Kingella kingae is a leading cause of bone and joint infections in young children. The pathogenesis of K. kingae disease requires translocation across the oropharyngeal epithelium, mediated by type IV pili (T4P). T4P are surface filaments that undergo extension and retraction, coordinated by a cell envelope-spanning protein complex, the T4P machine (T4PM). In K. kingae , T4P contain the pilus-associated proteins PilC1 and PilC2, which play key roles in T4P activity. Recent observations indicate that PilC2 interacts with PilC1 and may interfere with its function. It remains unknown how the K. kingae T4P fiber and T4PM structures may differ from those of other species, and the roles of PilC1 and PilC2 in K. kingae translocation are also unclear. We hypothesize that PilC1 and PilC2 form a dynamic complex that localizes to the tip of the T4P fiber to facilitate distinct cellular trafficking pathways mediated by their differential affinities for host cell factors, while also localizing within the T4PM to prime pilus biogenesis. Here, we resolved the structures of the K. kingae T4P fiber and T4PM using single-particle cryo-electron microscopy (cryo-EM) and cryo-electron tomography (cryo-ET), respectively. In ongoing work, cryo-EM will be used to determine the structures of PilC1 and PilC2 in isolation and in association with the T4P fiber; cryo-ET will be used to define their localizations and interactions with the T4PM; and volume SEM will be employed to characterize the cellular trafficking pathways mediated by PilC1 and PilC2 through the respiratory epithelium. These findings may inform the rational design of therapeutics to target the PilC family, thereby interfering with T4P function to prevent invasive disease caused by K. kingae and potentially other T4P-producing bacterial pathogens.
Shukla et al. (Sun,) studied this question.