Novel Role for PilNO in Type IV Pilus Retraction Revealed by Alignment Subcomplex Mutations.

Pseudomonas aeruginosa is an opportunistic pathogen that uses type IV pili (T4P) for host attachment. The T4P machinery is composed of four cell envelope-spanning subcomplexes. PilN and PilO heterodimers are part of the alignment subcomplex and essential for T4P function. Three potential PilNO interaction interfaces (the core-core, coiled-coil, and transmembrane segment interfaces) were probed using site-directed mutagenesis followed by functional assays in an Escherichia coli two-hybrid system and in P. aeruginosa. Several mutations blocked T4P assembly and/or motility, including two that revealed a novel role for PilNO in pilus retraction, while other mutations affected extension dynamics. These critical PilNO interaction interfaces represent novel targets for small-molecule inhibitors with the potential to disrupt T4P function.