest that a regulatory complex controlling gene expression that includes HSP90 proteins and a transcription factor that is modified by tetrapyrroles in response to changes in the environment is evolutionarily conserved between yeast and plants.

FERROCHELATASE: THE CONVERGENCE OF THE PORPHYRIN BIOSYNTHESIS AND IRON TRANSPORT PATHWAYS

Ferrochelatase (also known as PPIX ferrochelatase; Enzyme Commission number 4.9.9.1.1) catalyzes the insertion of ferrous iron into PPIX to form heme. This reaction unites the biochemically synchronized pathways of porphyrin synthesis and iron transport in nearly all living organisms. The ferrochelatases are an evolutionarily diverse family of enzymes with no more than six active site residues known to be perfectly conserved. The availability of over thirty different crystal structures, including many with bound metal ions or porphyrins, has added tremendously to our understanding of ferrochelatase structure and function. It is generally believed that ferrous iron is directly channeled to ferrochelatase in vivo, but the identity of the suspected chaperone remains uncertain despite much recent progress in this area. Identification of a conserved metal ion binding site at the base of the active site cleft may be an important clue as to how ferrochelatases acquire iron, and catalyze desolvation during transport to the catalytic site to complete heme synthesis.

Structure of the cyanobacterial Magnesium Chelatase H subunit determined by single particle reconstruction and small-angle X-ray scattering

The biosynthesis of chlorophyll, an essential cofactor for photosynthesis, requires the ATP-dependent insertion of Mg(2+) into protoporphyrin IX catalyzed by the multisubunit enzyme magnesium chelatase. This enzyme complex consists of the I subunit, an ATPase that forms a complex with the D subunit, and an H subunit that binds both the protoporphyrin substrate and the magnesium protoporphyrin product. In this study we used electron microscopy and small-angle x-ray scattering to investigate the structure of the magnesium chelatase H subunit, ChlH, from the thermophilic cyanobacterium Thermosynechococcus elongatus. Single particle reconstruction of negatively stained apo-ChlH and Chl-porphyrin proteins was used to reconstitute three-dimensional structures to a resolution of ∼30 Å. ChlH is a large, 148-kDa protein of 1326 residues, forming a cage-like assembly comprising the majority of the structure, attached to a globular N-terminal domain of ∼16 kDa by a narrow linker region. This N-terminal domain is adjacent to a 5 nm-diameter opening in the structure that allows access to a cavity. Small-angle x-ray scattering analysis of ChlH, performed on soluble, catalytically active ChlH, verifies the presence of two domains and their relative sizes. Our results provide a basis for the multiple regulatory and catalytic functions of ChlH of oxygenic photosynthetic organisms and for a chaperoning function that sequesters the enzyme-bound magnesium protoporphyrin product prior to its delivery to the next enzyme in the chlorophyll biosynthetic pathway, magnesium protoporphyrin methyltransferase.

A novel proteomic approach reveals a role for Mg-protoporphyrin IX in response to oxidative stress

The presence of genes encoding organellar proteins in different cellular compartments necessitates a tight coordination of expression by the different genomes of the eukaryotic cell. This coordination of gene expression is achieved by organelle-to-nucleus communication. Stress-induced perturbations of the tetrapyrrole pathway trigger large changes in nuclear gene expression. In order to investigate whether the tetrapyrrole Mg-ProtoIX itself is an important part of plastid-to-nucleus communication, we used an affinity column containing Mg-ProtoIX covalently linked to an Affi-Gel matrix. The proteins that bound to Mg-ProtoIX were analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis combined with nano liquid chromatography-mass spectrometry (MS)/MS. Thus, we present a novel proteomic approach to address the mechanisms involved in cellular signaling and we identified interactions between Mg-ProtoIX and a large number of proteins associated with oxidative stress responses. Our approach revealed an interaction between Mg-ProtoIX and the heat shock protein 90-type protein, HSP81-2 suggesting that a regulatory complex including HSP90 proteins and tetrapyrroles controlling gene expression is evolutionarily conserved between yeast and plants. In addition, our list of putative Mg-ProtoIX-binding proteins demonstrated that binding of tetrapyrroles does not depend on a specific amino acid motif but possibly on a specific fold of the protein.

Structural and electronic effects in the metalation of porphyrinoids. Theory and experiment

The structure-reactivity relationships in metalation reactions of porphyrinoids have been studied using experimental and theoretical methods. A series of eight porphyrinoic ligands, derivatives of chlorophylls, was prepared in which both the peripheral groups and the degrees of saturation of the macrocycle were systematically varied. To reveal the solvent and structural factors which control the interactions of these macroligands with metal centers, their interactions with reactive Zn(2+) and inert Pt(2+) ions were investigated using absorption spectroscopy. In parallel, quantum chemical calculations (density functional theory, DFT) were performed for the same set of molecules to examine the influence of structural and electronic factors on the energy of the frontier orbitals, the nucleophilicity/electronegativity of the macrocycle, its hardness, and conformation. These static descriptors of chemical reactivity, relevant to metalation reactions, were verified against the results obtained in the experimental model. The experimentally obtained kinetic data clearly show that the solvent has a crucial role in the activation of the incoming metal center. In terms of chelator structure, the largest effects concern the size of the delocalized pi-electron system and the presence of side groups. Both the DFT calculations and experimental results show the strong influence of the macrocycle rigidity and of the peripheral groups on the chelating ability of porphyrinoids. In particular, the peripheral functionalization of the macrocyclic system seems to drastically reduce its reactivity toward metal ions. The effect of peripheral groups is two-fold: (i) a lower electron density on the core nitrogens, and (ii) increased rigidity of the macrocycle. The outcomes of the theoretical and experimental analyses are discussed also in terms of their relevance to the mechanism of biological metal insertion in the biosynthesis of heme and chlorophyll.

