Interaction between cyanobacterial copper chaperone Atx1 and zinc homeostasis

Distinct function of Chlamydomonas CTRA-CTR transporters in Cu assimilation and intracellular mobilization

Successful acclimation to copper (Cu) deficiency involves a fine balance between Cu import and export. In the green alga Chlamydomonas reinhardtii, Cu import is dependent on a transcription factor, Copper Response Regulator 1 (CRR1), responsible for activating genes in Cu-deficient cells. Among CRR1 target genes are two Cu transporters belonging to the CTR/COPT gene family (CTR1 and CTR2) and a related soluble protein (CTR3). The ancestor of these green algal proteins was likely acquired from an ancient chytrid and contained conserved cysteine-rich domains (named the CTR-associated domains, CTRA) that are predicted to be involved in Cu acquisition. We show by reverse genetics that Chlamydomonas CTR1 and CTR2 are canonical Cu importers albeit with distinct affinities, while loss of CTR3 did not result in an observable phenotype under the conditions tested. Mutation of CTR1, but not CTR2, recapitulates the poor growth of crr1 in Cu-deficient medium, consistent with a dominant role for CTR1 in high-affinity Cu(I) uptake. On the other hand, the overaccumulation of Cu(I) (20 times the quota) in zinc (Zn) deficiency depends on CRR1 and both CTR1 and CTR2. CRR1-dependent activation of CTR gene expression needed for Cu over-accumulation can be bypassed by the provision of excess Cu in the growth medium. Over-accumulated Cu is sequestered into the acidocalcisome but can become remobilized by restoring Zn nutrition. This mobilization is also CRR1-dependent, and requires activation of CTR2 expression, again distinguishing CTR2 from CTR1 and consistent with the lower substrate affinity of CTR2.

ONE SENTENCE SUMMARY

Regulation of Cu uptake and sequestration by members of the CTR family of proteins in Chlamydomonas.

Unbalancing Zur (FurB)-mediated homeostasis in Anabaena sp. PCC7120: Consequences on metal trafficking, heterocyst development and biofilm formation

Zinc is required for the activity of many enzymes and plays an essential role in gene regulation and redox homeostasis. In Anabaena (Nostoc) sp. PCC7120, the genes involved in zinc uptake and transport are controlled by the metalloregulator Zur (FurB). Comparative transcriptomics of a zur mutant (Δzur) with the parent strain unveiled unexpected links between zinc homeostasis and other metabolic pathways. A notable increase in the transcription of numerous desiccation tolerance-related genes, including genes involved in the synthesis of trehalose and the transference of saccharide moieties, among many others, was detected. Biofilm formation analysis under static conditions revealed a reduced capacity of Δzur filaments to form biofilms compared to the parent strain, and such capacity was enhanced when Zur was overexpressed. Furthermore, microscopy analysis revealed that zur expression is required for the correct formation of the envelope polysaccharide layer in the heterocyst, as Δzur cells showed reduced staining with alcian blue compared to Anabaena sp. PCC7120. We suggest that Zur is an important regulator of the enzymes involved in the synthesis and transport of the envelope polysaccharide layer, influencing heterocyst development and biofilm formation, both relevant processes for cell division and interaction with substrates in its ecological niche.

Zn deficiency disrupts Cu and S homeostasis in Chlamydomonas resulting in over accumulation of Cu and Cysteine

Growth of Chlamydomonas reinhardtii in zinc (Zn) limited medium leads to disruption of copper (Cu) homeostasis, resulting in up to 40-fold Cu over-accumulation relative to its typical Cu quota. We show that Chlamydomonas controls its Cu quota by balancing Cu import and export, which is disrupted in a Zn deficient cell, thus establishing a mechanistic connection between Cu and Zn homeostasis. Transcriptomics, proteomics and elemental profiling revealed that Zn-limited Chlamydomonas cells up-regulate a subset of genes encoding "first responder" proteins involved in sulfur (S) assimilation and consequently accumulate more intracellular S, which is incorporated into L-cysteine, γ-glutamylcysteine, and homocysteine. Most prominently, in the absence of Zn, free L-cysteine is increased ∼80-fold, corresponding to ∼2.8 × 109 molecules/cell. Interestingly, classic S-containing metal binding ligands like glutathione and phytochelatins do not increase. X-ray fluorescence microscopy showed foci of S accumulation in Zn-limited cells that co-localize with Cu, phosphorus and calcium, consistent with Cu-thiol complexes in the acidocalcisome, the site of Cu(I) accumulation. Notably, cells that have been previously starved for Cu do not accumulate S or Cys, causally connecting cysteine synthesis with Cu accumulation. We suggest that cysteine is an in vivo Cu(I) ligand, perhaps ancestral, that buffers cytosolic Cu.

