Deciphering the Xcp Pseudomonas aeruginosa type II secretion machinery through multiple interactions with substrates.

The type II secretion system enables gram-negative bacteria to secrete exoproteins into the extracellular milieu. We performed biophysical and biochemical experiments to identify systematic interactions between Pseudomonas aeruginosa Xcp type II secretion system components and their substrates. We observed that three Xcp components, XcpP(C), the secretin XcpQ(D), and the pseudopilus tip, directly and specifically interact with secreted exoproteins. We established that XcpP(C), in addition to its interaction with the substrate, likely shields the entire periplasmic portion of the secreton. It can therefore be considered as the recruiter of the machinery. Moreover, the direct interaction observed between the substrate and the pseudopilus tip validates the piston model hypothesis, in which the pseudopilus pushes the substrate through the secretin pore during the secretion process. All together, our results allowed us to propose a model of the different consecutive steps followed by the substrate during the type II secretion process.