Characterization of a cobalt-specific P(1B)-ATPase

Comparative genome analyses uncovered the cadmium resistance mechanism of enterobacter cloacae

Cadmium (Cd) can be transported into plants from polluted soils and may cause animal and human diseases through food chains, which requires the development of highly efficient methods for soil Cd remediation. Although we isolated an Enterobacter cloacae strain Cu6 with Cd resistance, this strain cannot be used for soil Cd remediation due to its lower resistance. Here, we domesticated Cu6 and obtained a highly Cd-resistant strain, LPY6, and found that this strain can attenuate the toxic effects of Cd on wheat seedling growth. We deciphered the high Cd-resistance mechanism of LPY6 by genome comparative and genetic analysis. Compared with Cu6, 75 genes were mutated in LPY6. Thirty-four of these genes were deleted, and 41 had single nucleotide polymorphisms (SNPs). Most of these mutated proteins are involved in basic metabolism, substrate transport, stress response and formate and hydrogen metabolism. RNA quantitative analysis and promoter activity assays showed that the transcription or mRNA levels of two operons (cadA and norVW) in these mutated genes were regulated by Cd, zinc (Zn) or lead (Pb) ions, suggesting that these two operons might be required for Cd, Zn or Pb resistance. Expression of cadA and norVW operons in LPY6 partially recovered Cd susceptibility, demonstrating that CadA and NorVW are involved in Cd resistance in E. cloacae. Our findings illustrate that E. cloacae acquires Cd resistance through different pathways and lay a foundation for developing highly efficient methods for soil Cd remediation.

Reconsidering the czcD (NiCo) Riboswitch as an Iron Riboswitch

Recent work has proposed a new mechanism of bacterial iron regulation: riboswitches that undergo a conformational change in response to Fe^II. The czcD (NiCo) riboswitch was initially proposed to be specific for Ni^II and Co^II, but we recently showed via a czcD-based fluorescent sensor that Fe^II is also a plausible physiological ligand for this riboswitch class. Here, we provide direct evidence that this riboswitch class responds to Fe^II. Isothermal titration calorimetry studies of the native czcD riboswitches from three organisms show no response to Mn^II, a weak response to Zn^II, and similar dissociation constants (∼1 μM) and conformational responses for Fe^II, Co^II, and Ni^II. Only the iron response is in the physiological concentration regime; the riboswitches’ responses to Co^II, Ni^II, and Zn^II require 10³-, 10⁵-, and 10⁶-fold higher “free” metal ion concentrations, respectively, than the typical availability of those metal ions in cells. By contrast, the “Sensei” RNA, recently claimed to be an iron-specific riboswitch, exhibits no response to Fe^II. Our results demonstrate that iron responsiveness is a conserved property of czcD riboswitches and clarify that this is the only family of iron-responsive riboswitch identified to date, setting the stage for characterization of their physiological function.

A Novel Zinc Exporter CtpG Enhances Resistance to Zinc Toxicity and Survival in Mycobacterium bovis

Zinc is a microelement essential for the growth of almost all organisms, but it is toxic at high concentrations and represents an antimicrobial strategy for macrophages. Mycobacterium tuberculosis and Mycobacterium bovis are two well-known intracellular pathogens with strong environmental adaptability, including zinc toxicity. However, the signaling pathway and molecular mechanisms on sensing and resistance to zinc toxicity remains unclear in mycobacteria. Here, we first report that P_1B-type ATPase CtpG acts as a zinc efflux transporter and characterize a novel CmtR-CtpG-Zn²⁺ regulatory pathway that enhances mycobacterial resistance to zinc toxicity. We found that zinc upregulates ctpG expression via transcription factor CmtR and stimulates the ATPase activity of CtpG. The APC residues in TM6 is essential for CtpG to export zinc and enhance M. bovis BCG resistance to zinc toxicity. During infection, CtpG inhibits zinc accumulation in the mycobacteria, and aids bacterial survival in THP-1 macrophage and mice with elevated inflammatory responses. Our findings revealed the existence of a novel regulatory pathway on mycobacteria responding to and adapting to host-mediated zinc toxicity.

