Protein Secretion Systems in Pseudomonas aeruginosa: An Essay on Diversity, Evolution, and Function

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.

Keywords: cell envelope; channel; macromolecular complex; nanomachine; targeting.

Figure 1

Structure of the P. aeruginosa EstA autotransporter (T5aSS), reproduced from van den Berg (2010). (A) Backbone representation viewed from the side, with the protein colored by a gradient from blue at the N-terminus to red at the C-terminus. (B) Backbone view 90° rotated within the plane of the membrane relative to (A), with helices colored red, β-strands colored green, and loops colored gray. The catalytic triad residues are shown as blue stick models. Horizontal lines indicate the approximate location of the outer membrane core. (C) Surface view of EstA from the side with the β-barrel domain colored green and the passenger domain colored red. (D) Stereo view of the EstA passenger from the extracellular side, colored as a rainbow from dark blue at the N-terminus to dark red at the C-terminus. The numbers are those for the central residue of the α-helix.

Figure 2

Pseudomonas aeruginosa secretion gene clusters. Within the clusters shown, the gene encoding the ATPase is colored red, the gene encoding the outer membrane channel is colored green. The genes encoding secreted proteins are colored dark orange. Genes encoding components of the T2SS pseudopilus, the T3SS needle and the T6SS tail tube are colored dark blue. The genes in light orange encode either translocators of the T3SS or puncturing device of the T6SS. Shown in light blue are T3SS chaperones and in gray the T3SS ruler. Shown in yellow are genes encoding proteins involved in regulation. For more details see text.

Figure 3

A transmission electron-microscope image of isolated T3SS needle complexes from S. typhimurium reproduced from Schraidt et al. (2010).

Figure 4

Schematic model for the T6SS. (A) Structural-based homology prediction for the VgrG1a protein of P. aeruginosa reproduced from Hachani et al. (2011). The Hcp1 nanotube reproduced from Ballister et al. (2008). (B) Schematic representation of the T6SS, in which VgrG1a (puncturing device) and Hcp1 nanotube have been included as depicted in (A). Few other T6SS components are shown and the system punctures the bacterial cell envelope from inside to outside. See text for more details.

Figure 5

The Two partner secretion systems (T5bSSs). (A) Structural prediction and modeling of P. aeruginosa CdrB, the carrier domain, performed using Phyre (http://www.sbg.bio.ic.ac.uk/phyre). Figures were made using PyMOL (http://www.pymol.org). From left to right, top (facing extracellular milieu), side, and bottom (facing periplasm) view. (B) Amino-acid sequence alignment of the “TPS” domains of several TpsAs passenger including FHA (B. pertussis), HMW1 (H. influenzae), TpsA1–5 and CupB5 (P. aeruginosa). The NPNL motif in FHA and TpsA1–2 (red) or NxxGx motif in all TpsAs (blue) is indicated with bold characters. The alignment was performed using clustalW2 (http://www.ebi.ac.uk/Tools/msa/clustalw2).

Figure 6

XcpT pseudopilin overexpression results in pseudopilus formation. Shown are the results of TEM analysis of P. aeruginosa PAO1 overexpressing xcpT from pMTWT as described in Durand et al. (2003). The pseudopili are indicated with a black arrow and are labeled with gold particles coupled to antibodies against XcpT. Red arrows indicate unlabeled flagella for comparison.

Figure 7

Secretion systems in P. aeruginosa. The systems shown are discussed in the main text. For characteristics of exoproteins see also Table 1. The design of the secretion systems is as previously presented in Bleves et al. (2010). Color code is: brown (T1SS), light Orange (T2SS), red (T3SS), blue/purple (T5SS), green (T6SS).