Cysteine: an ancestral Cu binding ligand in green algae?

Growth of Chlamydomonas reinhardtii in zinc (Zn) limited medium leads to disruption of copper (Cu) homeostasis, resulting in up to 40-fold Cu over-accumulation relative to its typical Cu quota. We show that Chlamydomonas controls its Cu quota by balancing Cu import and export, which is disrupted in a Zn deficient cell, thus establishing a mechanistic connection between Cu and Zn homeostasis. Transcriptomics, proteomics and elemental profiling revealed that Zn-limited Chlamydomonas cells up-regulate a subset of genes encoding "first responder" proteins involved in sulfur (S) assimilation and consequently accumulate more intracellular S, which is incorporated into L-cysteine, γ-glutamylcysteine and homocysteine. Most prominently, in the absence of Zn, free L-cysteine is increased ~80-fold, corresponding to ~ 2.8 × 10 ⁹ molecules/cell. Interestingly, classic S-containing metal binding ligands like glutathione and phytochelatins do not increase. X-ray fluorescence microscopy showed foci of S accumulation in Zn-limited cells that co-localize with Cu, phosphorus and calcium, consistent with Cu-thiol complexes in the acidocalcisome, the site of Cu(I) accumulation. Notably, cells that have been previously starved for Cu do not accumulate S or Cys, causally connecting cysteine synthesis with Cu accumulation. We suggest that cysteine is an in vivo Cu(I) ligand, perhaps ancestral, that buffers cytosolic Cu.

Current knowledge and recent advances in understanding metabolism of the model cyanobacterium Synechocystis sp. PCC 6803

Cyanobacteria are key organisms in the global ecosystem, useful models for studying metabolic and physiological processes conserved in photosynthetic organisms, and potential renewable platforms for production of chemicals. Characterizing cyanobacterial metabolism and physiology is key to understanding their role in the environment and unlocking their potential for biotechnology applications. Many aspects of cyanobacterial biology differ from heterotrophic bacteria. For example, most cyanobacteria incorporate a series of internal thylakoid membranes where both oxygenic photosynthesis and respiration occur, while CO2 fixation takes place in specialized compartments termed carboxysomes. In this review, we provide a comprehensive summary of our knowledge on cyanobacterial physiology and the pathways in Synechocystis sp. PCC 6803 (Synechocystis) involved in biosynthesis of sugar-based metabolites, amino acids, nucleotides, lipids, cofactors, vitamins, isoprenoids, pigments and cell wall components, in addition to the proteins involved in metabolite transport. While some pathways are conserved between model cyanobacteria, such as Synechocystis, and model heterotrophic bacteria like Escherichia coli, many enzymes and/or pathways involved in the biosynthesis of key metabolites in cyanobacteria have not been completely characterized. These include pathways required for biosynthesis of chorismate and membrane lipids, nucleotides, several amino acids, vitamins and cofactors, and isoprenoids such as plastoquinone, carotenoids, and tocopherols. Moreover, our understanding of photorespiration, lipopolysaccharide assembly and transport, and degradation of lipids, sucrose, most vitamins and amino acids, and haem, is incomplete. We discuss tools that may aid our understanding of cyanobacterial metabolism, notably CyanoSource, a barcoded library of targeted Synechocystis mutants, which will significantly accelerate characterization of individual proteins.

