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Chapter 6 : Toxins and Type II Secretion Systems

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Toxins and Type II Secretion Systems, Page 1 of 2

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Abstract:

The type II secretion pathway is responsible for secretion of toxins and a variety of hydrolytic enzymes that include proteases, lipases, phospholipases, cellulases, and pectinases, which contribute to tissue damage and disease of animals and plants. Cholera toxin, exotoxin A, and aerolysin produced by , , and , respectively, are examples of three toxins that utilize the type II pathway for extracellular secretion. The biogenesis of all three toxins has been analyzed, and their crystal structures have been solved. The secretion of CT across the bacterial cell envelope occurs in two distinct steps that have different requirements; transport across the cytoplasmic membrane is followed by outer membrane translocation. A current hypothesis postulates that molecules secreted by the type II system may encode information critical to their secretion within their tertiary and quarternary structures. Restriction of proaerolysin to the poles could be evidence of periplasmic compartmentalization with concomitant secretion at the poles. The green fluorescent protein (GFP) was fused with Eps proteins to determine the cellular location, in living cells, of the type II secretion apparatus in . Recent intriguing data based on experiments accomplished with an active blue fluorescent protein (BFP) fused with ribosomal protein L1 in showed that ribosomes are located around the periphery of nucleoids, predominantly at the cell poles.

Citation: Scott M, Sandkvist M. 2003. Toxins and Type II Secretion Systems, p 81-94. In Burns D, Barbieri J, Iglewski B, Rappuoli R (ed), Bacterial Protein Toxins. ASM Press, Washington, DC. doi: 10.1128/9781555817893.ch6

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Figures

Image of Figure 1
Figure 1

Assembly and secretion of CT. The individual A and B subunits of CT are transported as precursor proteins across the cytoplasmic membrane via a Sec-like mechanism. The signal peptides are removed, and the subunits fold and assemble with the assistance of disulfide isomerase DsbA into the AB complex in the periplasmic compartment. The assembled toxin is then translocated across the outer membrane via the type II secretion apparatus that is encoded by the genes (C to N) and (O).

Citation: Scott M, Sandkvist M. 2003. Toxins and Type II Secretion Systems, p 81-94. In Burns D, Barbieri J, Iglewski B, Rappuoli R (ed), Bacterial Protein Toxins. ASM Press, Washington, DC. doi: 10.1128/9781555817893.ch6
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Image of Figure 2
Figure 2

Model of pilus-mediated secretion by the type II apparatus. Assembled CT (AB) is targeted to the secretion apparatus via specific recognition of B in a process that may involve components C and/or D (I). Upon binding, conformational changes in the type II apparatus may result in accommodation of the AB complex and lead to polymerization of component G and the other pilin-like proteins into a piston-like structure that extends from the E-F-L-M platform in the cytoplasmic membrane (II). Through the active process of pilin subunit polymerization, AB is then pushed through the channel to the extracellular environment in a process that is thought to be regulated and/or energized by the E component (III). Proteins A, B, N, and S are not shown in this model since they are not present in all organisms that encode and assemble a type II pathway and these proteins may not be required for secretion in every case.

Citation: Scott M, Sandkvist M. 2003. Toxins and Type II Secretion Systems, p 81-94. In Burns D, Barbieri J, Iglewski B, Rappuoli R (ed), Bacterial Protein Toxins. ASM Press, Washington, DC. doi: 10.1128/9781555817893.ch6
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Image of Figure 3
Figure 3

Polar localization of GFP-EpsL and GFP-EpsM protein fusions. Fluorescent microscopy, with the aid of an FITC filter, was used to determine the location within the cell of different GFP constructs. (A) GFP in mutant. (B) GFP-EpsL in mutant. (C) GFP-EpsM in mutant. (D) GFP-EpsM in MC1061. (E) GFP-EpsL in MC1061. (F) GFP-EpsL and EpsM in MC1061.

