Structure of the minor pseudopilin XcpW from the Pseudomonas aeruginosa type II secretion system

Pseudomonas aeruginosa utilizes the type II secretion machinery to transport virulence factors through the outer membrane into the extracellular space. Five proteins in the type II secretion system share sequence homology with pilin subunits of type IV pili and are called the pseudopilins. The major pseudopilin XcpT_G assembles into an intraperiplasmic pilus and is thought to act in a piston-like manner to push substrates through an outer membrane secretin. The other four minor pseudopilins, XcpU_H, XcpV_I, XcpW_J and XcpX_K, play less well defined roles in pseudopilus formation. It was recently discovered that these four minor pseudopilins form a quaternary complex that is presumed to initiate the formation of the pseudopilus and to localize to its tip. Here, the structure of XcpW_J was refined to 1.85 Å resolution. The structure revealed the type IVa pilin fold with an embellished variable antiparallel β-sheet as also found in the XcpW_J homologue enterotoxigenic Escherichia coli GspJ_W and the XcpU_H homologue Vibrio cholerae EpsU_H. It is proposed that the exposed surface of this sheet may cradle the long N-terminal α1 helix of another pseudopilin. The final 31 amino acids of the XcpW_J structure are instrinsically disordered. Deletion of this unstructured region of XcpW_J did not prevent type II secretion in vivo.