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A Noncanonical Role for Yersiniabactin in Bacterial Copper Acquisition Elizabeth M. Nolan* Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
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urinary tract. In the context of transition metals at the host− pathogen interface, the host employs copper as an antibacterial agent. Further detective work demonstrated that uropathogenic E. coli that produce Ybt are more resistant to copper toxicity than strains lacking a functional Ybt system. Taken together, these observations provided a new model by which Ybt contributes to E. coli virulence in the urinary tract by coordinating Cu2+ in the extracellular space and protecting the pathogen from copper toxicity. The presence of Cu2+−Ybt complexes also hinted that the Ybt acquisition machinery recognizes and transports this coordination complex. In preliminary experiments designed to probe this possibility, E. coli laboratory strain K-12 was employed.5 Because E. coli K-12 does not harbor the genes for Ybt biosynthesis and transport, it was transformed with a plasmid containing f yuA, which encodes the outer membrane protein that recognizes and transports Fe3+−Ybt. Following treatment with the Cu2+−Ybt complex, quantification of the cell-associated Cu2+−Ybt complex by mass spectrometry revealed negligible levels of the Cu2+−Ybt complex for E. coli K-12, whereas E. coli K-12 f yuA exhibited markedly higher levels of this metal complex. This work was extended to other Mn+−Ybt species, and similar results were obtained for all of the reported complexes, including Fe3+- and Ga3+-bound species. These results indicate that FyuA has the ability to recognize and transport a variety of metal-bound Ybt species. In the current work, Henderson and co-workers build upon these seminal studies and report that uropathogenic E. coli UTI89, which harbors the Ybt system and produces the siderophore when colonizing the urinary tract, imports Cu2+− Ybt and utilizes it as a nutrient copper source.1 An elegant series of experiments demonstrate that (i) UTI89 cultured under low-copper conditions produces Ybt and Cu2+−Ybt complexes form in the culture medium, (ii) UTI89 imports the Cu2+−Ybt complex and subsequently dissociates the Cu2+−Ybt complex, which allows for intracellular copper utilization and recycling of metal-free Ybt, and (iii) addition of Cu2+−Ybt to UTI89 that express TynA, a periplasmic copper amine oxidase, results in enhanced TynA activity in cell lysates relative to the untreated control. Additional uptake and recycling experiments showed that metal dissociation and Ybt recycling occur for the Fe3+−Ybt complex, whereas the Ga3+−Ybt complex is imported but not recycled. Because Ga3+ cannot be reduced, these experiments suggest that release of intracellular copper and iron from Ybt occurs via a reductive process that converts Cu2+ to Cu+ and Fe3+ to Fe2+.
ransition metal ions are both essential nutrients and toxic species, and organisms employ a variety of strategies to maintain metal homeostasis. Bacteria must obtain essential nutrient metals from the environment, and decades of investigations have uncovered a diversity of metal-ion acquisition systems that fulfill this important task. Along these lines, Fe3+-chelating secondary metabolites named “siderophores” were discovered more than 40 years ago. Bacteria biosynthesize and export these small molecules when confronted with conditions of iron limitation. In the canonical model, a siderophore scavenges Fe3+ in the extracellular space, and Fe3+−siderophore uptake machinery recognizes the coordination complex and transports it into the bacterial cell. This model has been and continues to be supported by many lines of inquiry, ranging from fundamental coordination chemistry to studies of iron homeostasis in the host−pathogen interaction. Nevertheless, as the bioinorganic community continues to explore siderophores and their contributions to metal homeostasis and microbial physiology, intriguing noncanonical roles for these metabolites are periodically discovered. In a recent article published in Nature Chemical Biology, Henderson and co-workers reported that the yersiniabactin (Ybt) system provides copper uptake in uropathogenic Escherichia coli.1 Ybt is a heterocyclic siderophore that was first isolated from Yersinia and is biosynthesized by various Enterobacteriaceae, including select commensal and pathogenic E. coli strains (Figure 1). The Ybt system, which includes biosynthetic and transport proteins, is encoded within a high-pathogenicity island. The Ybt synthetase is a hybrid nonribosomal peptide synthetase/polyketide synthase that biosynthesizes Ybt from one salicylic acid, three cysteines, and one malonyl-CoA. Ybt forms a 1:1 complex with Fe3+, and the outer membrane protein FyuA (ferric yersiniabactin uptake A, Psn in Yersinia pestis) recognizes the Fe3+−Ybt complex and provides its transport into the periplasm. The ATP-binding cassette transporter YbtPQ is located in the inner membrane and transports Ybt-bound Fe3+ into the cytoplasm. The Ybt system has been shown to be important for bacterial pathogenesis in animal models of infection, including bubonic plague caused by Y. pestis and urinary tract infection caused by E. coli.2,3 Prior investigations of siderophore utilization by uropathogenic E. coli demonstrated that many clinical isolates harbor the Ybt system. Moreover, mass spectrometric analyses of urine obtained from humans and mice suffering from E. coli urinary tract infection revealed Fe3+−Ybt complexes as well as Cu2+− Ybt complexes in this milieu.4 This discovery of Cu2+−Ybt complexes provided a clue that the contributions of Ybt to metal speciation and homeostasis extend beyond Fe 3+ coordination and uptake, at least for E. coli colonizing the © XXXX American Chemical Society
Received: October 4, 2017
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DOI: 10.1021/acs.biochem.7b01003 Biochemistry XXXX, XXX, XXX−XXX
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Biochemistry
Figure 1. Overview of Ybt and its role as a bacterial metallophore. In the chemical structure of Ybt, the metal-binding moieties are colored red.
These new findings are remarkable in several respects. They indicate that Ybt both protects E. coli from copper toxicity and provides E. coli with a nutrient copper source. Henderson and co-workers coin the term “nutritional passivation” to describe this phenomenon.1 The work also uncovers a copper uptake pathway in E. coli and highlights that Ybt is a versatile metal carrier or “metallophore”. Overall, the outcomes of this research provide inspiration for further out-of-the-box exploration of the Ybt system, as well as other siderophores and microbial metabolites that contribute to the biology of metals in the host−microbe and microbe−microbe interaction.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: 617-452-2495. ORCID
Elizabeth M. Nolan: 0000-0002-6153-8803 Notes
The author declares no competing financial interest.
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REFERENCES
(1) Koh, E.-I., Robinson, A. E., Bandara, N., Rogers, B. E., and Henderson, J. P. (2017) Copper import in Escherichia coli by the yersiniabactin metallophore system. Nat. Chem. Biol. 13, 1016−1021. (2) Perry, R. B., and Fetherston, J. D. (2011) Yersiniabactin iron uptake: mechanisms and role in Yersinia pestis pathogenesis. Microbes Infect. 13, 808−817. (3) Garcia, E. C., Brumbaugh, A. R., and Mobley, H. L. T. (2011) Redundancy and specificity of Escherichia coli ion acquisition systems during urinary tract infection. Infect. Immun. 79, 1225−1235. (4) Chaturvedi, K. S., Hung, C. S., Crowley, J. R., Stapleton, A. E., and Henderson, J. P. (2012) The siderophore yersiniabactin binds copper to protect pathogens during infection. Nat. Chem. Biol. 8, 731− 736. (5) Koh, E.-I., Hung, C. S., Parker, K. S., Crowley, J. R., Giblin, D. E., and Henderson, J. P. (2015) Metal selectivity by the virulenceassociated yersiniabactin metallophore system. Metallomics 7, 1011− 1022.
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DOI: 10.1021/acs.biochem.7b01003 Biochemistry XXXX, XXX, XXX−XXX
