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Chapter 11 : Ferrichrome- and Citrate-Mediated Iron Transport

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Ferrichrome- and Citrate-Mediated Iron Transport, Page 1 of 2

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Abstract:

Iron transport through the outer membrane as ferrichrome- and ferric citrate-siderophore complexes is catalyzed by the FhuA and FecA proteins and across the cytoplasmic membrane by the FhuCDB and FecBCDE proteins, respectively. The energy for transport across the outer membrane is provided by the proton motive force of the cytoplasmic membrane, and the energy for transport across the cytoplasmic membrane is provided by ATP hydrolysis. Energy transfer from the cytoplasmic membrane to the outer membrane is mediated by the TonB-ExbB-ExbD proteins. Knowledge about the proteins that transport ferric siderophores, their interaction, and their subcellular location provides insights into the transport mechanism, but much work remains to be done before an understanding of the molecular mechanism of substrate translocation across the outer membrane and the cytoplasmic membrane is gained. Although crystal structures of the proteins in the entire pathway are now available (FhuA in the outer membrane, FhuD in the periplasm, and BtuCD in the cytoplasmic membrane), they provide only a framework for a detailed biochemical and genetic analysis. Crystal structures are static, yet transport is a dynamic process involving many rearrangements of amino acid side chains and entire polypeptide regions.

Citation: Braun V, Braun M, Killmann H. 2004. Ferrichrome- and Citrate-Mediated Iron Transport, p 158-177. In Crosa J, Mey A, Payne S, Iron Transport in Bacteria. ASM Press, Washington, DC. doi: 10.1128/9781555816544.ch11
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Figures

Image of FIGURE 1
FIGURE 1

Structures of ferrichrome, albomycin, and rifamycin CGP 4832, as revealed by Xray analysis of the FhuA cocrystals.

Citation: Braun V, Braun M, Killmann H. 2004. Ferrichrome- and Citrate-Mediated Iron Transport, p 158-177. In Crosa J, Mey A, Payne S, Iron Transport in Bacteria. ASM Press, Washington, DC. doi: 10.1128/9781555816544.ch11
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Image of FIGURE 2
FIGURE 2

Crystal structures of the FhuA and FecA outer membrane proteins unloaded and loaded with their substrates. A portion of the β-strands (dark grey) was deleted to improve the view of the globular central domain (cork; light grey). FhuA1–18 and FecA1–79 are not seen in the crystal structure, which suggests a flexible structure. Structurally important regions and the amino termini (N) are indicated.

Citation: Braun V, Braun M, Killmann H. 2004. Ferrichrome- and Citrate-Mediated Iron Transport, p 158-177. In Crosa J, Mey A, Payne S, Iron Transport in Bacteria. ASM Press, Washington, DC. doi: 10.1128/9781555816544.ch11
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Image of FIGURE 3
FIGURE 3

Structure of the vitamin B transporter. The complete transporter is assembled from two membrane-spanning BtuC subunits and two ABC cassette BtuD subunits. Helices are drawn as cylinders and are labeled consecutively for each subunit, and strands are shown as ribbons. The amino and carboxy termini are indicated (N-ter and C-ter, respectively). (A) Side view of the full transporter. (B) View onto the cytoplasmic face of BtuC. (C) Bottom view onto the ABC cassette BtuD. Reprinted from Locher et al. (2002) with permission from the publisher.

Citation: Braun V, Braun M, Killmann H. 2004. Ferrichrome- and Citrate-Mediated Iron Transport, p 158-177. In Crosa J, Mey A, Payne S, Iron Transport in Bacteria. ASM Press, Washington, DC. doi: 10.1128/9781555816544.ch11
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Image of FIGURE 4
FIGURE 4

Sequence homology between the N-terminal half (FhuB[N]) and the C-terminal half (FhuB[C]) of FhuB and the BtuC protein. The transmembrane regions of BtuC ( Fig. 3 ) are indicated and compared with the predicted transmembrane arrangement of FhuB.

Citation: Braun V, Braun M, Killmann H. 2004. Ferrichrome- and Citrate-Mediated Iron Transport, p 158-177. In Crosa J, Mey A, Payne S, Iron Transport in Bacteria. ASM Press, Washington, DC. doi: 10.1128/9781555816544.ch11
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Image of FIGURE 5
FIGURE 5

Crystal structure of dinuclear ferric citrate as it is bound to the FecA protein.

Citation: Braun V, Braun M, Killmann H. 2004. Ferrichrome- and Citrate-Mediated Iron Transport, p 158-177. In Crosa J, Mey A, Payne S, Iron Transport in Bacteria. ASM Press, Washington, DC. doi: 10.1128/9781555816544.ch11
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Image of FIGURE 6
FIGURE 6

Subcellular location, transmembrane arrangement, and interaction of the FecIRABCDE proteins. (Fe -citrate) binds to FecA in the outer membrane and elicits a signal that is transmitted by FecR across the cytoplasmic membrane into the cytoplasm, where it activates the FecI σ factor. FecA also transports ferric citrate into the periplasm. Transport and signal transduction require energy from the proton motive force of the cytoplasmic membrane, mediated by the TonB, ExbB, and ExbD proteins. The N-proximal segment of FecA interacts with TonB and with FecR. FecI binds to the β′ subunit of the RNA polymerase, which binds to the promoter of the operon. The Fur protein loaded with Fe represses the transcription of and .

Citation: Braun V, Braun M, Killmann H. 2004. Ferrichrome- and Citrate-Mediated Iron Transport, p 158-177. In Crosa J, Mey A, Payne S, Iron Transport in Bacteria. ASM Press, Washington, DC. doi: 10.1128/9781555816544.ch11
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References

/content/book/10.1128/9781555816544.chap11
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