Role of Glutathione in Buffering Excess Intracellular Copper in Streptococcus pyogenes

Copper (Cu) is an essential metal for bacterial physiology but in excess it is bacteriotoxic. To limit Cu levels in the cytoplasm, most bacteria possess a transcriptionally responsive system for Cu export. In the Gram-positive human pathogen Streptococcus pyogenes (group A Streptococcus [GAS]), this system is encoded by the copYAZ operon. This study demonstrates that although the site of GAS infection represents a Cu-rich environment, inactivation of the copA Cu efflux gene does not reduce virulence in a mouse model of invasive disease. In vitro, Cu treatment leads to multiple observable phenotypes, including defects in growth and viability, decreased fermentation, inhibition of glyceraldehyde-3-phosphate dehydrogenase (GapA) activity, and misregulation of metal homeostasis, likely as a consequence of mismetalation of noncognate metal-binding sites by Cu. Surprisingly, the onset of these effects is delayed by ∼4 h even though expression of copZ is upregulated immediately upon exposure to Cu. Further biochemical investigations show that the onset of all phenotypes coincides with depletion of intracellular glutathione (GSH). Supplementation with extracellular GSH replenishes the intracellular pool of this thiol and suppresses all the observable effects of Cu treatment. These results indicate that GSH buffers excess intracellular Cu when the transcriptionally responsive Cu export system is overwhelmed. Thus, while the copYAZ operon is responsible for Cu homeostasis, GSH has a role in Cu tolerance and allows bacteria to maintain metabolism even in the presence of an excess of this metal ion.IMPORTANCE The control of intracellular metal availability is fundamental to bacterial physiology. In the case of copper (Cu), it has been established that rising intracellular Cu levels eventually fill the metal-sensing site of the endogenous Cu-sensing transcriptional regulator, which in turn induces transcription of a copper export pump. This response caps intracellular Cu availability below a well-defined threshold and prevents Cu toxicity. Glutathione, abundant in many bacteria, is known to bind Cu and has long been assumed to contribute to bacterial Cu handling. However, there is some ambiguity since neither its biosynthesis nor uptake is Cu-regulated. Furthermore, there is little experimental support for this physiological role of glutathione beyond measuring growth of glutathione-deficient mutants in the presence of Cu. Our work with group A Streptococcus provides new evidence that glutathione increases the threshold of intracellular Cu availability that can be tolerated by bacteria and thus advances fundamental understanding of bacterial Cu handling.

Bacterial sensors define intracellular free energies for correct enzyme metalation

There is a challenge for metalloenzymes to acquire their correct metals because some inorganic elements form more stable complexes with proteins than do others. These preferences can be overcome provided some metals are more available than others. However, while the total amount of cellular metal can be readily measured, the available levels of each metal have been more difficult to define. Metal-sensing transcriptional regulators are tuned to the intracellular availabilities of their cognate ions. Here we have determined the standard free energy for metal complex formation to which each sensor, in a set of bacterial metal sensors, is attuned: The less competitive the metal, the less favorable the free energy and hence greater availability to which the cognate allosteric mechanism is tuned. Comparing these free energies with values derived from the metal affinities of a metalloprotein reveals the mechanism of correct metalation exemplified here by a cobalt-chelatase for vitamin B₁₂.

Characterization of zinc stress response in Cyanobacterium Synechococcus sp. IU 625

The ability of cyanobacteria to survive many environmental stress factors is a testament to their resilience in nature. Of these environmental stress factors, overexposure to zinc is important to study since excessive zinc intake can be a severe hazard. Zinc toxicity in freshwater has been demonstrated to affects organisms such as invertebrates, algae and cyanobacteria. Cyanobacteria which possess increased resistance to zinc have been isolated. It is therefore important to elucidate the mechanism of survival and response to determine what factors allow their survival; as well as any remediation implications they may have. To characterize the effects of zinc in freshwater cyanobacteria, we investigated the response of Synechococcus sp. IU 625 (S. IU 625) over 29days to various concentrations (10, 25, and 50mg/L) of ZnCl₂. S. IU 625 was shown to be tolerant up to 25mg/L ZnCl₂ exposure, with 10mg/L ZnCl₂ having no outward physiological change and 50mg/L ZnCl₂ proving lethal to the cells. To determine a potential mechanism Inductive Coupled Plasma-Mass Spectrometry (ICP-MS) and RNA-seq analysis were performed on zinc exposed cells. Analysis performed on days 4 and 7 indicated that response is dose-dependent, with 10mg/L ZnCl₂ exhibiting nearly all zinc extracellular, corresponding with upregulation of cation transport response. Whereas the 25mg/L ZnCl₂ exhibited half of total zinc sequestered by the cells, which corresponds with the upregulation of sequestering proteins such as metallothionein and the downregulation of genes involved with ATP synthesis and phycobilisome assembly. These analyses were combined with growth monitoring, microscopy, quantitative polymerase chain reaction (qPCR) and flow cytometry to present a full spectrum of mechanisms behind zinc response in S. IU 625.