IMPORTANCE Tuberculosis is caused by the bacillus Mycobacterium tuberculosis and is one of the major sources of mortality. M. tuberculosis has developed unique mechanisms to adapt to host environments, including zinc deficiency and toxicity, during infection. However, the molecular mechanism by which mycobacteria promote detoxification of zinc, and the associated signaling pathways remains largely unclear. In this study, we first report that P_1B-type ATPase CtpG acts as a zinc efflux transporter and characterize a novel CmtR-CtpG-Zn²⁺ regulatory pathway that enhances mycobacterial resistance to zinc toxicity in M. bovis. Our findings reveal the existence of a novel excess zinc-triggered signaling circuit, provide new insights into mycobacterial adaptation to the host environment during infection, and might be useful targets for the treatment of tuberculosis.

Genomic Insights Into Cadmium Resistance of a Newly Isolated, Plasmid-Free Cellulomonas sp. Strain Y8

Our current knowledge on bacterial cadmium (Cd) resistance is mainly based on the functional exploration of specific Cd-resistance genes. In this study, we carried out a genomic study on Cd resistance of a newly isolated Cellulomonas strain with a MIC of 5 mM Cd. Full genome of the strain, with a genome size of 4.47 M bp and GC-content of 75.35%, was obtained through high-quality sequencing. Genome-wide annotations identified 54 heavy metal-related genes. Four potential Cd-resistance genes, namely zntAY8, copAY8, HMTY8, and czcDY8, were subjected to functional exploration. Quantitative PCR determination of in vivo expression showed that zntAY8, copAY8, and HMTY8 were strongly Cd-inducible. Expression of the three inducible genes against time and Cd concentrations were further quantified. It is found that zntAY8 responded more strongly to higher Cd concentrations, while expression of copAY8 and HMTY8 increased over time at lower Cd concentrations. Heterologous expression of the four genes in Cd-sensitive Escherichia coli led to different impacts on hosts’ Cd sorption, with an 87% reduction by zntAY8 and a 3.7-fold increase by HMTY8. In conclusion, a Cd-resistant Cellulomonas sp. strain was isolated, whose genome harbors a diverse panel of metal-resistance genes. Cd resistance in the strain is not controlled by a dedicated gene alone, but by several gene systems collectively whose roles are probably time- and dose-dependent. The plasmid-free, high-GC strain Y8 may provide a platform for exploring heavy metal genomics of the Cellulomonas genus.

Structure and ion-release mechanism of PIB-4-type ATPases

Transition metals, such as zinc, are essential micronutrients in all organisms, but also highly toxic in excessive amounts. Heavy-metal transporting P-type (P_IB) ATPases are crucial for homeostasis, conferring cellular detoxification and redistribution through transport of these ions across cellular membranes. No structural information is available for the P_IB-4-ATPases, the subclass with the broadest cargo scope, and hence even their topology remains elusive. Here we present structures and complementary functional analyses of an archetypal P_IB‑4‑ATPase, sCoaT from Sulfitobacter sp. NAS14-1. The data disclose the architecture, devoid of classical so-called heavy metal binding domains, and provides fundamentally new insights into the mechanism and diversity of heavy-metal transporters. We reveal several novel P-type ATPase features, including a dual role in heavy-metal release and as an internal counter ion of an invariant histidine. We also establish that the turn-over of P_IB‑ATPases is potassium independent, contrasting to many other P-type ATPases. Combined with new inhibitory compounds, our results open up for efforts in e.g. drug discovery, since P_IB-4-ATPases function as virulence factors in many pathogens.

The czcD (NiCo) Riboswitch Responds to Iron(II)

Iron is essential for nearly every organism, and mismanagement of its intracellular concentrations (either deficiency or excess) contributes to diminished virulence in human pathogens, necessitating intricate metalloregulatory mechanisms. To date, although several metal-responsive riboswitches have been identified in bacteria, none has been shown to respond to Fe^II. The czcD riboswitch, present in numerous human gut microbiota and pathogens, was recently shown to respond to Ni^II and Co^II but thought not to respond to Fe^II, on the basis of aerobic, in vitro assays; its function in vivo is not well understood. We constructed a fluorescent sensor using this riboswitch fused to the RNA aptamer, Spinach2. When assayed anaerobically, the resulting sensor responds in vitro to Fe^II, as well as to Mn^II, Co^II, Ni^II, and Zn^II, but only in the cases of Fe^II and Mn^II do the apparent K_d values (0.4 and 11 μM, respectively) fall within the range of labile metal concentrations maintained by known metalloregulators. We also show that the sensor-which is, to the best of our knowledge, the first reversible genetically encoded fluorescent sensor for Fe^II-responds to iron in Escherichia coli cells. Finally, we demonstrate that the putative metal exporters directly downstream of two czcD riboswitches efficiently rescue iron toxicity in a heterologous expression system. Together, our results indicate that iron merits consideration as a plausible physiological ligand for czcD riboswitches, although a response to general metal stress cannot be ruled out at present.

