An arsenic metallochaperone for an arsenic detoxification pump

Abstract

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.antimony ͉ resistance pump ͉ metalloid ͉ ArsD ͉ ArsA A ll cells possess regulatory mechanisms that control the intracellular concentration of metals, including essential metals, which become toxic in excess (copper and zinc), toxic metals (cadmium and lead), and toxic metalloids (arsenite and antimonite) (1-3). For example, metallothioneins are small proteins found in both eukaryotes and prokaryotes that bind transition metal ions such as copper and zinc for storage and heavy metals such as cadmium for detoxification (4). In plants and some fungi phytochelatins are cysteine-rich peptides that bind heavy metal ions for detoxification (5). In contrast to these types of metal-binding peptides and polypeptides are metallochaperones, which sequester metals in the cytoplasm and deliver them to protein targets (1, 6, 7). In some cases the targets are metalloenzymes such as superoxide dismutase, where the Saccharomyces cerevisiae metallochaperone Lys-7 (CCS in humans) is required for insertion of copper ions (8). Although the best characterized metallochaperones are for copper, UreE is a nickel chaperone that delivers Ni 2ϩ to urease (9), and frataxin is a chaperone that delivers iron to proteins for assembly of iron-sulfur clusters and heme (10). Other targets are membrane transporters, predominantly transport ATPases, that participate in metal ion homeostasis and detoxification such as S. cerevisiae Atx1 (Hah1 in humans), the metallochaperone for the yeast Ccc2p (ATP7A and ATP7B in humans) Cu(I)-translocating P-type ATPase (11). To date, no metallochaperones involved in detoxification of metals other than copper have been identified.

A C⋅As lyase for degradation of environmental organoarsenical herbicides and animal husbandry growth promoters

“…We analyzed the distribution of more than 100 arsI genes and found that all are in ars operons. There seems to be no specific ars gene associated with arsI, in contrast to arsD and arsA, which always go together because their functions are interrelated (32). MerB is a C·Hg lyase that confers resistance to organomercurials (33).…”

Identifying important structural characteristics of arsenic resistance proteins by using designed three-stranded coiled coils

“…Additionally, structural characterization of substrate and product complexes of the arsenate reductase ArsC, which reduces arsenate (AsO 4 3Ϫ ) to arsenite (AsO 2 Ϫ ), has helped elucidate this step of the arsenic detoxification pathway (3,4). Recent studies of E. coli ArsD indicate that it is a metallochaperone, transporting As(III) to ArsA for extrusion (5). Extended x-ray absorption fine structure and mutagenesis studies have shown that ArsR coordinates As(III) with three cysteine thiolates at a distance of 2.25 Å, a coordination mode that ArsD is also proposed to employ (1,5).…”

“…Recent studies of E. coli ArsD indicate that it is a metallochaperone, transporting As(III) to ArsA for extrusion (5). Extended x-ray absorption fine structure and mutagenesis studies have shown that ArsR coordinates As(III) with three cysteine thiolates at a distance of 2.25 Å, a coordination mode that ArsD is also proposed to employ (1,5). However, no x-ray or NMR structures of ArsR, the repressor protein, or ArsD have been reported to date.…”

The Arsenic Hyperaccumulator FernPteris vittataL.

“…However, based on the current evidence, limitation of As uptake is apparently not the way for P. vittata to tolerate high levels of As in the growth conditions because no altered (94) arsenate/phosphate uptake kinetics was detected in the stressed P. vittata (26). Arsenite extrusion as in E. coli (30) is also unlikely a tolerance mechanism for P. vittata. Compartmentation of toxic As into vacuoles of the target cells, such as fronds (27,31,32) and trochome cells (20), after its rapid uptake and translocation should be a key mechanism of As tolerance in this fern although the detailed mechanisms underlying As tolerance in P. vittata are largely unknown.…”