Illustration of four PilQ secretin proteins from the type IV pilus (T4P) of the bacterium Pseudomonas aeruginosa bound by T4P secretin-associated protein (TsaP). A pilus is a hair-like protein filament found on the surface of a bacterial cell. It is used for locomotion and to adhere to substrates. PilQ sits in the outer membrane of the bacterium creating a channel for the pilus machinery to pass through. TsaP plays a role in the assembly of PilQ.