Asymmetric functions of a binuclear metal center within the transport pathway of a human zinc transporter ZIP4

Metal clusters are exploited by numerous metalloenzymes for catalysis, but it is not common to utilize a metal cluster for substrate transport across membrane. The recent crystal structure of a prototypic Zrt-/Irt-like protein (ZIP) metal transporter from Bordetella bronchiseptica (BbZIP) revealed an unprecedented binuclear metal center (BMC) within the transport pathway. Here, through a combination of bioinformatics, biochemical and structural approaches, we concluded that the two physically associated metal-binding sites in the BMC of human ZIP4 (hZIP4) zinc transporter exert different functions: one conserved transition metal-binding site acts as the transport site essential for activity, whereas the variable metal-binding site is required for hZIP4's optimal activity presumably by serving as a secondary transport site and modulating the properties of the primary transport site. Sequential soaking experiments on BbZIP crystals clarified the process of metal release from the BMC to the bulky solvent. This work provides important insights into the transport mechanism of the ZIPs broadly involved in transition metal homeostasis and signaling, and also a paradigm on a novel function of metal cluster in metalloproteins.

Spectroscopic characterization of the Co-substituted C-terminal domain of rubredoxin-2

Pseudomonas putida rubredoxin-2 (Rxn2) is an essential member of the alkane hydroxylation pathway and transfers electrons from a reductase to the membrane-bound hydroxylase. The regioselective hydroxylation of linear alkanes is a challenging chemical transformation of great interest for the chemical industry. Herein, we report the preparation and spectroscopic characterization of cobalt-substituted P. putida Rxn2 and a truncated version of the protein consisting of the C-terminal domain of the protein. Our spectroscopic data on the Co-substituted C-terminal domain supports a high-spin Co(II) with a distorted tetrahedral coordination environment. Investigation of the two-domain protein Rxn2 provides insights into the metal-binding properties of the N-terminal domain, the role of which is not well understood so far. Circular dichroism, electron paramagnetic resonance and X-ray absorption spectroscopies support an alternative Co-binding site within the N-terminal domain, which appears to not be relevant in nature. We have shown that chemical reconstitution in the presence of Co leads to incorporation of Co(II) into the active site of the C-terminal domain, but not the N-terminal domain of Rxn2 indicating distinct roles for the two rubredoxin domains.

Cu+-specific CopB transporter: Revising P1B-type ATPase classification

The copper-transporting P_1B-ATPases, which play a key role in cellular copper homeostasis, have been divided traditionally into two subfamilies, the P_1B-1-ATPases or CopAs and the P_1B-3-ATPases or CopBs. CopAs selectively export Cu⁺ whereas previous studies and bioinformatic analyses have suggested that CopBs are specific for Cu²⁺ export. Biochemical and spectroscopic characterization of Sphaerobacter thermophilus CopB (StCopB) show that, while it does bind Cu²⁺, the binding site is not the prototypical P_1B-ATPase transmembrane site and does not involve sulfur coordination as proposed previously. Most important, StCopB exhibits metal-stimulated ATPase activity in response to Cu⁺, but not Cu²⁺, indicating that it is actually a Cu⁺ transporter. X-ray absorption spectroscopic studies indicate that Cu⁺ is coordinated by four sulfur ligands, likely derived from conserved cysteine and methionine residues. The histidine-rich N-terminal region of StCopB is required for maximal activity, but is inhibitory in the presence of divalent metal ions. Finally, reconsideration of the P_1B-ATPase classification scheme suggests that the P_1B-1- and P_1B-3-ATPase subfamilies both comprise Cu⁺ transporters. These results are completely consistent with the known presence of only Cu⁺ within the reducing environment of the cytoplasm, which should eliminate the need for a Cu²⁺ P_1B-ATPase.

