- Email: [email protected]
Autotransporters: protein contortionists whose carboxyl termini translocate their own amino-terminal domains
COMMENT
11 Rechnitzer, C. and Blom, J. (1989) APMIS 97, 105–114 12 Stevens, D.R. and Moulton, J.E. (1978) Infect. Immun. 19, 972–982 13 Chang, K.P. (1979) Exp. Parasitol. 48, 175–189 14 Bozue, J.A. and Johnson, W. (1996) Infect. Immun. 64, 668–673 15 Blackwell, J.M. et al. (1985) J. Exp. Med. 162, 324–331
16 Blank, C. et al. (1993) J. Infect. Dis. 167, 418–425 17 Mosser, D.M., Springer, T.A. and Diamond, M.S. (1992) J. Cell Biol. 116, 511–520 18 Rosenthal, L.A. et al. (1996) Infect. Immun. 64, 2206–2215 19 Payne, N.R. and Horwitz, M.A. (1987) J. Exp. Med. 166, 1377–1389 20 Marra, A., Horwitz, M.A. and
21 22 23 24
Shuman, H.A. (1990) J. Immunol. 144, 2738–2744 Brittingham, A. and Mosser, D.M. (1996) Parasitol. Today 12, 444–447 Krieger, M. and Herz, J. (1994) Annu. Rev. Biochem. 63, 601–637 Busch, D.H. et al. (1996) J. Immunol. 157, 3534–3541 Rittig, M., Häupl, T. and Burmester, G.R. (1994) Int. Arch. Allergy Immunol. 103, 4–10
Letters Species specificity and tissue tropism of EPEC and related pathogens
P
athogens that cause attaching and effacing (A/E) lesions in infected host cells, including enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC and EHEC, respectively), Hafnia alvei and Citrobacter rodantium, exhibit species specificity and tissue tropism1. In many cases, these involve specific attachment. In EPEC and related pathogens, intimin is the adhesin that has been implicated in mediating colonization and tissue tropism. Dr Kaper’s recent, thoughtful article in Trends Microbiol. on the virulence of EPEC (Ref. 2) describes new aspects of EPEC infection; among these is the realization that EPEC and EHEC translocate the intimin receptor (Tir/EspE) into the host cell3,4. Thus, intimin does not seem to recognize a tissue-specific component but instead recognizes a translocated receptor. Indeed, in their response to Kaper’s article, Finlay et al.5 ponder what mechanism is used by EPEC for tissue tropism. Recently, we demonstrated that the EPEC type III secretion system is activated upon contact with epithelial cells6. This is not surprising, as similar results have been obtained with type III secretion systems of Shigella, Salmonella and Yersinia7. An attractive hypothesis is that the elusive
recognition process that triggers type III secretion systems might be a tissue-specific event. This event would dictate where Tir and EspE are translocated and thus dictate the site of colonization. An additional layer of specificity might reside in initial attachment, as contact activation of the type III secretion system and translocation of EspB and Tir are dependent upon attachment to the host cell6. Currently, intimin is the only EPEC factor known to function as an adhesin in vivo8. It is possible that the presumably weak and Tir-independent attachment activity of intimin9, or some other adhesin, plays a role in jump-starting an efficient translocation process. Once a small amount of Tir is translocated, EPEC establishes an intimate contact and enters into a phase of enhanced translocation6. At this stage, Tir-independent attachment activity is no longer needed. A third layer of specificity might reside in the transcriptional regulation of expression of the type III secretion system in response to environmental cues. For example, human-specific EPEC secretes Esps at 378C but not at 428C. In contrast, 428C, which is the body temperature of rabbits, is the optimal temperature for Esp secretion by rabbit-specific EPEC (RDEC-1)10.
Autotransporters: protein contortionists whose carboxyl termini translocate their own amino-terminal domains
A
s originally described in a series of elegant experiments by Thomas Meyer and colleagues1, the autotransporter, immunoglobulin A (IgA) protease, in Neisseria gonorrhoeae seemed like a quirky,
one-off system for the secretion of an amino-terminal domain (passenger) through its own carboxy-terminal (transporter) inserted in the outer membrane of Gram-negative bacteria. In their excellent review,
It remains to be seen whether this hypothesis for EPEC species specificity and tissue tropism will stand up to testing. A critical point is to define, at the molecular level, the events that mediate the contact activation of the type III secretion system of EPEC.
