Cys-113 and Cys-422 form a high affinity metalloid binding site in the ArsA ATPase

Intradimeric Walker A ATPases: Conserved Features of A Functionally Diverse Family

Nucleoside-triphosphate hydrolases (NTPases) are a diverse, but essential group of enzymes found in all living organisms. NTPases that have a G-X-X-X-X-G-K-[S/T] consensus sequence (where X is any amino acid), known as the Walker A or P-loop motif, constitute a superfamily of P-loop NTPases. A subset of ATPases within this superfamily contains a modified Walker A motif, X-K-G-G-X-G-K-[S/T], wherein the first invariant lysine residue is essential to stimulate nucleotide hydrolysis. Although the proteins in this subset have vastly differing functions, ranging from electron transport during nitrogen fixation to targeting of integral membrane proteins to their correct membranes, they have evolved from a shared ancestor and have thus retained common structural features that affect their functions. These commonalities have only been disparately characterized in the context of their individual proteins systems, but have not been generally annotated as features that unite the members of this family. In this review, we report an analysis based on the sequences, structures, and functions of several members in this family that highlight their remarkable similarities. A principal feature of these proteins is their dependence on homodimerization. Since their functionalities are heavily influenced by changes that happen in conserved elements at the dimer interface, we refer to the members of this subclass as intradimeric Walker A ATPases.

NemA Catalyzes Trivalent Organoarsenical Oxidation and Is Regulated by the Trivalent Organoarsenical-Selective Transcriptional Repressor NemR

Synthetic aromatic arsenicals such as roxarsone (Rox(V)) and nitarsone (Nit(V)) have been used as animal growth enhancers and herbicides. Microbes contribute to redox cycling between the relatively less toxic pentavalent and highly toxic trivalent arsenicals. In this study, we report the identification of nemRA operon from Enterobacter sp. Z1 and show that it is involved in trivalent organoarsenical oxidation. Expression of nemA is induced by chromate (Cr(VI)), Rox(III), and Nit(III). Heterologous expression of NemA in Escherichia coli confers resistance to Cr(VI), methylarsenite (MAs(III)), Rox(III), and Nit(III). Purified NemA catalyzes simultaneous Cr(VI) reduction and MAs(III)/Rox(III)/Nit(III) oxidation, and oxidation was enhanced in the presence of Cr(VI). The results of electrophoretic mobility shift assays and fluorescence assays demonstrate that the transcriptional repressor, NemR, binds to either Rox(III) or Nit(III). NemR has three conserved cysteine residues, Cys21, Cys106, and Cys116. Mutation of any of the three resulted in loss of response to Rox(III)/Nit(III), indicating that they form an Rox(III)/Nit(III) binding site. These results show that NemA is a novel trivalent organoarsenical oxidase that is regulated by the trivalent organoarsenical-selective repressor NemR. This discovery expands our knowledge of the molecular mechanisms of organoarsenical oxidation and provides a basis for studying the redox coupling of environmental toxic compounds.

Functional and structural characterization of AntR, an Sb(III) responsive transcriptional repressor

The ant operon of the antimony-mining bacterium Comamonas testosterone JL40 confers resistance to Sb(III). The operon is transcriptionally regulated by the product of the first gene in the operon, antR. AntR is a member of ArsR/SmtB family of metal/metalloid-responsive repressors resistance. We purified and characterized C. testosterone AntR and demonstrated that it responds to metalloids in the order Sb(III) = methylarsenite (MAs(III) >> As(III)). The protein was crystallized, and the structure was solved at 2.1 Å resolution. The homodimeric structure of AntR adopts a classical ArsR/SmtB topology architecture. The protein has five cysteine residues, of which Cys103_a from one monomer and Cys113_b from the other monomer, are proposed to form one Sb(III) binding site, and Cys113_a and Cys103_b forming a second binding site. This is the first report of the structure and binding properties of a transcriptional repressor with high selectivity for environmental antimony.