Ferrous iron efflux systems in bacteria

Bacteria require iron for growth, with only a few reported exceptions. In many environments, iron is a limiting nutrient for growth and high affinity uptake systems play a central role in iron homeostasis. However, iron can also be detrimental to cells when it is present in excess, particularly under aerobic conditions where its participation in Fenton chemistry generates highly reactive hydroxyl radicals. Recent results have revealed a critical role for iron efflux transporters in protecting bacteria from iron intoxication. Systems that efflux iron are widely distributed amongst bacteria and fall into several categories: P1B-type ATPases, cation diffusion facilitator (CDF) proteins, major facilitator superfamily (MFS) proteins, and membrane bound ferritin-like proteins. Here, we review the emerging role of iron export in both iron homeostasis and as part of the adaptive response to oxidative stress.

A novel heavy metal ATPase peptide from Prosopis juliflora is involved in metal uptake in yeast and tobacco

Heavy metal pollution of agricultural soils is one of the most severe ecological problems in the world. Prosopis juliflora, a phreatophytic tree species, grows well in heavy metal laden industrial sites and is known to accumulate heavy metals. Heavy Metal ATPases (HMAs) are ATP driven heavy metal pumps that translocate heavy metals across biological membranes thus helping the plant in heavy metal tolerance and phytoremediation. In the present study we have isolated and characterized a novel 28.9 kDa heavy metal ATPase peptide (PjHMT) from P. juliflora which shows high similarity to the C-terminal region of P1B ATPase HMA1. It also shows the absence of the invariant signature sequence DKTGT, and the metal binding CPX motif but the presence of conserved regions like MVGEGINDAPAL (ATP binding consensus sequence), HEGGTLLVCLNS (metal binding domain) and MLTGD, GEGIND and HEGG motifs which play important roles in metal transport or ATP binding. PjHMT, was found to be upregulated under cadmium and zinc stress. Heterologous expression of PjHMT in yeast showed a higher accumulation and tolerance of heavy metals in yeast. Further, transgenic tobacco plants constitutively expressing PjHMT also showed increased accumulation and tolerance to cadmium. Thus, this study suggests that the transport peptide from P. juliflora may have an important role in Cd uptake and thus in phytoremediation.

Metal Selectivity of a Cd-, Co-, and Zn-Transporting P1B-type ATPase

The P_1B-ATPases, a family of transmembrane metal transporters important for transition metal homeostasis in all organisms, are subdivided into classes based on sequence conservation and metal specificity. The multifunctional P_1B-4-ATPase CzcP is part of the cobalt, zinc, and cadmium resistance system from the metal-tolerant, model organism Cupriavidus metallidurans. Previous work revealed the presence of an unusual soluble metal-binding domain (MBD) at the CzcP N-terminus, but the nature, extent, and selectivity of the transmembrane metal-binding site (MBS) of CzcP have not been resolved. Using homology modeling, we show that four wholly conserved amino acids from the transmembrane (TM) domain (Met²⁵⁴, Ser⁴⁷⁴, Cys⁴⁷⁶, and His⁸⁰⁷) are logical candidates for the TM MBS, which may communicate with the MBD via interactions with the first TM helix. Metal-binding analyses indicate that wild-type (WT) CzcP has three MBSs, and data on N-terminally truncated (ΔMBD) CzcP suggest the presence of a single TM MBS. Electronic absorption and electron paramagnetic resonance spectroscopic analyses of ΔMBD CzcP and variant proteins thereof provide insight into the details of Co²⁺ coordination by the TM MBS. These spectroscopic data, combined with in vitro functional studies of WT and variant CzcP proteins, show that the side chains of Met²⁵⁴, Cys⁴⁷⁶, and His⁸⁰⁷ contribute to Cd²⁺, Co²⁺, and Zn²⁺ binding and transport, whereas the side chain of Ser⁴⁷⁴ appears to play a minimal role. By comparison to other P_1B-4-ATPases, we suggest that an evolutionarily adapted flexibility in the TM region likely afforded CzcP the ability to transport Cd²⁺ and Zn²⁺ in addition to Co²⁺.