Ilan Rosenshine Dept of Molecular Genetics and Biotechnology, The Hebrew University Faculty of Medicine, PO Box 12272, Jerusalem 91120, Israel References 1 Donnenberg, M.S., Kaper, J.B. and Finlay, B.B. (1997) Trends Microbiol. 5, 109–114 2 Kaper, J.B. (1998) Trends Microbiol. 6, 169–172 3 Kenny, B. et al. (1997) Cell 91, 511–520 4 Deibel, C. et al. (1998) Mol. Microbiol. 28, 463–474 5 Finlay, B.B. et al. (1998) Trends Microbiol. 6, 172–173 6 Wolff, C. et al. (1998) Mol. Microbiol. 28, 143–155 7 Lee, C.A. (1997) Trends Microbiol. 5, 148–156 8 Hicks, S. et al. (1998) Infect. Immun. 66, 1570–1578 9 Frankel, G. et al. (1996) J. Biol. Chem. 271, 20359–20364 10 Abe, A. et al. (1997) Infect. Immun. 65, 3547–3555
Henderson et al.2 have detailed the burgeoning field of these so-called type IV secretion systems3,4, but have raised many more questions than anyone can yet answer about the precise mechanism involved. In particular, what remains completely unclear is how the hydrophilic passenger is targeted to and then translocated through the presumed
Copyright © 1998 Elsevier Science Ltd. All rights reserved. 0966 842X/98/$19.00 TRENDS
IN
MICROBIOLOGY
388
VOL. 6 NO. 10 OCTOBER 1998
COMMENT
b-barrel monomeric pore, which is formed by the carboxy-terminal of the autotransporter. Coincidentally, recent studies by Alan Finkelstein’s group5 with colicin Ia have come up with the equally surprising finding that even highly charged peptides can be translocated across lipid bilayers by the simplest of translocators – two downstream transmembrane domains – although this mechanism is also a complete mystery. Henderson et al.2 hint that the amino-terminal passenger of the autotransporter can be unfolded during transport. This requires verification, as it leads to two important questions: how is the unfolded state maintained and how does the transported domain then refold correctly and efficiently in the hostile external environment? The signal sequence of type IV proteins, essential for moving the pre-protein to the periplasm, is often highly unusual, being characterized by 10–15 additional residues present after the amino-terminal methionine and often preceding an unusually high number of positive charges compared with conventional amino-terminal export signals. Nevertheless, Henderson et al.2 suggest that the majority of these proteins are initially
transported by the inner membrane Sec system. But has this been unequivocally shown? It is conceivable that the amino-terminal signal sequence, acting as an antifoldase in the cytoplasm6, requires a particular structure when the mature protein (the passenger in this case) is particularly hydrophilic. Alternatively, perhaps, all type IV amino-terminal signal sequences recognize an alternative translocator to that of SecY. One refreshing aspect of their review is the frank and balanced way in which Henderson et al. present the facts, in particular highlighting what is not known. Their approach emphasizes that, since the early studies of Meyer and colleagues, the field has expanded massively – but sideways – with very little advance in deeper understanding of the mechanism of translocation of the passenger through its own carboxy-terminal pore or of the mysteries of the energy requirement for this mechanism. In this context, Henderson et al. allude to a conserved, potential nucleotidebinding motif in autotransporters; however, dogma demands that the periplasm is devoid of ATP, and the significance of such a motif would therefore have to be questioned.
This lack of real progress in understanding this secretion process extends to many systems for protein translocation. Frustratingly, very few laboratories appear to have the resources or skills, or even the willingness to forego rapid publications, to undertake the long haul to discover something of fundamental importance.