Comamonas testosteroni antA encodes an antimonite-translocating P-type ATPase

Antimony, like arsenic, is a toxic metalloid widely distributed in the environment. Microbial detoxification of antimony has recently been identified. Here we describe a novel bacterial P_1B-type antimonite (Sb(III))-translocating ATPase from the antimony-mining bacterium Comamonas testosterone JL40 that confers resistance to Sb(III). In a comparative proteomics analysis of strain JL40, an operon (ant operon) was up-regulated by Sb(III). The ant operon includes three genes, antR, antC and antA. AntR belongs to the ArsR/SmtB family of metalloregulatory proteins that regulates expression of the ant operon. AntA belongs to the P_1B family of the P-type cation-translocating ATPases. It has both similarities to and differences from other members of the P_1B-1 subfamily and appears to be the first identified member of a distinct subfamily that we designate P_1B-8. Expression AntA in E. coli AW3110 (Δars) conferred resistance to Sb(III) and reduced the intracellular concentration of Sb(III) but not As(III) or other metals. Everted membrane vesicles from cells expressing antA accumulated Sb(III) but not As(III), where uptake in everted vesicles reflects efflux from cells. AntC is a small protein with a potential Sb(III) binding site, and co-expression of AntC with AntA increased resistance to Sb(III). We propose that AntC functions as an Sb(III) chaperone to AntA, augmenting Sb(III) efflux. The identification of a novel Sb(III)-translocating ATPase enhances our understanding of the biogeochemical cycling of environmental antimony by bacteria.

Transition Metal Homeostasis

This chapter focuses on transition metals. All transition metal cations are toxic-those that are essential for Escherichia coli and belong to the first transition period of the periodic system of the element and also the "toxic-only" metals with higher atomic numbers. Common themes are visible in the metabolism of these ions. First, there is transport. High-rate but low-affinity uptake systems provide a variety of cations and anions to the cells. Control of the respective systems seems to be mainly through regulation of transport activity (flux control), with control of gene expression playing only a minor role. If these systems do not provide sufficient amounts of a needed ion to the cell, genes for ATP-hydrolyzing high-affinity but low-rate uptake systems are induced, e.g., ABC transport systems or P-type ATPases. On the other hand, if the amount of an ion is in surplus, genes for efflux systems are induced. By combining different kinds of uptake and efflux systems with regulation at the levels of gene expression and transport activity, the concentration of a single ion in the cytoplasm and the composition of the cellular ion "bouquet" can be rapidly adjusted and carefully controlled. The toxicity threshold of an ion is defined by its ability to produce radicals (copper, iron, chromate), to bind to sulfide and thiol groups (copper, zinc, all cations of the second and third transition period), or to interfere with the metabolism of other ions. Iron poses an exceptional metabolic problem due its metabolic importance and the low solubility of Fe(III) compounds, combined with the ability to cause dangerous Fenton reactions. This dilemma for the cells led to the evolution of sophisticated multi-channel iron uptake and storage pathways to prevent the occurrence of unbound iron in the cytoplasm. Toxic metals like Cd2+ bind to thiols and sulfide, preventing assembly of iron complexes and releasing the metal from iron-sulfur clusters. In the unique case of mercury, the cation can be reduced to the volatile metallic form. Interference of nickel and cobalt with iron is prevented by the low abundance of these metals in the cytoplasm and their sequestration by metal chaperones, in the case of nickel, or by B12 and its derivatives, in the case of cobalt. The most dangerous metal, copper, catalyzes Fenton-like reactions, binds to thiol groups, and interferes with iron metabolism. E. coli solves this problem probably by preventing copper uptake, combined with rapid efflux if the metal happens to enter the cytoplasm.

