Minor pseudopilins of the type II secretion system in P. aeruginosa: placing pieces in a large puzzle

Introduction: The type II secretion system (T2SS) is the unique identified machinery able to secrete a wide range of folded proteins from the periplasm to the extracellular milieu. The secretion process is carry-out by multiprotein complexes sharing high similarities with the type IV piliation system. The T2SS assembles in the periplasm an enigmatic pilus-like structure called the pseudopilus constituted by the helicoidal assembly of the major pseudopilin XcpT. Four other minor pseudopilins XcpU, V, W and X, structurally compatible with the pseudopilus, are also member of the T2SS. Previous structural studies have shown that three of them (XcpV, W and X homologs) are organized in an helicoidal ternary complex presumably located at the tip of the pseudopilus. Results and discussion: We combined affinity chromatography, SPR, SEC MALS and NMR chemical shift perturbation to investigate the interaction network between the soluble domains of the five pseudopilins of the P. aeruginosa Xcp machinery. We revealed an unprecedented strictly ordered quaternary complex, including the so far not assigned minor pseudopilin XxcpU. In order to understand at a molecular level the role of the minor pseudopilins and their organization within the pseudopilus, we investigated the quaternary complex using SAXS, crystallography and Mass-spectroscopy. We generated the SAXS envelop of the complex in which we were able to dock a model of the XcpV-W-X ternary complex. In our model, the fourth minor pseudopilin XcpU integrate the ternary complex by binding the lower part of XcpW following the helicoidal arrangement of the pseudopilus, thus suggesting for XcpU a linker function between the body and the tip of the pseudopilus.