Joined: May 2002
Note that while flagella do not apparently make use of a secretin as the outer membrane pore, type III virulence systems do. And furthermore, the outer membrane ring of the flagellum *is* secreted by a type II rather than Type III virulence system, an interesting similarity.
Other points here:
- 6+ secretion systems in one critter
- both type II and type III SS have virulent and non-virulent uses
Secretins of Pseudomonas aeruginosa: large holes in the outer membrane
Abstract Pseudomonas aeruginosa produces a large number of exoproteins, ranging from the ADP-ribosyltransferases exotoxin A and ExoS to degradative enzymes, such as elastase and chitinase. As it is a gram-negative bacterium, P. aeruginosa must be able to transport these exoproteins across both membranes of the cell envelope. In addition, also proteins that are part of cellular appendages, such as type IV pili and flagella, have to cross the cell envelope. Whereas the majority of the proteins transported across the inner membrane are dependent on the Sec channel, the systems for translocation across the outer membrane seem to be more diverse. Gram-negative bacteria have invented a number of different strategies during the course of evolution to achieve this goal. Although these transport machineries seem to be radically different, many of them actually depend on a member of the secretin protein family for their function. Recent results show that secretins form a large complex in the outer membrane, which constitutes the actual translocation channel. Understanding the working mechanism of this protein translocation channel could open up new strategies to target molecular machineries at the heart of many important virulence factors.
Keywords Secretin - Outer membrane - Exoprotein - Pseudomonas aeruginosa - Pili
Secreted proteins of Pseudomonas aeruginosa
Gram-negative bacteria are efficiently protected against many harmful compounds in the environment by the presence of a second membrane, the outer membrane, which functions as a molecular sieve because of the presence of both specific and general pore-forming proteins (Hancock 1997). These porins form channels in the outer membrane through which small hydrophilic molecules with a molecular mass up to 250 Dalton can diffuse. However, gram-negative bacteria also have to transport a range of macromolecules across the outer membrane. Today, at least 19 different soluble exoproteins are known to be secreted by P. aeruginosa (Table 1). Most of these soluble exoproteins, such as exotoxin A, S, U and Y, elastase, staphylolytic protease, lipase and phospholipase C, are well-known virulence factors (Sandkvist 2001a; Cornelis and Van Gijsegem 2000), whereas others are at least suspected to be involved in virulence. This means that, among the gram-negative bacteria, P. aeruginosa is one of the most active secreting species. Proteins that are part of cellular appendages also have to be transported across the outer membrane. These compounds include the subunits that compose the flagella and the type IV pili. Both these appendages are used for binding and motility in P. aeruginosa and are essential for pathogenicity (Hahn 1997). Adhesins and surface-associated enzymes may also belong to this category.
All the proteins and protein structures described above have to be secreted through outer membrane channel(s). The opening of these channels will have to be strictly regulated in order to retain proper functioning of the outer membrane as a molecular sieve. In the last decade, it has become clear that there are multiple outer membrane channels and transport machineries for the translocation of proteins across this second membrane. If one only considers the soluble exoproteins, already six different pathways have been identified: the type I pathway (Andersen et al. 2000); the type II pathway (Filloux et al. 1998); the type III or contact secretion pathway (Cornelis and Van Gijsegem 2000); the type IV pathway (Christie 2001); the autotransporter pathway (Henderson et al. 1998); and the two-partner secretion pathway (Jacob-Dubuisson et al. 2001). Apart from the type IV secretion pathway, all of these different systems are used (Table 1) or are at least present in P. aeruginosa. In addition, chitinase is probably secreted via a novel pathway (Folders et al. 2001). Although there are many different transport systems, one family of outer membrane proteins has been shown to be particularly useful for P. aeruginosa, the secretins. Members of this family are involved in two of the secretion systems described above: the type II and the type III secretion pathways. These two systems seem to be completely different: the type III system mediates the secretion of virulence factors directly from the cytoplasm into eukaryotic target cells and is homologous to the flagellar assembly system, whereas the type II system secretes folded proteins from the periplasm into the surrounding and is highly homologous to the type IV pili biogenesis machinery. The only common denominator between these systems is the outer membrane component, which belongs to the secretin family (Genin and Boucher 1994). Secretin family members are also used for the biogenesis of type IV pili (Mattick et al. 1996), but not in flagella synthesis. In addition, secretins are involved in other processes, such as the biosynthesis of another class of pili (Skerker and Shapiro 2000) and the biogenesis of filamentous phage particles (Linderoth et al. 1997). These phages, such as Pseudomonas phage Pf3 and Escherichia coli phages M13 and f1, are continuously produced by infected bacterial cells without disrupting the integrity of the bacterial cell. Again, only the outer membrane component of this machinery shows homology with the other secretin-dependent secretion systems described above. Finally, in several other gram-negative bacteria, such as Haemophilus influenza and Neisseria species, secretin family members have been implicated in the process of natural competence, i.e. the uptake of DNA from the environment (Dubnau 1999). However, this dependence can be explained by the fact that, in these cases, type IV pili are involved in competence and secretin family members are essential components in the biosynthesis of this class of pili.
These data show that secretins are involved in many different processes in Proteobacteria. Recently, genome sequencing projects have demonstrated that secretins can also be found in a wide variety of other bacterial species, including such diverse species as Chlamydia trachomatis, Deinococcus radiodurans, and even the deep-branching species Aquifex aeolicus. This means that the use of secretins for outer membrane transport is widespread among the bacteria and that some of these new secretins could very well be involved in transport processes that have not been characterised thus far. What makes the secretin family members such useful proteins to be employed in protein transport? Part of the answer is probably related to the functional unit of secretins: the oligomeric complex.