Citation: Scott M, Sandkvist M. 2003. Toxins and Type II Secretion Systems, p 81-94. In Burns D, Barbieri J, Iglewski B, Rappuoli R (ed), Bacterial Protein Toxins. ASM Press, Washington, DC. doi: 10.1128/9781555817893.ch6
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Image of Figure 4
Figure 4

Polar Eps-dependent protease secretion in single cells. Cells of wildtype TRH7000 (A) and mutant PU3 (B) that expressed IPTGinducible HA/protease from plasmid (pHAP) were embedded in agarose supplemented with M9 salts, amino acids, IPTG, and intramolecularly quenched casein. Casein became highly fluorescent when cleaved by secreted HA/protease. The location of HA/protease secreted from single cells via the Eps apparatus was determined by fluorescence microscopy with the use of a TRITC filter after overnight incubation at 37°C. Next to panels (A) and (B) are the corresponding phase-contrast images.

Citation: Scott M, Sandkvist M. 2003. Toxins and Type II Secretion Systems, p 81-94. In Burns D, Barbieri J, Iglewski B, Rappuoli R (ed), Bacterial Protein Toxins. ASM Press, Washington, DC. doi: 10.1128/9781555817893.ch6
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References

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1. Burr, S. E.,, D. B. Diep,, and J. T. Buckley. 2001. Type II secretion by Aeromonas salmonicida: evidence for two periplasmic pools of proaerolysin. J. Bacteriol. 183:59565963.
2. Filloux, A.,, G. Michel,, and M. Bally. 1998. GSP-dependent protein secretion in gramnegative bacteria: the Xcp system of Pseudomonas aeruginosa. FEMS Microbiol. Rev. 22: 177198.
3. Hirst, T. R.,, and J. Holmgren. 1987. Conformation of protein secreted across bacterial outer membranes: a study of enterotoxin translocation from Vibrio cholerae. Proc. Natl. Acad. Sci. USA 84:74187422.
4. LaPointe, C. F.,, and R. K. Taylor. 2000. The type 4 prepilin peptidases comprise a novel family of aspartic acid proteases. J. Biol. Chem. 275:15021510.
5. Lybarger, S. R.,, and J. R. Maddock. 2001. Polarity in action: asymmetric protein localization in bacteria. J. Bacteriol. 183:32613267.
6. Marciano, D. K.,, M. Russel,, and S. M. Simon. 2001. Assembling filamentous phage occlude pIV channels. Proc. Natl. Acad. Sci. USA 98:93599364.
7. Mascarenhas, J.,, M. H. Weber,, and P. L. Graumann. 2001. Specific polar localization of ribosomes in Bacillus subtilis depends on active transcription. EMBO Rep. 2:685689.
8. Merz, A. J.,, M. So,, and M. P. Sheetz. 2000. Pilus retraction powers bacterial twitching motility. Nature 407:98102.
9. Sakai, D.,, T. Horiuchi,, and T. Komano. 2001. ATPase activity and multimer formation of Pilq protein are required for thin pilus biogenesis in plasmid R64. J. Biol. Chem. 276:1796817975.
10. Sandkvist, M. 2001. Biology of type II secretion. Mol. Microbiol. 40:271283.
11. Sandkvist, M. 2001. Type II secretion and pathogenesis. Infect. Immun. 69:35233535.
12. Sauvonnet, N.,, G. Vignon,, A. P. Pugsley,, and P. Gounon. 2000. Pilus formation and protein secretion by the same machinery in Escherichia coli. EMBO J. 19:22212228.
13. Scott, M. E.,, Z. Y. Dossani,, and M. Sandkvist. 2001. Directed polar secretion of protease from single cells of Vibrio cholerae via the type II secretion pathway. Proc. Natl. Acad. Sci. USA 98:1397813983.

Tables

Generic image for table
Table 1

Subcellular location and putative function of the components of the type II secretion apparatus

Citation: Scott M, Sandkvist M. 2003. Toxins and Type II Secretion Systems, p 81-94. In Burns D, Barbieri J, Iglewski B, Rappuoli R (ed), Bacterial Protein Toxins. ASM Press, Washington, DC. doi: 10.1128/9781555817893.ch6
Generic image for table
Table 2

Interacting partners within secretion apparatus

Citation: Scott M, Sandkvist M. 2003. Toxins and Type II Secretion Systems, p 81-94. In Burns D, Barbieri J, Iglewski B, Rappuoli R (ed), Bacterial Protein Toxins. ASM Press, Washington, DC. doi: 10.1128/9781555817893.ch6

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