AztD, a Periplasmic Zinc Metallochaperone to an ATP-binding Cassette (ABC) Transporter System in Paracoccus denitrificans

Bacterial ATP-binding cassette (ABC) transporters of transition metals are essential for acquisition of necessary elements from the environment. A large number of Gram-negative bacteria, including human pathogens, have a fourth conserved gene of unknown function adjacent to the canonical permease, ATPase, and solute-binding protein (SBP) genes of the AztABC zinc transporter system. To assess the function of this putative accessory factor (AztD) from Paracoccus denitrificans, we have analyzed its transcriptional regulation, metal binding properties, and interaction with the SBP (AztC). Transcription of the aztD gene is significantly up-regulated under conditions of zinc starvation. Recombinantly expressed AztD purifies with slightly substoichiometric zinc from the periplasm of Escherichia coli and is capable of binding up to three zinc ions with high affinity. Size exclusion chromatography and a simple intrinsic fluorescence assay were used to determine that AztD as isolated is able to transfer bound zinc nearly quantitatively to apo-AztC. Transfer occurs through a direct, associative mechanism that prevents loss of metal to the solvent. These results indicate that AztD is a zinc chaperone to AztC and likely functions to maintain zinc homeostasis through interaction with the AztABC system. This work extends our understanding of periplasmic zinc trafficking and the function of chaperones in this process.

Generating a Metal-responsive Transcriptional Regulator to Test What Confers Metal Sensing in Cells

FrmR from Salmonella enterica serovar typhimurium (a CsoR/RcnR-like transcriptional de-repressor) is shown to repress the frmRA operator-promoter, and repression is alleviated by formaldehyde but not manganese, iron, cobalt, nickel, copper, or Zn(II) within cells. In contrast, repression by a mutant FrmRE64H (which gains an RcnR metal ligand) is alleviated by cobalt and Zn(II). Unexpectedly, FrmR was found to already bind Co(II), Zn(II), and Cu(I), and moreover metals, as well as formaldehyde, trigger an allosteric response that weakens DNA affinity. However, the sensory metal sites of the cells' endogenous metal sensors (RcnR, ZntR, Zur, and CueR) are all tighter than FrmR for their cognate metals. Furthermore, the endogenous metal sensors are shown to out-compete FrmR. The metal-sensing FrmRE64H mutant has tighter metal affinities than FrmR by approximately 1 order of magnitude. Gain of cobalt sensing by FrmRE64H remains enigmatic because the cobalt affinity of FrmRE64H is substantially weaker than that of the endogenous cobalt sensor. Cobalt sensing requires glutathione, which may assist cobalt access, conferring a kinetic advantage. For Zn(II), the metal affinity of FrmRE64H approaches the metal affinities of cognate Zn(II) sensors. Counter-intuitively, the allosteric coupling free energy for Zn(II) is smaller in metal-sensing FrmRE64H compared with nonsensing FrmR. By determining the copies of FrmR and FrmRE64H tetramers per cell, then estimating promoter occupancy as a function of intracellular Zn(II) concentration, we show how a modest tightening of Zn(II) affinity, plus weakened DNA affinity of the apoprotein, conspires to make the relative properties of FrmRE64H (compared with ZntR and Zur) sufficient to sense Zn(II) inside cells.

