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J. Biol. Chem., Vol. 280, Issue 10, 9283-9290, March 11, 2005

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Anion-independent Iron Coordination by the Campylobacter jejuni Ferric Binding Protein^*

Stacey A. L. Tom-Yew, Diana T. Cui, Elena G. Bekker, and Michael E. P. Murphy¶

From the Department of Microbiology and Immunology, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada

Campylobacter jejuni, the leading cause of human gastroenteritis, expresses a ferric binding protein (cFbpA) that in many pathogenic bacteria functions to acquire iron as part of their virulence repertoire. Recombinant cFbpA is isolated with ferric iron bound from Escherichia coli. The crystal structure of cFbpA reveals unprecedented iron coordination by only five protein ligands. The histidine and one tyrosine are derived from the N-terminal domain, whereas the three remaining tyrosine ligands are from the C-terminal domain. Surprisingly, a synergistic anion present in all other characterized ferric transport proteins is not observed in the cFbpA iron-binding site, suggesting a novel role for this protein in iron uptake. Furthermore, cFbpA is shown to bind iron with high affinity similar to Neisserial FbpA and exhibits an unusual preference for ferrous iron (oxidized subsequently to the ferric form) or ferric iron chelated by oxalate. Sequence and structure analyses reveal that cFbpA is a member of a new class of ferric binding proteins that includes homologs from invasive and intracellular bacteria as well as cyanobacteria. Overall, six classes are defined based on clustering within the tree and by their putative iron coordination. The absence of a synergistic anion in the iron coordination sphere of cFbpA also suggests an alternative model of evolution for FbpA homologs involving an early iron-binding ancestor instead of a requirement for a preexisting anion-binding ancestor.

Received for publication, November 4, 2004 , and in revised form, December 20, 2004.

^* This work is supported by Canadian Institutes of Health Research Grant MOP49597(to M. E. P. M.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental figures and tables.

The atomic coordinates and structure factors (code 1Y4T and 1Y9U) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).

Recipient of a University Graduate Fellowship from the University of British Columbia and a Natural Sciences and Engineering Research Council of Canada (NSERC) Postgraduate Scholarship.

Recipient of an NSERC Undergraduate Student Research Award.

^¶ A Canadian Institutes of Health Research scholar. To whom correspondence should be addressed: Dept. of Microbiology and Immunology, University of British Columbia, #300-6174 University Blvd., Vancouver, BC V6T 1Z3, Canada. Tel.: 604-822-8022; Fax: 604-822-6041; E-mail: michael.murphy{at}ubc.ca.

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