Interactions between alkaline earth cations and oxo ligands. DFT study of the affinity of the Mg²+ cation for phosphoryl ligands

DFT (B3LYP/6-31+G(d)) calculations of Mg(2+) affinities for a set of phosphoryl ligands were performed. Two types of ligands were studied: a set of trivalent [O = P(R)] and a set of pentavalent phosphoryl ligands [O = P(R)(3)] (R = H, F, Cl, Br, OH, OCH(3), CH(3), CN, NH(2) and NO(2)), with R either bound directly to the phosphorus atom or to the para position of a phenyl ring. The affinity of the Mg(2+) cation for the ligands was quantified by means of the enthalpy for the substitution of one water molecule in the [Mg(H(2)O)(6)](2+) complex for a ligand. The enthalpy of substitution was correlated with electronic and geometric parameters. Electron-donor groups increase the interaction between the cation and the ligand, while electron-acceptor groups decrease the interaction enthalpy.

BchJ and BchM interact in a 1 : 1 ratio with the magnesium chelatase BchH subunit of Rhodobacter capsulatus

Substrate channeling between the enzymatic steps in the (bacterio)chlorophyll biosynthetic pathway catalyzed by magnesium chelatase (BchI/ChlI, BchD/ChlD and BchH/ChlH subunits) and S-adenosyl-L-methionine:magnesium-protoporphyrin IX O-methyltransferase (BchM/ChlM) has been suggested. This involves delivery of magnesium-protoporphyrin IX from the BchH/ChlH subunit of magnesium chelatase to BchM/ChlM. Stimulation of BchM/ChlM activity by BchH/ChlH has previously been shown, and physical interaction of the two proteins has been demonstrated. In plants and cyanobacteria, there is an added layer of complexity, as Gun4 serves as a porphyrin (protoporphyrin IX and magnesium-protoporphyrin IX) carrier, but this protein does not exist in anoxygenic photosynthetic bacteria. BchJ may play a similar role to Gun4 in Rhodobacter, as it has no currently assigned function in the established pathway. Purified recombinant Rhodobacter capsulatus BchJ and BchM were found to cause a shift in the equilibrium amount of Mg-protoporphyrin IX formed in a magnesium chelatase assay. Analysis of this shift revealed that it was always in a 1 : 1 ratio with either of these proteins and the BchH subunit of the magnesium chelatase. The establishment of the new equilibrium was faster with BchM than with BchJ in a coupled magnesium chelatase assay. BchJ bound magnesium-protoporphyrin IX or formed a ternary complex with BchH and magnesium-protoporphyrin IX. These results suggest that BchJ may play a role as a general magnesium porphyrin carrier, similar to one of the roles of GUN4 in oxygenic organisms.

Putting metal in the middle

Placing metal ions into the center of a porphyrin ring is a significant cellular challenge. Lundqvist et al. (2010) provide snapshots of the AAA+ motor unit of magnesium chelatase, an enzyme that inserts a magnesium ion into the porphyrin ring accompanied by ATP expenditure, in different states of binding to adenosyl nucleotides.

ATP-induced conformational dynamics in the AAA+ motor unit of magnesium chelatase

Mg-chelatase catalyzes the first committed step of the chlorophyll biosynthetic pathway, the ATP-dependent insertion of Mg(2+) into protoporphyrin IX (PPIX). Here we report the reconstruction using single-particle cryo-electron microscopy of the complex between subunits BchD and BchI of Rhodobacter capsulatus Mg-chelatase in the presence of ADP, the nonhydrolyzable ATP analog AMPPNP, and ATP at 7.5 A, 14 A, and 13 A resolution, respectively. We show that the two AAA+ modules of the subunits form a unique complex of 3 dimers related by a three-fold axis. The reconstructions demonstrate substantial differences between the conformations of the complex in the presence of ATP and ADP, and suggest that the C-terminal integrin-I domains of the BchD subunits play a central role in transmitting conformational changes of BchI to BchD. Based on these data a model for the function of magnesium chelatase is proposed.

