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Infection and Immunity, December 2000, p. 6988-6996, Vol. 68, No. 12
0019-9567/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Identification of Discrete Domains within Gonococcal Transferrin-Binding Protein A That Are Necessary for Ligand Binding and Iron Uptake Functions

Ian C. Boulton,1,dagger Mary Kate Yost,1 James E. Anderson,2 and Cynthia Nau Cornelissen1,*

Department of Microbiology and Immunology, Medical College of Virginia Campus of Virginia Commonwealth University, Richmond, Virginia 23298,1 and Department of Medicine, School of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 275992

Received 5 June 2000/Returned for modification 11 August 2000/Accepted 29 August 2000

The availability of free iron in vivo is strictly limited, in part by the iron-binding protein transferrin. The pathogenic Neisseria spp. can sequester iron from this protein, dependent upon two iron-repressible, transferrin-binding proteins (TbpA and TbpB). TbpA is a TonB-dependent, integral, outer membrane protein that may form a beta -barrel exposing multiple surface loops, some of which are likely to contain ligand-binding motifs. In this study we propose a topological model of gonococcal TbpA and then test some of the hypotheses set forth by the model by individually deleting three putative loops (designated loops 4, 5, and 8). Each mutant TbpA could be expressed without toxicity and was surface exposed as assessed by immunoblotting, transferrin binding, and protease accessibility. Deletion of loop 4 or loop 5 abolished transferrin binding to whole cells in solid- and liquid-phase assays, while deletion of loop 8 decreased the affinity of the receptor for transferrin without affecting the copy number. Strains expressing any of the three mutated TbpAs were incapable of growth on transferrin as a sole iron source. These data implicate putative loops 4 and 5 as critical determinants for receptor function and transferrin-iron uptake by gonococcal TbpA. The phenotype of the Delta L8TbpA mutant suggests that high-affinity ligand interaction is required for transferrin-iron internalization.

* Corresponding author. Mailing address: Department of Microbiology and Immunology, Medical College of Virginia Campus of Virginia Commonwealth University, Box 980678, Richmond, VA 23298-0678. Phone: (804) 225-4121. Fax: (804) 828-9946. E-mail: cncornel{at}hsc.vcu.edu.

dagger Present address: Department of Medical Genetics and Microbiology, University of Toronto, Toronto, Ontario, Canada.

Infection and Immunity, December 2000, p. 6988-6996, Vol. 68, No. 12
0019-9567/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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