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Chapter 5 : The Type I Export Mechanism

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The Type I Export Mechanism, Page 1 of 2

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Abstract:

In gram-negative bacteria such as , proteins destined for the cell surface or surrounding medium must cross the cytoplasmic (inner) and outer membranes and the periplasm between them. Several mechanisms employ periplasmic intermediates, e.g., for assembly of adhesion pili, or utilize a large number of proteins to span the envelope, e.g., in the assembly of flagella. The type I export mechanism contrasts with these as it does not generate periplasmic intermediates and employs a dedicated secretory apparatus of just three proteins. Although acylation is essential for toxin interaction with mammalian cell target membranes, it is not required for export across the prokaryotic envelope. The type I export machinery requires only three export components. These are all integral membrane proteins, a traffic ATPase, an accessory or ‘‘adaptor’’ protein (HlyD), and the outer membrane protein TolC. Cell membrane traffic ATPases provide energy from ATP hydrolysis for movement of various molecules, large polypeptides to small ions, across membranes. The hemolysin export ATPase HlyB has 707 residues and is assumed to function as a homodimer. Putatively, six transmembrane helices between amino acids 158 and 432 interact with the bacterial membrane, whereas the C-terminal ca. 200 residues form the ATPase domain located in the cytoplasm. Each TolC monomer contributes four antiparallel β-strands and four antiparallel α-helical strands to form the channel and tunnel domains, respectively. Whereas a β-barrel is a typical feature of outer membrane proteins, the TolC channel domain is different in that the three monomers form a single β-barrel.

Citation: Koronakis V, Eswaran J, Hughes C. 2003. The Type I Export Mechanism, p 71-80. In Burns D, Barbieri J, Iglewski B, Rappuoli R (ed), Bacterial Protein Toxins. ASM Press, Washington, DC. doi: 10.1128/9781555817893.ch5
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Figures

Image of Figure 1
Figure 1

Representation of the 1,024-residue hemolysin HlyA. HlyC fatty acylation recognition domains FAI and FAII span the target lysines K564 and K690, upstream of the glycine-rich Ca-binding repeats (G-repeats) and the C-terminal secretion signal (sec).

Citation: Koronakis V, Eswaran J, Hughes C. 2003. The Type I Export Mechanism, p 71-80. In Burns D, Barbieri J, Iglewski B, Rappuoli R (ed), Bacterial Protein Toxins. ASM Press, Washington, DC. doi: 10.1128/9781555817893.ch5
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Image of Figure 2
Figure 2

Topology of the traffic ATPase HlyB in the inner membrane.

Citation: Koronakis V, Eswaran J, Hughes C. 2003. The Type I Export Mechanism, p 71-80. In Burns D, Barbieri J, Iglewski B, Rappuoli R (ed), Bacterial Protein Toxins. ASM Press, Washington, DC. doi: 10.1128/9781555817893.ch5
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Image of Figure 3
Figure 3

Topology of the adaptor protein HlyD in the inner membrane.

Citation: Koronakis V, Eswaran J, Hughes C. 2003. The Type I Export Mechanism, p 71-80. In Burns D, Barbieri J, Iglewski B, Rappuoli R (ed), Bacterial Protein Toxins. ASM Press, Washington, DC. doi: 10.1128/9781555817893.ch5
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Image of Figure 4
Figure 4

The structures of TolC porins (e.g., OmpF and SecY) and the siderophore transporters FhuA and FepA. (See Color Plates following p. 256.)

Citation: Koronakis V, Eswaran J, Hughes C. 2003. The Type I Export Mechanism, p 71-80. In Burns D, Barbieri J, Iglewski B, Rappuoli R (ed), Bacterial Protein Toxins. ASM Press, Washington, DC. doi: 10.1128/9781555817893.ch5
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Image of Figure 5
Figure 5

Schematic presentation of the proposed interaction of trimeric TolC (blue) with substrate-specific inner membrane complexes containing an adaptor protein (red) and a traffic ATPase (green). When the substrate binds to the inner membrane complex, the trimeric adaptor protein contacts the periplasmic tunnel, possibly via the predicted coiled-coil structures, triggering the conformational change that opens the entrance and presents the exit duct. Following export, the components revert to resting state. An animated model of protein export is available at http://archive.bmn.com/supp/ceb/ain1.html. (See Color Plates following p. 256.)

Citation: Koronakis V, Eswaran J, Hughes C. 2003. The Type I Export Mechanism, p 71-80. In Burns D, Barbieri J, Iglewski B, Rappuoli R (ed), Bacterial Protein Toxins. ASM Press, Washington, DC. doi: 10.1128/9781555817893.ch5
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Image of Figure 6
Figure 6

The TolC tunnel entrance closed and modeled open state showing the four monomers colored in green, blue, and red. (See Color Plates following p. 256.)

Citation: Koronakis V, Eswaran J, Hughes C. 2003. The Type I Export Mechanism, p 71-80. In Burns D, Barbieri J, Iglewski B, Rappuoli R (ed), Bacterial Protein Toxins. ASM Press, Washington, DC. doi: 10.1128/9781555817893.ch5
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References

/content/book/10.1128/9781555817893.chap5
1. Balakrishnan, L.,, C. Hughes,, and V. Koronakis. 2001. Substrate-triggered recruitment of the TolC channel-tunnel during type I export of hemolysin by Escherichia coli. J. Mol. Biol. 313:501510.
2. Johnson, J. M.,, and G. M. Church. 1999. Alignment and structure prediction of divergent protein families: periplasmic and outer membrane proteins of bacterial efflux pumps. J. Mol. Biol. 287:695715.
3. Koronakis, V.,, C. Andersen,, and C. Hughes. 2001. Channel-tunnels. Curr. Opin. Struct. Biol. 4:403407.
4. Koronakis, V.,, A. Sharff,, E. Koronakis,, B. Luisi,, and C. Hughes. 2000. Crystal structure of the bacterial membrane protein TolC central to multidrug efflux and protein export. Nature 405:914919.
5. Nikaido, H. 1998. Multiple antibiotic resistance and efflux. Curr. Opin. Microbiol. 1: 516523.
6. Stanley, P.,, V. Koronakis,, and C. Hughes. 1998. Acylation of Escherichia coli hemolysin: a unique protein lipidation mechanism underlying toxin function. Microbiol. Mol. Biol. Rev. 62:309333.
7. Stanley, P.,, L. C. Packman,, V. Koronakis,, and C. Hughes. 1994. Fatty acylation of two internal lysine residues required for the toxic activity of Escherichia coli hemolysin. Science 266:19921996.
8. Thanabalu, T.,, E. Koronakis,, C. Hughes,, and V. Koronakis. 1998. Substrate-induced assembly of a contiguous channel for protein export from E. coli: reversible bridging of an inner-membrane translocase to an outer membrane exit pore. EMBO J. 17:64876496.

Tables

Generic image for table
Table 1

The substrates of type I export systems

Citation: Koronakis V, Eswaran J, Hughes C. 2003. The Type I Export Mechanism, p 71-80. In Burns D, Barbieri J, Iglewski B, Rappuoli R (ed), Bacterial Protein Toxins. ASM Press, Washington, DC. doi: 10.1128/9781555817893.ch5

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