The Listeria monocytogenes Fur-regulated virulence protein FrvA is an Fe(II) efflux P1B4 -type ATPase

Listeria monocytogenes FrvA (Lmo0641) is critical for virulence in the mouse model and is an ortholog of the Bacillus subtilis Fur- and PerR-regulated Fe(II) efflux P1B4 -type ATPase PfeT. Previously, FrvA was suggested to protect against heme toxicity. Here, we demonstrate that an frvA mutant is sensitive to iron intoxication, but not to other metals. Expression of frvA is induced by high iron and this induction requires Fur. FrvA functions in vitro as a divalent cation specific ATPase most strongly activated by ferrous iron. When expressed in B. subtilis, FrvA increases resistance to iron both in wild-type and in a pfeT null strain. FrvA is a high affinity Fe(II) exporter and its induction imposes severe iron limitation in B. subtilis resulting in derepression of both Fur- and PerR-regulated genes. FrvA also recognizes Co(II) and Zn(II) as substrates and can complement B. subtilis strains defective in the endogenous export systems for these cations. Building on these results, we conclude that FrvA functions in the efflux of Fe(II), and not heme during listerial infection.

Fine-tuning of Substrate Affinity Leads to Alternative Roles of Mycobacterium tuberculosis Fe2+-ATPases

Little is known about iron efflux transporters within bacterial systems. Recently, the participation of Bacillus subtilis PfeT, a P1B4-ATPase, in cytoplasmic Fe(2+) efflux has been proposed. We report here the distinct roles of mycobacterial P1B4-ATPases in the homeostasis of Co(2+) and Fe(2+) Mutation of Mycobacterium smegmatis ctpJ affects the homeostasis of both ions. Alternatively, an M. tuberculosis ctpJ mutant is more sensitive to Co(2+) than Fe(2+), whereas mutation of the homologous M. tuberculosis ctpD leads to Fe(2+) sensitivity but no alterations in Co(2+) homeostasis. In vitro, the three enzymes are activated by both Fe(2+) and Co(2+) and bind 1 eq of either ion at their transport site. However, equilibrium binding affinities and activity kinetics show that M. tuberculosis CtpD has higher affinity for Fe(2+) and twice the Fe(2+)-stimulated activity than the CtpJs. These parameters are paralleled by a lower activation and affinity for Co(2+) Analysis of Fe(2+) and Co(2+) binding to CtpD by x-ray absorption spectroscopy shows that both ions are five- to six-coordinate, constrained within oxygen/nitrogen environments with similar geometries. Mutagenesis studies suggest the involvement of invariant Ser, His, and Glu residues in metal coordination. Interestingly, replacement of the conserved Cys at the metal binding pocket leads to a large reduction in Fe(2+) but not Co(2+) binding affinity. We propose that CtpJ ATPases participate in the control of steady state Fe(2+) levels. CtpD, required for M. tuberculosis virulence, is a high affinity Fe(2+) transporter involved in the rapid response to iron dyshomeostasis generated upon redox stress.

Assay of Copper Transfer and Binding to P1B-ATPases

P1B-type ATPases transport transition metals across biological membranes. The chemical characteristics of these substrates, as well as, physiological requirements have contributed to the evolution of high metal binding affinities (fM) in these enzymes. Metal binding determinations are consequently facilitated by the stable metal-protein interaction, while affinity measurements require careful analysis of metal levels. In the cell, transition metals are associated with chaperone proteins. Metals reach the ATPase transport sites following specific protein-protein interactions and ligand exchange enabling the metal transfer from the chaperone to the transporter. Here, we describe methods for analyzing the binding of Cu(+) to Cu(+)-ATPases, as well as the approach to monitor Cu(+) transfer from soluble Cu(+)-chaperones donors to and from membrane Cu(+)-ATPases.

Structure and Function of Cu(I)- and Zn(II)-ATPases

Copper and zinc are micronutrients essential for the function of many enzymes while also being toxic at elevated concentrations. Cu(I)- and Zn(II)-transporting P-type ATPases of subclass 1B are of key importance for the homeostasis of these transition metals, allowing ion transport across cellular membranes at the expense of ATP. Recent biochemical studies and crystal structures have significantly improved our understanding of the transport mechanisms of these proteins, but many details about their structure and function remain elusive. Here we compare the Cu(I)- and Zn(II)-ATPases, scrutinizing the molecular differences that allow transport of these two distinct metal types, and discuss possible future directions of research in the field.