Mutational analysis of three bchH paralogs in (bacterio-)chlorophyll biosynthesis in Chlorobaculum tepidum

The first committed step in the biosynthesis of (bacterio-)chlorophyll is the insertion of Mg2+ into protoporphyrin IX by Mg-chelatase. In all known (B)Chl-synthesizing organisms, Mg-chelatase is encoded by three genes that are homologous to bchH, bchD, and bchI of Rhodobacter spp. The genomes of all sequenced strains of green sulfur bacteria (Chlorobi) encode multiple bchH paralogs, and in the genome of Chlorobaculum tepidum, there are three bchH paralogs, denoted CT1295 (bchT), CT1955 (bchS), and CT1957 (bchH). Cba. tepidum mutants lacking one or two of these paralogs were constructed and characterized. All of the mutants lacking only one of these BchH homologs, as well as bchS bchT and bchH bchT double mutants, which can only produce BchH or BchS, respectively, were viable. However, attempts to construct a bchH bchS double mutant, in which only BchT was functional, were consistently unsuccessful. This result suggested that BchT alone is unable to support the minimal (B)Chl synthesis requirements of cells required for viability. The pigment compositions of the various mutant strains varied significantly. The BChl c content of the bchS mutant was only approximately 10% of that of the wild type, and this mutant excreted large amounts of protoporphyrin IX into the growth medium. The observed differences in BChl c production of the mutant strains were consistent with the hypothesis that the three BchH homologs function in end product regulation and/or substrate channeling of intermediates in the BChl c biosynthetic pathway.

Does the different proteomic profile found in apical and basal leaves of spinach reveal a strategy of this plant toward cadmium pollution response?

Chlorosis develops in Spinacia oleracea L. plants exposed to Cd and is prevalently localized in the basal leaves. A proteomic comparison of basal and apical leaves from Cd-treated plants showed modified profiles that are different and complementary in the two locations. Total chlorophyll increased in apical leaves as did photosynthetic complexes and enzymes involved in CO2 fixation and carbohydrate metabolism. Thus, apical leaves seem to supply the plant's energy requirements and, consistent with this, remain green after 40 days. In contrast, basal leaves experienced reduced chlorophyll a synthesis and photosynthesis, and later on an over production of ROS, which induces a cell defense response, leading to senescence and cell death. There was also an over production of GSH and phytochelatins, whose main role is in chelating Cd. These chelate-polypeptide complexes accumulate in the vacuole, limiting the distribution of Cd to apical leaves. In line, we found that many proteins involved in carbon metabolism were less abundant, whereas proteins involved in remobilizing carbon from other energy sources were up-regulated. We suggest that phytochelatin production has priority in Cd-stressed basal leaves and the nitrogen and sulfur metabolic pathways are activated for this purpose. Finally, as dead leaves detach from the plant, they carry away the sequestered Cd, thereby removing it completely from the plant and preventing any future access to the apical leaves. These events may represent an active detoxification strategy in higher plants.

Physiological functions of mineral macronutrients

Plants require calcium, magnesium, nitrogen, phosphorous, potassium and sulfur in relatively large amounts (>0.1% of dry mass) and each of these so-called macronutrients is essential for a plant to complete its life cycle. Normally, these minerals are taken up by plant roots from the soil solution in ionic form with the metals Ca(2+), Mg(2+) and K(+) present as free cations, P and S as their oxyanions phosphate (PO(4)(3-)) and sulfate (SO(4)(2-)) and N as anionic nitrate (NO(3)(-)) or cation ammonium (NH(4)(+)). Recently, important progress has been made in identifying transport and regulatory mechanisms for macronutrients and the mechanisms of uptake and distribution. These and the main physiological roles of each nutrient will be discussed.

Kinetic analyses of the magnesium chelatase provide insights into the mechanism, structure, and formation of the complex

The metabolic pathway known as (bacterio)chlorophyll biosynthesis is initiated by magnesium chelatase (BchI, BchD, BchH). This first step involves insertion of magnesium into protoporphyrin IX (proto), a process requiring ATP hydrolysis. Structural information shows that the BchI and BchD subunits form a double hexameric enzyme complex, whereas BchH binds proto and can be purified as BchH-proto. We utilized the Rhodobacter capsulatus magnesium chelatase subunits using continuous magnesium chelatase assays and treated the BchD subunit as the enzyme with both BchI and BchH-proto as substrates. Michaelis-Menten kinetics was observed with the BchI subunit, whereas the BchH subunit exhibited sigmoidal kinetics (Hill coefficient of 1.85). The BchI.BchD complex had intrinsic ATPase activity, and addition of BchH greatly increased ATPase activity. This was concentration-dependent and gave sigmoidal kinetics, indicating there is more than one binding site for the BchH subunit on the BchI.BchD complex. ATPase activity was approximately 40-fold higher than magnesium chelatase activity and continued despite cessation of magnesium chelation, implying one or more secondary roles for ATP hydrolysis and possibly an as yet unknown switch required to terminate ATPase activity. One of the secondary roles for BchH-stimulated ATP hydrolysis by a BchI.BchD complex is priming of BchH to facilitate correct binding of proto to BchH in a form capable of participating in magnesium chelation. This porphyrin binding is the rate-limiting step in catalysis. These data suggest that ATP hydrolysis by the BchI.BchD complex causes a series of conformational changes in BchH to effect substrate binding, magnesium chelation, and product release.