Channel-tunnels: outer membrane components of type I secretion systems and multidrug efflux pumps of Gram-negative bacteria

  • C. AndersenEmail author
Part of the Reviews of Physiology, Biochemistry and Pharmacology book series (REVIEWS, volume 147)


For translocation across the cell envelope of Gram-negative bacteria, substances have to overcome two permeability barriers, the inner and outer membrane. Channel-tunnels are outer membrane proteins, which are central to two distinct export systems: the type I secretion system exporting proteins such as toxins or proteases, and efflux pumps discharging antibiotics, dyes, or heavy metals and thus mediating drug resistance. Protein secretion is driven by an inner membrane ATP-binding cassette (ABC) transporter while drug efflux occurs via an inner membrane proton antiporter. Both inner membrane transporters are associated with a periplasmic accessory protein that recruits an outer membrane channel-tunnel to form a functional export complex. Prototypes of these export systems are the hemolysin secretion system and the AcrAB/TolC drug efflux pump of Escherichia coli, which both employ TolC as an outer membrane component. Its remarkable conduit-like structure, protruding 100 Å into the periplasmic space, reveals how both systems are capable of transporting substrates across both membranes directly from the cytosol into the external environment. Proteins of the channel-tunnel family are widespread within Gram-negative bacteria. Their involvement in drug resistance and in secretion of pathogenic factors makes them an interesting system for further studies. Understanding the mechanism of the different export apparatus could help to develop new drugs, which block the efflux pumps or the secretion system.


Efflux Pump Secretion System Outer Membrane Protein Accessory Protein Secretion Signal 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.



ATP-binding cassette


Circular dichroism


Single channel conductance


General secretory pathway


Inner membrane


Major facilitator


Minimum inhibitory concentration


Nuclear magnetic resonance


Outer membrane


Proton motif force


Resistance nodulation cell division


Repeats in toxins


Small multidrug resistance


Transmembrane segment


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© Springer-Verlag 2003

Authors and Affiliations

  1. 1.Department of BiotechnologyUniversity of WürzburgWürzburgGermany

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