Barry Holland Institut de Genetique et Microbiologie, Unité de Recherche Associée au CNRS 2225, Bâtiment 409, 91405 Orsay Cedex, France References 1 Halter, R., Pohlner, J. and Meyer, T. (1984) EMBO J. 3, 1595–1601 2 Henderson, I.R., Navarro-Garcia, F. and Nataro, J.P. (1998) Trends Microbiol. 6, 370–378 3 Genin, S. and Boucher, C.A. (1994) Mol. Gen. Genet. 243, 112–118 4 Pimenta, A. et al. (1997) in Unusual Routes for Protein Secretion (Kuchler, K., Holland, I.B. and Rubartelli, A., eds), pp. 1–48, R.G. Landes 5 Jakes, K.S. et al. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 4321–4326 6 Liu, G., Topping, T.B. and Randall, L.L. (1989) Proc. Natl. Acad. Sci. U. S. A. 86, 9213–9217
Horizons Microbial genomics Francisella tularensis genome sequencing project Francisella tularensis is a Gramnegative bacillus that causes a plaguelike illness, mainly in North America, Scandinavia and Russia. A consortium of labs has begun to sequence the 2.0-Mb genome of a highly virulent Biovar A strain, Schu 4. Participants include Rick Titball, Petra Oyston and Kerri Mack from the Defence Evaluation Research Agency, Porton Down, UK (http://www.dra. hmg.gb/dera.htm); Siv Andersson at the University of Uppsala, Sweden (http://evolution.bmc.uu.se/ ~siv/gnomics/); Gunnar Sandstrom, Karin Hjalmarsson and Thomas Svensson at the NDRE, Umeå, Sweden; Luther Lindler at Walter
Reed Army Institute for Research, Frederick, MD, USA (http://wrairwww.army.mil/); and Brendan Wren and Kerstin Williams at St Bartholomew’s and the Royal London Hospital School of Medicine and Dentistry, UK (http:// www.medmicro.mds.qmw.ac. uk/~bwren). For more information, see the F. tularensis genome project Web site on http://www.medmicro.mds.qmw. ac.uk/ft/. Clostridium difficile genome sequencing project The Sanger Centre has been awarded funding by Beowulf Genomics to sequence the 4.4-Mb genome of Clostridium difficile strain 630 (epidemic type X), a nosocomial pathogen that causes antibiotic-
associated diarrhoea (http://www. sanger.ac.uk/Projects/C_ difficile/). Work will be carried out in collaboration with Brendan Wren; Neil Fairweather (http: //www.bc.ic.ac.uk/research/ fairweather/fairweather_intro. html) and Gordon Dougan (http://www.bc.ic.ac.uk/ research/dougan/dougan.html) at Imperial College, London, UK; and Peter Mullany at the Eastman Dental Institute , London (http:// www.eastman.ucl.ac.uk/staff/ pmullany.html). For more information, see the C. difficile genome project Web site on http:// www.medmicro.mds.qmw.ac.uk/ cdiff/. Mark Pallen e-mail: [email protected]
Copyright © 1998 Elsevier Science Ltd. All rights reserved. 0966 842X/98/$19.00 TRENDS
IN
MICROBIOLOGY
389
VOL. 6 NO. 10 OCTOBER 1998
11 Rechnitzer, C. and Blom, J. (1989) APMIS 97, 105–114 12 Stevens, D.R. and Moulton, J.E. (1978) Infect. Immun. 19, 972–982 13 Chang, K.P. (1979) Exp. Parasitol. 48, 175–189 14 Bozue, J.A. and Johnson, W. (1996) Infect. Immun. 64, 668–673 15 Blackwell, J.M. et al. (1985) J. Exp. Med. 162, 324–331
16 Blank, C. et al. (1993) J. Infect. Dis. 167, 418–425 17 Mosser, D.M., Springer, T.A. and Diamond, M.S. (1992) J. Cell Biol. 116, 511–520 18 Rosenthal, L.A. et al. (1996) Infect. Immun. 64, 2206–2215 19 Payne, N.R. and Horwitz, M.A. (1987) J. Exp. Med. 166, 1377–1389 20 Marra, A., Horwitz, M.A. and
21 22 23 24
Shuman, H.A. (1990) J. Immunol. 144, 2738–2744 Brittingham, A. and Mosser, D.M. (1996) Parasitol. Today 12, 444–447 Krieger, M. and Herz, J. (1994) Annu. Rev. Biochem. 63, 601–637 Busch, D.H. et al. (1996) J. Immunol. 157, 3534–3541 Rittig, M., Häupl, T. and Burmester, G.R. (1994) Int. Arch. Allergy Immunol. 103, 4–10
Letters Species specificity and tissue tropism of EPEC and related pathogens
P
athogens that cause attaching and effacing (A/E) lesions in infected host cells, including enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC and EHEC, respectively), Hafnia alvei and Citrobacter rodantium, exhibit species specificity and tissue tropism1. In many cases, these involve specific attachment. In EPEC and related pathogens, intimin is the adhesin that has been implicated in mediating colonization and tissue tropism. Dr Kaper’s recent, thoughtful article in Trends Microbiol. on the virulence of EPEC (Ref. 2) describes new aspects of EPEC infection; among these is the realization that EPEC and EHEC translocate the intimin receptor (Tir/EspE) into the host cell3,4. Thus, intimin does not seem to recognize a tissue-specific component but instead recognizes a translocated receptor. Indeed, in their response to Kaper’s article, Finlay et al.5 ponder what mechanism is used by EPEC for tissue tropism. Recently, we demonstrated that the EPEC type III secretion system is activated upon contact with epithelial cells6. This is not surprising, as similar results have been obtained with type III secretion systems of Shigella, Salmonella and Yersinia7. An attractive hypothesis is that the elusive