Predicting copper-, iron-, and zinc-binding proteins in pathogenic species of the Paracoccidioides genus

Approximately one-third of all proteins have been estimated to contain at least one metal cofactor, and these proteins are referred to as metalloproteins. These represent one of the most diverse classes of proteins, containing metal ions that bind to specific sites to perform catalytic, regulatory and structural functions. Bioinformatic tools have been developed to predict metalloproteins encoded by an organism based only on its genome sequence. Its function and the type of metal binder can also be predicted via a bioinformatics approach. Paracoccidioides complex includes termodimorphic pathogenic fungi that are found as saprobic mycelia in the environment and as yeast, the parasitic form, in host tissues. They are the etiologic agents of Paracoccidioidomycosis, a prevalent systemic mycosis in Latin America. Many metalloproteins are important for the virulence of several pathogenic microorganisms. Accordingly, the present work aimed to predict the copper, iron and zinc proteins encoded by the genomes of three phylogenetic species of Paracoccidioides (Pb01, Pb03, and Pb18). The metalloproteins were identified using bioinformatics approaches based on structure, annotation and domains. Cu-, Fe-, and Zn-binding proteins represent 7% of the total proteins encoded by Paracoccidioides spp. genomes. Zinc proteins were the most abundant metalloproteins, representing 5.7% of the fungus proteome, whereas copper and iron proteins represent 0.3 and 1.2%, respectively. Functional classification revealed that metalloproteins are related to many cellular processes. Furthermore, it was observed that many of these metalloproteins serve as virulence factors in the biology of the fungus. Thus, it is concluded that the Cu, Fe, and Zn metalloproteomes of the Paracoccidioides spp. are of the utmost importance for the biology and virulence of these particular human pathogens.

A chemical potentiator of copper-accumulation used to investigate the iron-regulons of Saccharomyces cerevisiae

The extreme resistance of Saccharomyces cerevisiae to copper is overcome by 2-(6-benzyl-2-pyridyl)quinazoline (BPQ), providing a chemical-biology tool which has been exploited in two lines of discovery. First, BPQ is shown to form a red (BPQ)₂Cu(I) complex and promote Ctr1-independent copper-accumulation in whole cells and in mitochondria isolated from treated cells. Multiple phenotypes, including loss of aconitase activity, are consistent with copper-BPQ mediated damage to mitochondrial iron–sulphur clusters. Thus, a biochemical basis of copper-toxicity in S. cerevisiae is analogous to other organisms. Second, iron regulons controlled by Aft1/2, Cth2 and Yap5 that respond to mitochondrial iron–sulphur cluster status are modulated by copper-BPQ causing iron hyper-accumulation via upregulated iron-import. Comparison of copper-BPQ treated, untreated and copper-only treated wild-type and fra2Δ by RNA-seq has uncovered a new candidate Aft1 target-gene (LSO1) and paralogous non-target (LSO2), plus nine putative Cth2 target-transcripts. Two lines of evidence confirm that Fra2 dominates basal repression of the Aft1/2 regulons in iron-replete cultures. Fra2-independent control of these regulons is also observed but CTH2 itself appears to be atypically Fra2-dependent. However, control of Cth2-target transcripts which is independent of CTH2 transcript abundance or of Fra2, is also quantified. Use of copper-BPQ supports a substantial contribution of metabolite repression to iron-regulation.

Metal specificity of cyanobacterial nickel-responsive repressor InrS: cells maintain zinc and copper below the detection threshold for InrS

InrS is a Ni(II)-responsive, CsoR/RcnR-like, DNA-binding transcriptional repressor of the nrsD gene, but the Ni(II) co-ordination sphere of InrS is unlike Ni(II)-RcnR. We show that copper and Zn(II) also bind tightly to InrS and in vitro these ions also impair InrS binding to the nrsD operator-promoter. InrS does not respond to Zn(II) (or copper) in vivo after 48 h, when Zn(II) sensor ZiaR responds, but InrS transiently responds (1 h) to both metals. InrS conserves only one (of two) second co-ordination shell residues of CsoR (Glu98 in InrS). The allosteric mechanism of InrS is distinct from Cu(I)-CsoR and conservation of deduced second shell residues better predicts metal specificity than do the metal ligands. The allosteric mechanism of InrS permits greater promiscuity in vitro than CsoR. The factors dictating metal-selectivity in vivo are that K_Ni(II) and ΔG_C^{Ni(II)-InrS·DNA} are sufficiently high, relative to other metal sensors, for InrS to detect Ni(II), while the equivalent parameters for copper may be insufficient for copper-sensing in S ynechocystis (at 48 h). InrS K_Zn(II) (5.6 × 10⁻¹³ M) is comparable to the sensory sites of ZiaR (and Zur), but ΔG_C^{Zn(II)-InrS·DNA} is less than ΔG_C^{Zn(II)-ZiaR·DNA} implying that relative to other sensors, ΔG_C^{Zn(II)-Sensor·DNA} rather than K_Zn(II) determines the final detection threshold for Zn(II).

