Type IV pili (T4P) are filamentous appendages that are present on the surfaces of a diverse range of bacteria, including the opportunistic human pathogen Pseudomonas aeruginosa. T4P can polymerize and depolymerize, which leads to extension and retraction cycles, and they have crucial roles in, for example, bacterial twitching motility. Single-stranded RNA (ssRNA) phages exploit the T4P to gain cell entry and infect bacteria. Specifically, the phage-encoded maturation protein (Mat) has been shown previously to attach to the side of the bacterial pilus. However, the underlying mechanisms of how this interaction mediates phage entry into the host cell is unclear. In this new study, Thongchol, Yu et al. show that the ssRNA phage PP7 binds to the P. aeruginosa T4P, leading to phage entry into the cell and the detachment of the pilus, which impairs bacterial motility.

Next, structural studies revealed that each PP7 virion contains two Mat molecules, one exposed and the other internalized. The two molecules form a heterotypic homodimer that binds to the 3′ end of viral RNA. In addition, further structural analysis showed that the minor β-sheet region of the external PP7 Mat molecule interacts with a single pilin subunit of the T4P filament, and the authors termed this β-sheet region the pilus-interacting region (PIR).