recognition process that triggers type III secretion systems might be a tissue-specific event. This event would dictate where Tir and EspE are translocated and thus dictate the site of colonization. An additional layer of specificity might reside in initial attachment, as contact activation of the type III secretion system and translocation of EspB and Tir are dependent upon attachment to the host cell6. Currently, intimin is the only EPEC factor known to function as an adhesin in vivo8. It is possible that the presumably weak and Tir-independent attachment activity of intimin9, or some other adhesin, plays a role in jump-starting an efficient translocation process. Once a small amount of Tir is translocated, EPEC establishes an intimate contact and enters into a phase of enhanced translocation6. At this stage, Tir-independent attachment activity is no longer needed. A third layer of specificity might reside in the transcriptional regulation of expression of the type III secretion system in response to environmental cues. For example, human-specific EPEC secretes Esps at 378C but not at 428C. In contrast, 428C, which is the body temperature of rabbits, is the optimal temperature for Esp secretion by rabbit-specific EPEC (RDEC-1)10.
Autotransporters: protein contortionists whose carboxyl termini translocate their own amino-terminal domains
A
s originally described in a series of elegant experiments by Thomas Meyer and colleagues1, the autotransporter, immunoglobulin A (IgA) protease, in Neisseria gonorrhoeae seemed like a quirky,
one-off system for the secretion of an amino-terminal domain (passenger) through its own carboxy-terminal (transporter) inserted in the outer membrane of Gram-negative bacteria. In their excellent review,
It remains to be seen whether this hypothesis for EPEC species specificity and tissue tropism will stand up to testing. A critical point is to define, at the molecular level, the events that mediate the contact activation of the type III secretion system of EPEC.
Ilan Rosenshine Dept of Molecular Genetics and Biotechnology, The Hebrew University Faculty of Medicine, PO Box 12272, Jerusalem 91120, Israel References 1 Donnenberg, M.S., Kaper, J.B. and Finlay, B.B. (1997) Trends Microbiol. 5, 109–114 2 Kaper, J.B. (1998) Trends Microbiol. 6, 169–172 3 Kenny, B. et al. (1997) Cell 91, 511–520 4 Deibel, C. et al. (1998) Mol. Microbiol. 28, 463–474 5 Finlay, B.B. et al. (1998) Trends Microbiol. 6, 172–173 6 Wolff, C. et al. (1998) Mol. Microbiol. 28, 143–155 7 Lee, C.A. (1997) Trends Microbiol. 5, 148–156 8 Hicks, S. et al. (1998) Infect. Immun. 66, 1570–1578 9 Frankel, G. et al. (1996) J. Biol. Chem. 271, 20359–20364 10 Abe, A. et al. (1997) Infect. Immun. 65, 3547–3555
Henderson et al.2 have detailed the burgeoning field of these so-called type IV secretion systems3,4, but have raised many more questions than anyone can yet answer about the precise mechanism involved. In particular, what remains completely unclear is how the hydrophilic passenger is targeted to and then translocated through the presumed
Copyright © 1998 Elsevier Science Ltd. All rights reserved. 0966 842X/98/$19.00 TRENDS
IN
MICROBIOLOGY
388
VOL. 6 NO. 10 OCTOBER 1998
COMMENT
b-barrel monomeric pore, which is formed by the carboxy-terminal of the autotransporter. Coincidentally, recent studies by Alan Finkelstein’s group5 with colicin Ia have come up with the equally surprising finding that even highly charged peptides can be translocated across lipid bilayers by the simplest of translocators – two downstream transmembrane domains – although this mechanism is also a complete mystery. Henderson et al.2 hint that the amino-terminal passenger of the autotransporter can be unfolded during transport. This requires verification, as it leads to two important questions: how is the unfolded state maintained and how does the transported domain then refold correctly and efficiently in the hostile external environment? The signal sequence of type IV proteins, essential for moving the pre-protein to the periplasm, is often highly unusual, being characterized by 10–15 additional residues present after the amino-terminal methionine and often preceding an unusually high number of positive charges compared with conventional amino-terminal export signals. Nevertheless, Henderson et al.2 suggest that the majority of these proteins are initially
transported by the inner membrane Sec system. But has this been unequivocally shown? It is conceivable that the amino-terminal signal sequence, acting as an antifoldase in the cytoplasm6, requires a particular structure when the mature protein (the passenger in this case) is particularly hydrophilic. Alternatively, perhaps, all type IV amino-terminal signal sequences recognize an alternative translocator to that of SecY. One refreshing aspect of their review is the frank and balanced way in which Henderson et al. present the facts, in particular highlighting what is not known. Their approach emphasizes that, since the early studies of Meyer and colleagues, the field has expanded massively – but sideways – with very little advance in deeper understanding of the mechanism of translocation of the passenger through its own carboxy-terminal pore or of the mysteries of the energy requirement for this mechanism. In this context, Henderson et al. allude to a conserved, potential nucleotidebinding motif in autotransporters; however, dogma demands that the periplasm is devoid of ATP, and the significance of such a motif would therefore have to be questioned.
