XcpX controls biogenesis of the Pseudomonas aeruginosa XcpT-containing pseudopilus

Pseudomonas aeruginosa is an opportunistic Gram-negative pathogen equipped with multiple secretion systems. The type II secretion machinery (Xcp secreton) is involved in the release of toxins and enzymes. The Xcp secreton is a multiprotein complex, and most of its components share homology with proteins involved in type IV pili biogenesis. Among them, the XcpT-X pseudopilins possess characteristics of the major constituent of the type IV pili, the pilin PilA. We have shown previously that XcpT can be assembled in a multifibrillar structure that was called the pseudopilus. By using two different microscopic approaches, we show here that the pseudopili are preferentially isolated fibers rather than tight bundles. Moreover, none of the other four pseudopilins are able to form a pseudopilus, suggesting that the assembly of such a structure is a unique property of XcpT. Moreover, we show that 5 of the 12 Xcp proteins are not required for pseudopilus biogenesis, whereas they are for type II secretion. Most interestingly, we showed that one pseudopilin, XcpX, controls the assembly of XcpT into a pseudopilus. Indeed, when the number of XcpX subunits increases, the length of the pseudopilus decreases. Conversely, in the absence of XcpX, the pseudopilus length is abnormally long. Our results indicate that XcpT and XcpX directly interact with each other. Furthermore, this interaction induces a clear destabilization of XcpT. The interaction between XcpT and XcpX could be part of the molecular mechanism underlying the dynamic control of pseudopilus elongation, which could be crucial for type II-dependent protein secretion.