Recent developments in copper and zinc homeostasis in bacterial pathogens

Copper and zinc homeostasis systems in pathogenic bacteria are required to resist host efforts to manipulate the availability and toxicity of these metal ions. Central to this microbial adaptive response is the involvement of metal-trafficking and metal-sensing proteins that ultimately exercise control of metal speciation in the cell. Cu-specific and Zn-specific metalloregulatory proteins regulate the transcription of metal-responsive genes while metallochaperones and related proteins ensure that these metals are appropriately buffered by the intracellular milieu and delivered to correct intracellular targets. In this review, we summarize recent findings on how bacterial pathogens mount a metal-specific response to derail host efforts to win the 'fight over metals.'

Investigating the role of zinc and copper binding motifs of trafficking sites in the cyanobacterium Synechocystis PCC 6803

Although zinc and copper are required by proteins with very different functions, these metals can be delivered to cellular locations by homologous metal transporters within the same organism, as demonstrated by the cyanobacterial ( Synechocystis PCC 6803) zinc exporter ZiaA and thylakoidal copper importer PacS. The N-terminal metal-binding domains of these transporters (ZiaAN and PacSN, respectively) have related ferredoxin folds also found in the metallochaperone Atx1, which delivers copper to PacS, but differ in the residues found in their M/IXCXXC metal-binding motifs. To investigate the role of the nonconserved residues in this region on metal binding, the sequence from ZiaAN has been introduced into Atx1 and PacSN, and the motifs of Atx1 and PacSN swapped. The motif sequence can tune Cu(I) affinity only approximately 3-fold. However, the introduction of the ZiaAN motif (MDCTSC) dramatically increases the Zn(II) affinity of both Atx1 and PacSN by up to 2 orders of magnitude. The Atx1 mutant with the ZiaAN motif crystallizes as a side-to-side homodimer very similar to that found for [Cu(I)2-Atx1]2 ( Badarau et al. Biochemistry 2010 , 49 , 7798 ). In a crystal structure of the PacSN mutant possessing the ZiaAN motif (PacSN(ZiaAN)), the Asp residue from the metal-binding motif coordinates Zn(II). This demonstrates that the increased Zn(II) affinity of this variant and the high Zn(II) affinity of ZiaAN are due to the ability of the carboxylate to ligate this metal ion. Comparison of the Zn(II) sites in PacSN(ZiaAN) structures provides additional insight into Zn(II) trafficking in cyanobacteria.

Co(ll)-detection does not follow Kco(ll) gradient: channelling in Co(ll)-sensing

The MerR-like transcriptional activator CoaR detects surplus Co(ll) to regulate Co(ll) efflux in a cyanobacterium. This organism also has cytosolic metal-sensors from three further families represented by Zn(ll)-sensors ZiaR and Zur plus Ni(ll)-sensor InrS. Here we discover by competition with Fura-2 that CoaR has KCo(ll) weaker than 7 × 10(-8) M, which is weaker than ZiaR, Zur and InrS (KCo(ll) = 6.94 ± 1.3 × 10(-10) M; 4.56 ± 0.16 × 10(-10) M; and 7.69 ± 1.1 × 10(-9) M respectively). KCo(ll) for CoaR is also weak in the CoaR-DNA adduct. Further, Co(ll) promotes DNA-dissociation by ZiaR and DNA-association by Zur in vitro in a manner analogous to Zn(ll), as monitored by fluorescence anisotropy. After 48 h exposure to maximum non-inhibitory [Co(ll)], CoaR responds in vivo yet the two Zn(ll)-sensors do not, despite their tighter KCo(ll) and despite Co(ll) triggering allostery in ZiaR and Zur in vitro. These data imply that the two Zn(ll) sensors fail to respond because they fail to gain access to Co(ll) under these conditions in vivo. Several lines of evidence suggest that CoaR is membrane associated via a domain with sequence similarity to precorrin isomerase, an enzyme of vitamin B12 biosynthesis. Moreover, site directed mutagenesis reveals that transcriptional activation requires CoaR residues that are predicted to form hydrogen bonds to a tetrapyrrole. The Co(ll)-requiring vitamin B12 biosynthetic pathway is also membrane associated suggesting putative mechanisms by which Co(ll)-containing tetrapyrroles and/or Co(ll) ions are channelled to CoaR.