This lack of real progress in understanding this secretion process extends to many systems for protein translocation. Frustratingly, very few laboratories appear to have the resources or skills, or even the willingness to forego rapid publications, to undertake the long haul to discover something of fundamental importance.
Barry Holland Institut de Genetique et Microbiologie, Unité de Recherche Associée au CNRS 2225, Bâtiment 409, 91405 Orsay Cedex, France References 1 Halter, R., Pohlner, J. and Meyer, T. (1984) EMBO J. 3, 1595–1601 2 Henderson, I.R., Navarro-Garcia, F. and Nataro, J.P. (1998) Trends Microbiol. 6, 370–378 3 Genin, S. and Boucher, C.A. (1994) Mol. Gen. Genet. 243, 112–118 4 Pimenta, A. et al. (1997) in Unusual Routes for Protein Secretion (Kuchler, K., Holland, I.B. and Rubartelli, A., eds), pp. 1–48, R.G. Landes 5 Jakes, K.S. et al. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 4321–4326 6 Liu, G., Topping, T.B. and Randall, L.L. (1989) Proc. Natl. Acad. Sci. U. S. A. 86, 9213–9217
Horizons Microbial genomics Francisella tularensis genome sequencing project Francisella tularensis is a Gramnegative bacillus that causes a plaguelike illness, mainly in North America, Scandinavia and Russia. A consortium of labs has begun to sequence the 2.0-Mb genome of a highly virulent Biovar A strain, Schu 4. Participants include Rick Titball, Petra Oyston and Kerri Mack from the Defence Evaluation Research Agency, Porton Down, UK (http://www.dra. hmg.gb/dera.htm); Siv Andersson at the University of Uppsala, Sweden (http://evolution.bmc.uu.se/ ~siv/gnomics/); Gunnar Sandstrom, Karin Hjalmarsson and Thomas Svensson at the NDRE, Umeå, Sweden; Luther Lindler at Walter
Reed Army Institute for Research, Frederick, MD, USA (http://wrairwww.army.mil/); and Brendan Wren and Kerstin Williams at St Bartholomew’s and the Royal London Hospital School of Medicine and Dentistry, UK (http:// www.medmicro.mds.qmw.ac. uk/~bwren). For more information, see the F. tularensis genome project Web site on http://www.medmicro.mds.qmw. ac.uk/ft/. Clostridium difficile genome sequencing project The Sanger Centre has been awarded funding by Beowulf Genomics to sequence the 4.4-Mb genome of Clostridium difficile strain 630 (epidemic type X), a nosocomial pathogen that causes antibiotic-
associated diarrhoea (http://www. sanger.ac.uk/Projects/C_ difficile/). Work will be carried out in collaboration with Brendan Wren; Neil Fairweather (http: //www.bc.ic.ac.uk/research/ fairweather/fairweather_intro. html) and Gordon Dougan (http://www.bc.ic.ac.uk/ research/dougan/dougan.html) at Imperial College, London, UK; and Peter Mullany at the Eastman Dental Institute , London (http:// www.eastman.ucl.ac.uk/staff/ pmullany.html). For more information, see the C. difficile genome project Web site on http:// www.medmicro.mds.qmw.ac.uk/ cdiff/. Mark Pallen e-mail: [email protected]
Copyright © 1998 Elsevier Science Ltd. All rights reserved. 0966 842X/98/$19.00 TRENDS
IN
MICROBIOLOGY
389
VOL. 6 NO. 10 OCTOBER 1998