A new metal binding domain involved in cadmium, cobalt and zinc transport

The P_1B-ATPases, which couple cation transport across membranes to ATP hydrolysis, are central to metal homeostasis in all organisms. An important feature of P_1B-ATPases is the presence of soluble metal binding domains that regulate transport activity. Only one type of MBD has been characterized extensively, but bioinformatics analyses indicate that a diversity of MBDs may exist in nature. Here we report the biochemical, structural, and functional characterization of a new MBD from the Cupriavidus metallidurans P_1B-4-ATPase CzcP (CzcP MBD). The CzcP MBD binds two Cd²⁺, Co²⁺, or Zn²⁺ ions in distinct and unique sites, and adopts an unexpected fold consisting of two fused ferredoxin-like domains. Both in vitro and in vivo activity assays using full length CzcP, truncated CzcP, and several variants indicate a regulatory role for the MBD and distinct functions for the two metal binding sites. Taken together, these findings elucidate a previously unknown MBD and suggest new regulatory mechanisms for metal transport by P_1B-ATPases.

A sulfur-based transport pathway in Cu+-ATPases

Cells regulate copper levels tightly to balance the biogenesis and integrity of copper centers in vital enzymes against toxic levels of copper. PIB -type Cu(+)-ATPases play a central role in copper homeostasis by catalyzing the selective translocation of Cu(+) across cellular membranes. Crystal structures of a copper-free Cu(+)-ATPase are available, but the mechanism of Cu(+) recognition, binding, and translocation remains elusive. Through X-ray absorption spectroscopy, ATPase activity assays, and charge transfer measurements on solid-supported membranes using wild-type and mutant forms of the Legionella pneumophila Cu(+)-ATPase (LpCopA), we identify a sulfur-lined metal transport pathway. Structural analysis indicates that Cu(+) is bound at a high-affinity transmembrane-binding site in a trigonal-planar coordination with the Cys residues of the conserved CPC motif of transmembrane segment 4 (C382 and C384) and the conserved Met residue of transmembrane segment 6 (M717 of the MXXXS motif). These residues are also essential for transport. Additionally, the studies indicate essential roles of other conserved intramembranous polar residues in facilitating copper binding to the high-affinity site and subsequent release through the exit pathway.

Diversity of the metal-transporting P1B-type ATPases

The P1B-ATPases are integral membrane proteins that couple ATP hydrolysis to metal cation transport. Widely distributed across all domains of life, these enzymes have been previously shown to transport copper, zinc, cobalt, and other thiophilic heavy metals. Recent data suggest that these enzymes may also be involved in nickel and/or iron transport. Here we have exploited large amounts of genomic data to examine and classify the various P1B-ATPase subfamilies. Specifically, we have combined new methods of data partitioning and network visualization known as Transitivity Clustering and Protein Similarity Networks with existing biochemical data to examine properties such as length, speciation, and metal-binding motifs of the P1B-ATPase subfamily sequences. These data reveal interesting relationships among the enzyme sequences of previously established subfamilies, indicate the presence of two new subfamilies, and suggest the existence of new regulatory elements in certain subfamilies. Taken together, these findings underscore the importance of P1B-ATPases in homeostasis of nearly every biologically relevant transition metal and provide an updated framework for future studies.

Differential roles for the Co(2+) /Ni(2+) transporting ATPases, CtpD and CtpJ, in Mycobacterium tuberculosis virulence

The genome of Mycobacterium tuberculosis encodes two paralogous P1 B 4 -ATPases, CtpD (Rv1469) and CtpJ (Rv3743). Both proteins showed ATPase activation by Co(2+) and Ni(2+) , and both appear to be required for metal efflux from the cell. However, using a combination of biochemical and genetic studies we found that these proteins play non-redundant roles in virulence and metal efflux. CtpJ expression is induced by Co(2+) and this protein possesses a relatively high turnover rate. A ctpJ deletion mutant accumulated Co(2+) , indicating that this ATPase controls cytoplasmic metal levels. In contrast, CtpD expression is induced by redox stressors and this protein displays a relatively low turnover rate. A ctpD mutant failed to accumulate metal, suggesting an alternative cellular function. ctpD is cotranscribed with two thioredoxin genes trxA (Rv1470), trxB (Rv1471), and an enoyl-coA hydratase (Rv1472), indicating a possible role for CtpD in the metallation of these redox-active proteins. Supporting this, in vitro metal binding assays showed that TrxA binds Co(2+) and Ni(2+) . Mutation of ctpD, but not ctpJ, reduced bacterial fitness in the mouse lung, suggesting that redox maintenance, but not Co(2+) accumulation, is important for growth in vivo.