Crosstalk between Cu(I) and Zn(II) homeostasis via Atx1 and cognate domains

The copper metallochaperone Atx1 and the N-terminal metal-binding domain of a copper-transporting ATP-ase can form tight Zn(II)-mediated hetero-complexes in both cyanobacteria and humans. Copper and zinc homeostasis could be linked by metal binding to these CXXC-containing proteins.

Cytosolic Ni(II) sensor in cyanobacterium: nickel detection follows nickel affinity across four families of metal sensors

Efflux of surplus Ni(II) across the outer and inner membranes of Synechocystis PCC 6803 is mediated by the Nrs system under the control of a sensor of periplasmic Ni(II), NrsS. Here, we show that the product of ORF sll0176, which encodes a CsoR/RcnR-like protein now designated InrS (for internal nickel-responsive sensor), represses nrsD (NrsD is deduced to efflux Ni(II) across the inner membrane) from a cryptic promoter between the final two ORFs in the nrs operon. Transcripts initiated from the newly identified nrsD promoter accumulate in response to nickel or cobalt but not copper, and recombinant InrS forms specific, Ni(II)-inhibited complexes with the nrsD promoter region. Metal-dependent difference spectra of Ni(II)- and Cu(I)-InrS are similar to Cu(I)-sensing CsoR and dissimilar to Ni(II)/Co(II)-sensing RcnR, consistent with factors beyond the primary coordination sphere switching metal selectivity. Competition with chelators mag-fura-2, nitrilotriacetic acid, EDTA, and EGTA estimate K_D Ni(II) for the tightest site of InrS as 2.05 (±1.5) × 10⁻¹⁴m, and weaker K_D Ni(II) for the cells' metal sensors of other types: Zn(II) co-repressor Zur, Co(II) activator CoaR, and Zn(II) derepressor ZiaR. Ni(II) transfer to InrS occurs upon addition to Ni(II) forms of each other sensor. InrS binds Ni(II) sufficiently tightly to derepress Ni(II) export at concentrations below K_D Ni(II) of the other sensors.

Cyanobacterial metallochaperone inhibits deleterious side reactions of copper

Copper metallochaperones supply copper to cupro-proteins through copper-mediated protein-protein-interactions and it has been hypothesized that metallochaperones thereby inhibit copper from causing damage en route. Evidence is presented in support of this latter role for cyanobacterial metallochaperone, Atx1. In cyanobacteria Atx1 contributes towards the supply of copper to plastocyanin inside thylakoids but it is shown here that in copper-replete medium, copper can reach plastocyanin without Atx1. Unlike metallochaperone-independent copper-supply to superoxide dismutase in eukaryotes, glutathione is not essential for Atx1-independent supply to plastocyanin: Double mutants missing atx1 and gshB (encoding glutathione synthetase) accumulate the same number of atoms of copper per cell in the plastocyanin pool as wild type. Critically, Δatx1ΔgshB are hypersensitive to elevated copper relative to wild type cells and also relative to ΔgshB single mutants with evidence that hypersensitivity arises due to the mislocation of copper to sites for other metals including iron and zinc. The zinc site on the amino-terminal domain (ZiaA(N)) of the P(1)-type zinc-transporting ATPase is especially similar to the copper site of the Atx1 target PacS(N), and ZiaA(N) will bind Cu(I) more tightly than zinc. An NMR model of a substituted-ZiaA(N)-Cu(I)-Atx1 heterodimer has been generated making it possible to visualize a juxtaposition of residues surrounding the ZiaA(N) zinc site, including Asp(18), which normally repulse Atx1. Equivalent repulsion between bacterial copper metallochaperones and the amino-terminal regions of P(1)-type ATPases for metals other than Cu(I) is conserved, again consistent with a role for copper metallochaperones to withhold copper from binding sites for other metals.