Mutations in the ArsA ATPase that restore interaction with the ArsD metallochaperone

The ArsA ATPase is the catalytic subunit of the ArsAB As(III) efflux pump. It receives trivalent As(III) from the intracellular metallochaperone ArsD. The interaction of ArsA and ArsD allows for resistance to As(III) at environmental concentrations. A quadruple mutant in the arsD gene encoding a K2A/K37A/K62A/K104A ArsD is unable to interact with ArsA. An error-prone mutagenesis approach was used to generate random mutations in the arsA gene that restored interaction with the quadruple arsD mutant in yeast two-hybrid assays. A number of arsA genes with multiple mutations were isolated. These were analyzed in more detail by separation into single arsA mutants. Three such mutants encoding Q56R, F120I and D137V ArsA were able to restore interaction with the quadruple ArsD mutant in yeast two-hybrid assays. Each of the three single ArsA mutants also interacted with wild type ArsD. Only the Q56R ArsA derivative exhibited significant metalloid-stimulated ATPase activity in vitro. Purified Q56R ArsA was stimulated by wild type ArsD and to a lesser degree by the quadruple ArsD derivative. The F120I and D137V ArsAs did not show metalloid-stimulated ATPase activity. Structural models generated by in silico docking suggest that an electrostatic interface favors reversible interaction between ArsA and ArsD. We predict that mutations in ArsA propagate changes in hydrogen bonding and salt bridges to the ArsA-ArsD interface that affect their interactions.

Pathways of arsenic uptake and efflux

Arsenic is the most prevalent environmental toxic substance and ranks first on the U.S. Environmental Protection Agency's Superfund List. Arsenic is a carcinogen and a causative agent of numerous human diseases. Paradoxically arsenic is used as a chemotherapeutic agent for treatment of acute promyelocytic leukemia. Inorganic arsenic has two biological important oxidation states: As(V) (arsenate) and As(III) (arsenite). Arsenic uptake is adventitious because the arsenate and arsenite are chemically similar to required nutrients. Arsenate resembles phosphate and is a competitive inhibitor of many phosphate-utilizing enzymes. Arsenate is taken up by phosphate transport systems. In contrast, at physiological pH, the form of arsenite is As(OH)(3), which resembles organic molecules such as glycerol. Consequently, arsenite is taken into cells by aquaglyceroporin channels. Arsenic efflux systems are found in nearly every organism and evolved to rid cells of this toxic metalloid. These efflux systems include members of the multidrug resistance protein family and the bacterial exchangers Acr3 and ArsB. ArsB can also be a subunit of the ArsAB As(III)-translocating ATPase, an ATP-driven efflux pump. The ArsD metallochaperone binds cytosolic As(III) and transfers it to the ArsA subunit of the efflux pump. Knowledge of the pathways and transporters for arsenic uptake and efflux is essential for understanding its toxicity and carcinogenicity and for rational design of cancer chemotherapeutic drugs.

Microbial interactions in the arsenic cycle: adoptive strategies and applications in environmental management

Arsenic (As) is a nonessential element that is often present in plants and in other organisms. However, it is one of the most hazardous of toxic elements globally. In many parts of the world, arsenic contamination in groundwater is a serious and continuing threat to human health. Microbes play an important role in regulating the environmental fate of arsenic. Different microbial processes influence the biogeochemical cycling of arsenic in ways that affect the accumulation of different arsenic species in various ecosystem compartments. For example, in soil, there are bacteria that methylate arsenite to trimethylarsine gas, thereby releasing arsenic to the atmosphere.In marine ecosystems, microbes exist that can convert inorganic arsenicals to organic arsenicals (e.g., di- and tri-methylated arsenic derivatives, arsenocholine,arsenobetaine, arsenosugars, arsenolipids). The organo arsenicals are further metabolized to complete the arsenic cycle.Microbes have developed various strategies that enable them to tolerate arsenic and to survive in arsenic-rich environments. Such strategies include As exclusion from cells by establishing permeability barrier, intra- and extracellular sequestration,active efflux pumps, enzymatic reduction, and reduction in the sensitivity of cellular targets. These strategies are used either singly or in combination. In bacteria,the genes for arsenic resistance/detoxification are encoded by the arsenic resistance operons (ars operon).In this review, we have addressed and emphasized the impact of different microbial processes (e.g., arsenite oxidation, cytoplasmic arsenate reduction, respiratory arsenate reduction, arsenite methylation) on the arsenic cycle. Microbes are the only life forms reported to exist in heavy arsenic-contaminated environments. Therefore,an understanding of the strategies adopted by microbes to cope with arsenic stress is important in managing such arsenic-contaminated sites. Further future insights into the different microbial genes/proteins that are involved in arsenic resistance may also be useful for developing arsenic resistant crop plants.

