PilC

PilC is a protein associated with type IV pilus systems in several Gram-negative bacteria, most notably Neisseria meningitidis and Neisseria gonorrhoeae, as well as Haemophilus influenzae. In these organisms, PilC refers to one or two surface-exposed, outer membrane–associated proteins that can act both as assembly factors for type IV pili and as adhesins that promote contact with host cells. In Neisseria, two paralogs, PilC1 and PilC2, can be differentially expressed and contribute to distinct adhesion properties and pilus functions.

PilC proteins are typically encoded within pilus operons alongside the major pilin subunit (PilE) and other pilus biogenesis factors. They are anchored to the outer membrane, usually via a Sec-dependent pathway and lipid modification, positioning them at the base or tip region of pilus fibers that extend from the bacterial surface. Through interactions with other components of the pilus assembly machinery, PilC helps assemble pili and also mediates binding to receptors on human cells, contributing to colonization and virulence. The exact adhesion receptors can vary by species and PilC variant.

Expression of PilC can be regulated by environmental cues and phase variation, enabling pathogens to modulate pili expression and adhesive properties. Because PilC is exposed on the bacterial surface and contributes to host interaction, it is a focus of ongoing research as a potential target for vaccines or therapeutics. The term PilC may also be used for similar PilC-like proteins in related bacteria that deploy type IV pili.