Thermodynamics of copper and zinc distribution in the cyanobacterium Synechocystis PCC 6803

Copper is supplied to plastocyanin for photosynthesis and cytochrome c oxidase for respiration in the thylakoids of Synechocystis PCC 6803 by the membrane-bound P-type ATPases CtaA and PacS, and the metallochaperone Atx1. We have determined the Cu(I) affinities of all of the soluble proteins and domains in this pathway. The Cu(I) affinities of the trafficking proteins range from 5 × 10(16) to 5 × 10(17) M(-1) at pH 7.0, consistent with values for homologues. Unusually, Atx1 binds Cu(I) significantly tighter than the metal-binding domains (MBDs) of CtaA and PacS (CtaA(N) and PacS(N)), and equilibrium copper exchange constants of approximately 0.2 are obtained for transfer to the MBDs. Dimerization of Atx1 increases the affinity for Cu(I), but the loop 5 His61 residue has little influence. The MBD of the zinc exporter ZiaA (ZiaA(N)) exhibits an almost identical Cu(I) affinity, and Cu(I) exchange with Atx1, as CtaA(N) and PacS(N), and the relative stabilities of the complexes must enable the metallochaperone to distinguish between the MBDs. The binding of potentially competing zinc to the trafficking proteins has been studied. ZiaA(N) has the highest Zn(II) affinity and thermodynamics could be important for zinc removal from the cell. Plastocyanin has a Cu(I) affinity of 2.6 × 10(17) M(-1), 15-fold tighter than that of the Cu(A) site of cytochrome c oxidase, highlighting the need for specific mechanisms to ensure copper delivery to both of these targets. The narrow range of Cu(I) affinities for the cytoplasmic copper proteins in Synechocystis will facilitate relocation when copper is limiting.

Metalloregulatory proteins: metal selectivity and allosteric switching

Prokaryotic organisms have evolved the capacity to quickly adapt to a changing and challenging microenvironment in which the availability of both biologically required and non-essential transition metal ions can vary dramatically. In all bacteria, a panel of metalloregulatory proteins controls the expression of genes encoding membrane transporters and metal trafficking proteins that collectively manage metal homeostasis and resistance. These "metal sensors" are specialized allosteric proteins, in which the direct binding of a specific or small number of "cognate" metal ion(s) drives a conformational change in the regulator that allosterically activates or inhibits operator DNA binding, or alternatively, distorts the promoter structure thereby converting a poor promoter to a strong one. In this review, we discuss our current understanding of the features that control metal specificity of the allosteric response in these systems, and the role that structure, thermodynamics and conformational dynamics play in mediating allosteric activation or inhibition of DNA binding.

The CRR1 nutritional copper sensor in Chlamydomonas contains two distinct metal-responsive domains

Copper response regulator 1 (CRR1), an SBP-domain transcription factor, is a global regulator of nutritional copper signaling in Chlamydomonas reinhardtii and activates genes necessary during periods of copper deficiency. We localized Chlamydomonas CRR1 to the nucleus in mustard (Sinapis alba) seedlings, a location consistent with its function as a transcription factor. The Zn binding SBP domain of CRR1 binds copper ions in vitro. Cu(I) can replace Zn(II), but the Cu(II) form is unstable. The DNA binding activity is inhibited in vitro by Cu(II) or Hg(II) ions, which also prevent activation of transcription in vivo, but not by Co(II) or Ni(II), which have no effect in vivo. Copper inhibition of DNA binding is reduced by mutation of a conserved His residue. These results implicate the SBP domain in copper sensing. Deletion of a C-terminal metallothionein-like Cys-rich domain impacted neither nutritional copper signaling nor the effect of mercuric supplementation, but rendered CRR1 insensitive to hypoxia and to nickel supplementation, which normally activate the copper deficiency regulon in wild-type cells. Strains carrying the crr1-ΔCys allele upregulate ZRT genes and hyperaccumulate Zn(II), suggesting that the effect of nickel ions may be revealing a role for the C-terminal domain of CRR1 in zinc homeostasis in Chlamydomonas.