PROJECT SUMMARY Neisseria gonorrhoeae (GC) is a growing public health problem which is likely to worsen due to increasing antibiotic resitance and the lack of a vaccine. Type IV pili (Tfp) are known to be essential for GC virulence in humans, and there is strong evidence that Tfp bind to high-affinity receptors on human cells. However, the identities of both the specific GC protein(s) and host receptor(s) responsible for high-affinity adherence are not known. Twenty years of pubished experiments strongly support the hypothesis that the low-abundance protein PilC is the high-affinity adhesin presented by GC Tfp. However, little is known about the overall structure of PilC, the mechanisms through which it associates with Tfp, and the host receptors that it presumably engages. In this project we will use in vivo genetic tests and in vitro biochemical analyses to test models of PilC functional organization, and we will use the resulting information and reagents to begin the process of identifying high-affinity receptors on human mucosal epithelial cells. The impacts of this project will include clarifying the mechanisms of PilC association with pili, the mechanisms through which PilC influences pilus biogenesis, and the mechanisms through which GC and the closely related pathogen N. meningitidis adhere to human epithelial tissues to initiate infection.