Efflux permease CgAcr3-1 of Corynebacterium glutamicum is an arsenite-specific antiporter

Resistance to arsenite (As(III)) by cells is generally accomplished by arsenite efflux permeases from Acr3 or ArsB unrelated families. We analyzed the function of three Acr3 proteins from Corynebacterium glutamicum, CgAcr3-1, CgAcr3-2, and CgAcr3-3. CgAcr3-1 conferred the highest level of As(III) resistance and accumulation in vivo. CgAcr3-1 was also the most active when everted membranes vesicles from Escherichia coli or C. glutamicum mutants were assayed for efflux with different energy sources. As(III) and antimonite (Sb(III)) resistance and accumulation studies using E. coli or C. glutamicum arsenite permease mutants clearly show that CgAcr3-1 is specific for As(III). In everted membrane vesicles expressing CgAcr3-1, dissipation of either the membrane potential or the pH gradient of the proton motive force did not prevent As(III) uptake, whereas dissipation of both components eliminated uptake. Further, a mutagenesis study of CgAcr3-1 suggested that a conserved cysteine and glutamate are involved in active transport. Therefore, we propose that CgAcr3-1 is an antiporter that catalyzes arsenite-proton exchange with residues Cys129 and Glu305 involved in efflux.

Mammalian metallothionein in toxicology, cancer, and cancer chemotherapy

The present paper centers on mammalian metallothionein 1 and 2 in relationship to cell and tissue injury beginning with its reaction with Cd²⁺ and then considering its role in the toxicology and chemotherapy of both metals and non-metal electrophiles and oxidants. Intertwined is a consideration of MTs role in tumor cell Zn²⁺ metabolism. The paper updates and expands on our recent review by Petering et al. (Met Ions Life Sci 5:353-398, 2009).

The ArsD As(III) metallochaperone

Arsenic, a toxic metalloid widely existing in the environment, causes a variety of health problems. The ars operon encoded by Escherichia coli plasmid R773 has arsD and arsA genes, where ArsA is an ATPase that is the catalytic subunit of the ArsAB As(III) extrusion pump, and ArsD is an arsenic chaperone for ArsA. ArsD transfers As(III) to ArsA and increases the affinity of ArsA for As(III), allowing resistance to environmental concentrations of arsenic. Cys12, Cys13 and Cys18 in ArsD form a three sulfur-coordinated As(III) binding site that is essential for metallochaperone activity. ATP hydrolysis by ArsA is required for transfer of As(III) from ArsD to ArsA, suggesting that transfer occurs with a conformation of ArsA that transiently forms during the catalytic cycle. The 1.4 Å x-ray crystal structure of ArsD shows a core of four β-strands flanked by four α-helices in a thioredoxin fold. Docking of ArsD with ArsA was modeled in silico. Independently ArsD mutants exhibiting either weaker or stronger interaction with ArsA were selected. The locations of the mutations mapped on the surface of ArsD are consistent with the docking model. The results suggest that the interface with ArsA involves one surface of α1 helix and metalloid binding site of ArsD.

Genetic mapping of the interface between the ArsD metallochaperone and the ArsA ATPase

The ArsD metallochaperone delivers trivalent metalloids, As(III) or Sb(III), to the ArsA ATPase, the catalytic subunit of the ArsAB As(III) efflux pump. Transfer of As(III) increases the affinity of ArsA for As(III), allowing resistance to environmental arsenic concentrations. As(III) transfer is channelled from chaperone to ATPase, implying that ArsD and ArsA form an interface at their metal binding sites. A genetic approach was used to test this hypothesis. Thirteen ArsD mutants exhibiting either weaker or stronger interaction with ArsA were selected by either repressed transactivator yeast two-hybrid or reverse yeast two-hybrid assays. Additionally, Lys-37 and Lys-62 were identified as being involved in ArsD function by site-directed mutagenesis and chemical modification. Substitution at either position with arginine was tolerated, suggesting participation of a positive charge. By yeast two-hybrid analysis K37A and K62A mutants lost interaction with ArsA. All 15 mutations were mapped on the surface of the ArsD structure, and their locations are consistent with a structural model generated by in silico docking. Four are close to metalloid binding site residues Cys-12, Cys-13 and Cys-18, and seven are on the surface of helix 1. These results suggest that the interface involves one surface of helix 1 and the metalloid binding site.

