Abstract
Protein secretion systems are molecular nanomachines used by Gram-negative bacteria to thrive within their environment. They are used to release enzymes that hydrolyze complex carbon sources into usable compounds, or to release proteins that capture essential ions such as iron. They are also used to colonize and survive within eukaryotic hosts, causing acute or chronic infections, subverting the host cell response and escaping the immune system. In this article, the opportunistic human pathogen Pseudomonas aeruginosa is used as a model to review the diversity of secretion systems that bacteria have evolved to achieve these goals. This diversity may result from a progressive transformation of cell envelope complexes that initially may not have been dedicated to secretion. The striking similarities between secretion systems and type IV pili, flagella, bacteriophage tail, or efflux pumps is a nice illustration of this evolution. Differences are also needed since various secretion configurations call for diversity. For example, some proteins are released in the extracellular medium while others are directly injected into the cytosol of eukaryotic cells. Some proteins are folded before being released and transit into the periplasm. Other proteins cross the whole cell envelope at once in an unfolded state. However, the secretion system requires conserved basic elements or features. For example, there is a need for an energy source or for an outer membrane channel. The structure of this review is thus quite unconventional. Instead of listing secretion types one after each other, it presents a melting pot of concepts indicating that secretion types are in constant evolution and use basic principles. In other words, emergence of new secretion systems could be predicted the way Mendeleïev had anticipated characteristics of yet unknown elements.
Highlights
Protein secretion systems are molecular nanomachines used by Gram-negative bacteria to thrive within their environment
PROTEIN SECRETION SYSTEMS IN GRAM-NEGATIVE BACTERIA In Gram-negative bacteria, such as P. aeruginosa, the cell envelope is composed of two hydrophobic membranes
The diversity may exist within one single organism such as P. aeruginosa that possesses all the secretion systems known to date but the T4SS (Figure 7)
Summary
Division of Cell and Molecular Biology, Centre for Molecular Microbiology and Infection, Imperial College London, London, UK. The opportunistic human pathogen Pseudomonas aeruginosa is used as a model to review the diversity of secretion systems that bacteria have evolved to achieve these goals This diversity may result from a progressive transformation of cell envelope complexes that initially may not have been dedicated to secretion. The secreted proteins, enzymes, or toxins, do have to travel through the hydrophobic environment of the membranes These proteins are usually large hydrophilic molecules and need to be accommodated into aqueous channels or other type of conduits that are spanning the cell envelope. G. with the pullulanase from Klebsiella species in which a series of motifs have been proposed to be essential for recognition by the Pul system (Francetic and Pugsley, 2005) Another issue about targeting of substrates to the secretion machine is the identification of the T2SS component involved in the specific recognition. OutD of Erwinia chrysanthemi binds to the PelB pectate lyase (Shevchik et al, 1997) and an interaction has been demonstrated between the cholera toxin and the secretin
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