Sinorhizobium meliloti Nia is a P(1B-5)-ATPase expressed in the nodule during plant symbiosis and is involved in Ni and Fe transport

The P1B-ATPases are a ubiquitous family of metal transporters. These transporters are classified into subfamilies on the basis of substrate specificity, which is conferred by conserved amino acids in the last three transmembrane domains. Five subfamilies have been identified to date, and representative members of four (P1B-1 to P1B-4) have been studied. The fifth family (P1B-5), of which some members contain a C-terminal hemerythrin (Hr) domain, is less well characterized. The S. meliloti Sma1163 gene encodes for a P1B-5-ATPase, denoted Nia (Nickel-iron ATPase), that is induced by exogenous Fe(2+) and Ni(2+). The nia mutant accumulates nickel and iron, suggesting a possible role in detoxification of these two elements under free-living conditions, as well as in symbiosis, when the highest expression levels are measured. This function is supported by an inhibitory effect of Fe(2+) and Ni(2+) on the pNPPase activity, and by the ability of Nia to bind Fe(2+) in the transmembrane domain. Optical and X-ray absorption spectroscopic studies of the isolated Hr domain confirm the presence of a dinuclear iron center and suggest that this domain might function as an iron sensor.

A novel P(1B)-type Mn2+-transporting ATPase is required for secreted protein metallation in mycobacteria

Transition metals are central for bacterial virulence and host defense. P(1B)-ATPases are responsible for cytoplasmic metal efflux and play roles either in limiting cytosolic metal concentrations or in the maturation of secreted metalloproteins. The P(1B)-ATPase, CtpC, is required for Mycobacterium tuberculosis survival in a mouse model (Sassetti, C. M., and Rubin, E. J. (2003) Genetic requirements for mycobacterial survival during infection. Proc. Natl. Acad. Sci. U.S.A. 100, 12989-12994). CtpC prevents Zn(2+) toxicity, suggesting a role in Zn(2+) export from the cytosol (Botella, H., Peyron, P., Levillain, F., Poincloux, R., Poquet, Y., Brandli, I., Wang, C., Tailleux, L., Tilleul, S., Charriere, G. M., Waddell, S. J., Foti, M., Lugo-Villarino, G., Gao, Q., Maridonneau-Parini, I., Butcher, P. D., Castagnoli, P. R., Gicquel, B., de Chastellièr, C., and Neyrolles, O. (2011) Mycobacterial P1-type ATPases mediate resistance to zinc poisoning in human macrophages. Cell Host Microbe 10, 248-259). However, key metal-coordinating residues and the overall structure of CtpC are distinct from Zn(2+)-ATPases. We found that isolated CtpC has metal-dependent ATPase activity with a strong preference for Mn(2+) over Zn(2+). In vivo, CtpC is unable to complement Escherichia coli lacking a functional Zn(2+)-ATPase. Deletion of M. tuberculosis or Mycobacterium smegmatis ctpC leads to cytosolic Mn(2+) accumulation but no alterations in other metals levels. Whereas ctpC-deficient M. tuberculosis is sensitive to extracellular Zn(2+), the M. smegmatis mutant is not. Both ctpC mutants are sensitive to oxidative stress, which might explain the Zn(2+)-sensitive phenotype of the M. tuberculosis ctpC mutant. CtpC is a high affinity/slow turnover ATPase, suggesting a role in protein metallation. Consistent with this hypothesis, mutation of CtpC leads to a decrease of Mn(2+) bound to secreted proteins and of the activity of secreted Fe/Mn-superoxide dismutase, particularly in M. smegmatis. Alterations in the assembly of metalloenzymes involved in redox stress response might explain the sensitivity of M. tuberculosis ctpC mutants to oxidative stress and growth and persistence defects in mice infection models.