The 1.4 A crystal structure of the ArsD arsenic metallochaperone provides insights into its interaction with the ArsA ATPase

Arsenic is a carcinogen that tops the Superfund list of hazardous chemicals. Bacterial resistance to arsenic is facilitated by ArsD, which delivers As(III) to the ArsA ATPase, the catalytic subunit of the ArsAB pump. Here we report the structure of the arsenic metallochaperone ArsD at 1.4 A and a model for its binding of metalloid. There are two ArsD molecules in the asymmetric unit. The overall structure of the ArsD monomer has a thioredoxin fold, with a core of four beta-strands flanked by four alpha-helices. Based on data from structural homologues, ArsD was modeled with and without bound As(III). ArsD binds one arsenic per monomer coordinated with the three sulfur atoms of Cys12, Cys13, and Cys18. Using this structural model, an algorithm was used to dock ArsD and ArsA. The resulting docking model provides testable predictions of the contact points of the two proteins and forms the basis for future experiments.

Biochemical characterization of a novel ArsA ATPase complex from Alkaliphilus metalliredigens QYMF

The two putative ars operons in Alkaliphilus metalliredigens QYMF are distinctive in that the arsA gene is split in halves, amarsA1 and amarsA2, and, acr3 but not an arsB gene coexists with arsA. Heterologous expression of one of the A. metalliredigensars operons (ars1) conferred arsenite but not antimonite resistance to DeltaarsEscherichia coli. Only the co-expressed AmArsA1 and AmArsA2 displayed arsenite or antimonite stimulated ATPase activity. The results show that AmArsA1-AmArsA2 interaction is needed to form the functional ArsA ATPase. This novel AmArsA1-AmArsA2 complex may provide insight in how it participates with Acr3 in arsenite detoxification.

Arsenic binding and transfer by the ArsD As(III) metallochaperone

ArsD is a metallochaperone that delivers trivalent metalloids [As(III) or Sb(III)] to the ArsA ATPase, the catalytic subunit of the ArsAB pump encoded by the arsRDABC operon of Escherichia coli plasmid R773. Interaction with ArsD increases the affinity of ArsA for As(III), conferring resistance to environmental concentrations of arsenic. Previous genetic analysis suggested that ArsD residues Cys12, Cys13, and Cys18 are involved in the transfer of As(III) to ArsA. Here X-ray absorption spectroscopy was used to show that As(III) is coordinated with three sulfur atoms, consistent with the three cysteine residues forming the As(III) binding site. Two single-tryptophan derivatives of ArsD exhibited quenching of intrinsic protein fluorescence upon binding of As(III) or Sb(III), which allowed estimation of the rates of binding and affinities for metalloids. Substitution of Cys12, Cys13, or Cys18 decreased the affinity for As(III) more than 10-fold. Reduced glutathione greatly increased the rate of binding of As(III) to ArsD but did not affect binding of As(III) to ArsA. This suggests that in vivo cytosolic As(III) might be initially bound to GSH and transferred to ArsD and then to ArsAB, which pumps the metalloid out of the cell. The As(III) chelator dimercaptosuccinic acid did not block the transfer from ArsD to ArsA, consistent with channeling of the metalloid from one protein to the other, as opposed to release and rebinding of the metalloid. Finally, transfer of As(III) from ArsD to ArsA occurred in the presence of MgATP at 23 degrees C but not at 4 degrees C. Neither MgADP nor MgATP-gamma-S could replace MgATP. These results suggest that transfer occurs with a conformation of ArsA that transiently forms during the catalytic cycle.

