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Journal of Bacteriology, June 2001, p. 3476-3487, Vol. 183, No. 11
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.11.3476-3487.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

FhuA Barrel-Cork Hybrids Are Active Transporters and Receptors

Helmut Killmann, Michael Braun, Christina Herrmann, and Volkmar Braun*

Mikrobiologie/Membranphysiologie, Universität Tübingen, D-72076 Tübingen, Germany

Received 9 January 2001/Accepted 20 March 2001

The crystal structure of Escherichia coli FhuA reveals a beta -barrel domain that is closed by a globular cork domain. It has been assumed that the proton motive force of the cytoplasmic membrane through the interaction of the TonB protein with the TonB box of the cork opens the FhuA channel. Yet, deletion of the cork results in an FhuA derivative, FhuADelta 5-160, that still displays TonB-dependent substrate transport and phage receptor activity. To investigate this unexpected finding further, we constructed FhuADelta 5-160 derivatives of FhuA proteins from Salmonella paratyphi B, Salmonella enterica serovar Typhimurium, and Pantoea agglomerans. The FhuADelta 5-160 proteins inserted correctly into the outer membrane, and with the exception of the P. agglomerans protein, transported ferrichrome and albomycin. FhuA hybrids consisting of the beta -barrel of one strain and the cork of another strain were active and showed higher TonB-dependent ferrichrome transport rates than the corkless derivatives. Exceptions were the E. coli beta -barrel/Salmonella serovar Typhimurium cork hybrid protein and the Salmonella serovar Typhimurium beta -barrel/P. agglomerans cork hybrid protein, both of which were less active than the beta -barrels alone. Each of the FhuA mutant proteins displayed activity for each of their ligands, except for phage T5, only when coupled to TonB. The hybrid FhuA proteins displayed a similar activity with the E. coli TonB protein as with their cognate TonB proteins. Sensitivity to phages T1, T5, and phi 80, rifamycin CGP 4832, and colicin M was determined by the beta -barrel, whereas sensitivity to phage ES18 and microcin J25 required both the beta -barrel and cork domains. These results demonstrate that the beta -barrel domain of FhuA confers activity and specificity and responds to TonB and that the cork domains of various FhuA proteins can be interchanged and contribute to the activities of the FhuA hybrids.

* Corresponding author. Mailing address: Mikrobiologie/Membranphysiologie, Universität Tübingen, Auf der Morgenstelle 28, D-72076 Tübingen, Germany. Phone: (49) 7071 2972096. Fax: (49) 7071 295843. E-mail: volkmar.braun{at}mikrobio.uni-tuebingen.de.

Journal of Bacteriology, June 2001, p. 3476-3487, Vol. 183, No. 11
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.11.3476-3487.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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Copyright © 2001 by the American Society for Microbiology. All rights reserved.