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Infection and Immunity, January 2005, p. 62-69, Vol. 73, No. 1
0019-9567/05/$08.00+0     doi:10.1128/IAI.73.1.62-69.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

An Immunoreactive 38-Kilodalton Protein of Ehrlichia canis Shares Structural Homology and Iron-Binding Capacity with the Ferric Ion-Binding Protein Family

C. Kuyler Doyle,1 Xiaofeng Zhang,1 Vsevolod L. Popov,1 and Jere W. McBride1,2,3,4*

Departments of Pathology,1 Microbiology and Immunology,2 Center for Biodefense and Emerging Infectious Diseases,3 Sealy Center for Vaccine Development, University of Texas Medical Branch, Galveston, Texas4

Received 15 June 2004/ Returned for modification 29 July 2004/ Accepted 26 September 2004

Ehrlichiae are tick-transmitted, gram-negative, obligately intracellular bacteria that live and replicate in cytoplasmic vacuoles, but little is known about iron acquisition mechanisms necessary for their survival. In this study, a genus-conserved immunoreactive ferric ion-binding protein (Fbp) of Ehrlichia canis was identified and its iron-binding capability was investigated. E. canis Fbp was homologous to a family of periplasmic Fbp's involved in iron acquisition and transport in gram-negative bacteria. E. canis Fbp had a molecular mass (38 kDa) consistent with those of Fbp's in other bacteria and exhibited substantial immunoreactivity in its native conformation. The predicted three-dimensional structure of E. canis Fbp demonstrated conservation of important Fbp family structural motifs: two domains linked with a polypeptide "hinge" region. Under iron-binding conditions, the recombinant Fbp exhibited an intense red color and an absorbance spectrum indicative of iron binding, and it bound Fe(III) but not Fe(II). Fbp was observed primarily in the cytoplasm of the reticulate forms of E. canis and Ehrlichia chaffeensis but was notably found on extracellular morula fibers in morulae containing dense-cored organisms. Although expression of Fbp is regulated through an operon of three functionally linked genes in other gram-negative bacteria, the absence of an intact fbp operon in Ehrlichia spp. suggests that genes involved in ehrlichial iron acquisition have been subject to reductive evolution.

* Corresponding author. Mailing address: Department of Pathology, University of Texas Medical Branch, Galveston, TX 77555. Phone: (409) 747-2498. Fax: (409) 747-2455. E-mail: jemcbrid{at}utmb.edu.

Editor: W. A. Petri, Jr.

Infection and Immunity, January 2005, p. 62-69, Vol. 73, No. 1
0019-9567/05/$08.00+0     doi:10.1128/IAI.73.1.62-69.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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