Role of signature lysines in the deviant walker a motifs of the ArsA ATPase

The ArsA ATPase belongs to the P-loop GTPase subgroup within the GTPase superfamily of proteins. Members of this subgroup have a deviant Walker A motif which contains a signature lysine that is predicted to make intermonomer contact with the bound nucleotides and to play a role in ATP hydrolysis. ArsA has two signature lysines located at positions 16 and 335. The role of Lys16 in the A1 half and Lys335 in the A2 half was investigated by altering the lysines individually to alanine, arginine, leucine, methionine, glutamate, and glutamine by site-directed mutagenesis. While Lys16 mutants show similar resistance phenotypes as the wild type, the Lys335 mutants are sensitive to higher concentrations of arsenite. K16Q ArsA shows 70% of wild-type ATPase activity while K335Q ArsA is inactive. ArsA is activated by binding of Sb(III), and both wild-type and mutant ArsAs bind Sb(III) with a 1:1 stoichiometry. Although each ArsA binds nucleotide, the binding affinity decreases in the order wild type > K16Q > K335Q. The results of limited trypsin digestion analysis indicate that both wild type and K16Q adopt a similar conformation during activated catalysis, whereas K335Q adopts a conformation that is resistant to trypsin cleavage. These biochemical data along with structural modeling suggest that, although Lys16 is not critical for ATPase activity, Lys335 is involved in intersubunit interaction and activation of ATPase activity in both halves of the protein. Taken together, the results indicate that Lys16 and Lys335, located in the A1 and A2 halves of the protein, have different roles in ArsA catalysis, consistent with our proposal that the nucleotide binding domains in these two halves are functionally nonequivalent.

Properties of arsenite efflux permeases (Acr3) from Alkaliphilus metalliredigens and Corynebacterium glutamicum

Members of the Acr3 family of arsenite permeases confer resistance to trivalent arsenic by extrusion from cells, with members in every phylogenetic domain. In this study bacterial Acr3 homologues from Alkaliphilus metalliredigens and Corynebacterium glutamicum were cloned and expressed in Escherichia coli. Modification of a single cysteine residue that is conserved in all analyzed Acr3 homologues resulted in loss of transport activity, indicating that it plays a role in Acr3 function. The results of treatment with thiol reagents suggested that the conserved cysteine is located in a hydrophobic region of the permease. A scanning cysteine accessibility method was used to show that Acr3 has 10 transmembrane segments, and the conserved cysteine would be predicted to be in the fourth transmembrane segment.

Molecular basis of antimony treatment in leishmaniasis

Leishmaniasis is a disease that affects 2 million people and kills 70000 persons every year. It is caused by Leishmania species, which are human protozoan parasites of the trypanosomatidae family. Trypanosomatidae differ from the other eukaryotes in their specific redox metabolism because the glutathione/glutathione reductase system is replaced by the unique trypanothione/trypanothione reductase system. The current treatment of leishmaniasis relies mainly on antimonial drugs. The crystal structures of oxidized trypanothione reductase (TR) from Leishmania infantum and of the complex of reduced TR with NADPH and Sb(III), reported in this paper, disclose for the first time the molecular mechanism of action of antimonial drugs against the parasite. Sb(III), which is coordinated by the two redox-active catalytic cysteine residues (Cys52 and Cys57), one threonine residue (Thr335), and His461' of the 2-fold symmetry related subunit in the dimer, strongly inhibits TR activity. Because TR is essential for the parasite survival and virulence and it is absent in mammalian cells, these findings provide insights toward the design of new more affordable and less toxic drugs against Leishmaniasis.

