Infection and Immunity, May 2008, p. 1960-1969, Vol. 76, No. 5

Identification of TbpA Residues Required for Transferrin-Iron
Utilization by Neisseria gonorrhoeae
Jennifer M. Noto and Cynthia Nau Cornelissen*
Department of Microbiology and Immunology, Virginia Commonwealth
University Medical Center, Richmond, Virginia 23298-0678

Received 7 January 2008/ Returned for modification 15 February 2008/
Accepted 6 March 2008

Neisseria gonorrhoeae requires iron for survival in the human host and
therefore expresses high-affinity receptors for iron acquisition from
host iron-binding proteins. The gonococcal transferrin-iron uptake
system is composed of two transferrin binding proteins, TbpA and TbpB.
TbpA is a TonB-dependent, outer membrane transporter critical for iron
acquisition, while TbpB is a surface-exposed lipoprotein that
increases the efficiency of iron uptake. The precise mechanism by
which TbpA mediates iron acquisition has not been elucidated; however,
the process is distinct from those of characterized siderophore
transporters. Similar to these TonB-dependent transporters, TbpA is
proposed to have two distinct domains, a β-barrel and a plug domain.
We hypothesize that the TbpA plug coordinates iron and therefore
potentially functions in multiple steps of transferrin-mediated iron
acquisition. To test this hypothesis, we targeted a conserved motif
within the TbpA plug domain and generated single, double, and triple
alanine substitution mutants. Mutagenized TbpAs were expressed on the
gonococcal cell surface and maintained wild-type transferrin binding
affinity. Single alanine substitution mutants internalized iron at
wild-type levels, while the double and triple mutants showed a
significant decrease in iron uptake. Moreover, the triple alanine
substitution mutant was unable to grow on transferrin as a sole iron
source; however, expression of TbpB compensated for this defect. These
data indicate that the conserved motif between residues 120 and 122 of
the TbpA plug domain is critical for transferrin-iron utilization,
suggesting that this region plays a role in iron acquisition that is
shared by both TbpA and TbpB.

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* Corresponding author. Mailing address: P.O. Box 980678, Richmond, VA
23298-0678. Phone: (804) 827-1754. Fax: (804) 828-9946. E-mail:
cncornel@vcu.edu

Published ahead of print on 17 March 2008.

Editor: J. N. Weiser
--------------------------------------------------------------------------------
Infection and Immunity, May 2008, p. 1960-1969, Vol. 76, No. 5
0019-9567/08/$08.00+0     doi:10.1128/IAI.00020-08
Copyright © 2008, American Society for Microbiology. All Rights
Reserved.

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