Role of conserved aspartates in the ArsA ATPase

The ArsA ATPase is the catalytic subunit of the arsenite-translocating ArsAB pump that is responsible for resistance to arsenicals and antimonials in Escherichia coli. ATPase activity is activated by either arsenite or antimonite. ArsA is composed of two homologous halves A1 and A2, each containing a nucleotide binding domain, and a single metalloid binding or activation domain is located at the interface of the two halves of the protein. The metalloid binding domain is connected to the two nucleotide binding domains through two DTAPTGH sequences, one in A1 and the other in A2. The DTAPTGH sequences are proposed to be involved in information communication between the metal and catalytic sites. The roles of Asp142 in A1 D 142TAPTGH sequence, and Asp447 in A2 D 447TAPTGH sequence was investigated after altering the aspartates individually to alanine, asparagine, and glutamate by site-directed mutagenesis. Asp142 mutants were sensitive to As(III) to varying degrees, whereas the Asp447 mutants showed the same resistance phenotype as the wild type. Each altered protein exhibited varying levels of both basal and metalloid-stimulated activity, indicating that neither Asp142 nor Asp447 is essential for catalysis. Biochemical characterization of the altered proteins imply that Asp142 is involved in Mg (2+) binding and also plays a role in signal transduction between the catalytic and activation domains. In contrast, Asp447 is not nearly as critical for Mg (2+) binding as Asp142 but appears to be in communication between the metal and catalytic sites. Taken together, the results indicate that Asp142 and Asp447, located on the A1 and A2 halves of the protein, have different roles in ArsA catalysis, consistent with our proposal that these two halves are functionally nonequivalent.

ArsD: an As(III) metallochaperone for the ArsAB As(III)-translocating ATPase

The toxic metalloid arsenic is widely disseminated in the environment and causes a variety of health and environment problems. As an adaptation to arsenic-contaminated environments, organisms have developed resistance systems. Many ars operons contain only three genes, arsRBC. Five gene ars operons have two additional genes, arsD and arsA, and these two genes are usually adjacent to each other. ArsA from Escherichia coli plasmid R773 is an ATPase that is the catalytic subunit of the ArsAB As(III) extrusion pump. ArsD was recently identified as an arsenic chaperone to the ArsAB pump, transferring the trivalent metalloids As(III) and Sb(III) to the ArsA subunit of the pump. This increases the affinity of ArsA for As(III), resulting in increased rates if extrusion and resistance to environmentally relevant concentrations of arsenite. ArsD is a homodimer with three vicinal cysteine pairs, Cys12-Cys13, Cys112-Cys113 and Cys119-Cys120, in each subunit. Each vicinal pair binds one As(III) or Sb(III). ArsD mutants with alanines substituting for Cys112, Cys113, Cys119 or Cys120, individually or in pairs or truncations lacking the vicinal pairs, retained ability to interact with ArsA, to activate its ATPase activity. Cells expressing these mutants retained ArsD-enhanced As(III) efflux and resistance. In contrast, mutants with substitutions of conserved Cys12, Cys13 or Cys18, individually or in pairs, were unable to activate ArsA or to enhance the activity of the ArsAB pump. It is proposed that ArsD residues Cys12, Cys13 and Cys18, but not Cys112, Cys113, Cys119 or Cys120, are required for delivery of As(III) to and activation of the ArsAB pump.

Biogeochemical cyclic activity of bacterial arsB in arsenic-contaminated mines

Biogeochemical cyclic activity of the ars (arsenic resistance system) operon is arsB influx/efflux encoded by the ecological of Pseudomonas putida. This suggests that studying arsenite-oxidizing bacteria may lead to a better understanding of molecular geomicrobiology, which can be applied to the bioremediation of arsenic-contaminated mines. This is the first report in which multiple arsB-binding mechanisms have been used on indigenous bacteria. In ArsB (strains OS-5; ABB83931; OS-19; ABB04282 and RW-28; ABB88574), there are ten putative enzyme, Histidine (His) 131, His 133, His 137, Arginine (Arg) 135, Arg 137, Arg 161, Trptohan (Trp) 142, Trp 164, Trp 166, and Trp 171, which are each located in different regions of the partial sequence. The adenosine triphosphate (ATP)-binding cassette transports, binding affinities and associating ratable constants show that As-binding is comparatively insensitive to the location of the residues within the moderately stable alpha-helical structure. The alpha-helical structures in ArsB-permease and anion permease arsB have been shown to import/export arsenic in P. putida. We proposed that arsB residues, His 131, His 133, His 137, Arg 135, Arg 137, Arg 161, Trp 142, Trp 164, Trp 166, and Trp 171 are required for arsenic binding and activation of arsA/arsB or arsAB. This arsB influx/efflux pum-ping is important, and the effect in arsenic species change and mobility in mine soil has got a significantly ecological role because it allows arsenic oxidizing/reducing bacteria to control biogeochemical cycle of abandoned mines.

Characterization of the metalloactivation domain of an arsenite/antimonite resistance pump

The ArsAB extrusion pump encoded by the ars operon of Escherichia coli plasmid R773 confers resistance to the toxic trivalent metalloids arsenite [As(III)] and antimonite [Sb(III)]. The ArsA ATPase, the catalytic subunit of the pump, has two homologous halves, A1 and A2. At the interface of these two halves are two nucleotide-binding domains and a metalloid-binding domain. Cys-113 and Cys-422 have been shown to form a high-affinity metalloid binding site. The crystal structure of ArsA shows two other bound metalloid atoms, one liganded to Cys-172 and His-453, and the other liganded to His-148 and Ser-420. The contribution of those putative metalloid sites was examined. There was little effect of mutagenesis of residues His-148 and Ser-420 on metalloid binding. However, a C172A ArsA mutant and C172A/H453A double mutant exhibited significantly decreased affinity for Sb(III). These results suggest first that there is only a single high-affinity metalloid binding site in ArsA, and second that Cys-172 controls the affinity of this site for metalloid and hence the efficiency of metalloactivation of the ArsAB efflux pump.

ArsD Residues Cys12, Cys13, and Cys18 Form an As(III)-binding Site Required for Arsenic Metallochaperone Activity

The ArsA ATPase is the catalytic subunit of the ArsAB pump encoded by the arsRDABC operon of Escherichia coli plasmid R773. ArsD is a metallochaperone that delivers As(III) to ArsA, increasing its affinity for As(III), thus conferring resistance to environmental concentrations of arsenic. R773 ArsD is a homodimer with three vicinal cysteine pairs, Cys(12)-Cys(13), Cys(112)-Cys(113), and Cys(119)-Cys(120), in each subunit. Each vicinal pair binds As(III) or Sb(III). Alignment of the primary sequence of homologues of ArsD indicates that only the first vicinal cysteine pair, Cys(12)-Cys(13), and an additional cysteine, Cys(18), are conserved. The effect of cysteine-to-alanine substitutions and truncations were examined. By yeast two-hybrid analysis, nearly all of the ArsD mutants were able to interact with wild type ArsD, indicating that the mutations do not interfere with dimerization. ArsD mutants with alanines substituting for Cys(112), Cys(113), Cys(119), or Cys(120) individually or in pairs or truncations lacking the vicinal pairs retained ability to interact with ArsA and to activate its ATPase activity. Cells expressing these mutants retained ArsD-enhanced As(III) efflux and resistance. In contrast, mutants with substitutions of conserved Cys(12), Cys(13), or Cys(18), individually or in pairs, were unable to activate ArsA or to enhance the activity of the ArsAB pump. We propose that ArsD residues Cys(12), Cys(13), and Cys(18), but not Cys(112), Cys(113), Cys(119), or Cys(120), are required for delivery of As(III) to and activation of the ArsAB pump.

An arsenic metallochaperone for an arsenic detoxification pump

Environmental arsenic is a world-wide health issue, making it imperative for us to understand mechanisms of metalloid uptake and detoxification. The predominant intracellular form is the highly mephitic arsenite, which is detoxified by removal from cytosol. What prevents arsenite toxicity as it diffuses through cytosol to efflux systems? Although intracellular copper is regulated by metallochaperones, no chaperones involved in conferring resistance to other metals have been identified. In this article, we report identification of an arsenic chaperone, ArsD, encoded by the arsRDABC operon of Escherichia coli. ArsD transfers trivalent metalloids to ArsA, the catalytic subunit of an As(III)/Sb(III) efflux pump. Interaction with ArsD increases the affinity of ArsA for arsenite, thus increasing its ATPase activity at lower concentrations of arsenite and enhancing the rate of arsenite extrusion. Cells are consequently resistant to environmental concentrations of arsenic. This report of an arsenic chaperone suggests that cells regulate the intracellular concentration of arsenite to prevent toxicity.