Soluble pilin of Neisseria gonorrhoeae interacts with human target cells and tissue

Pili of Neisseria gonorrhoeae are phase-variable surface structures that me diate adherence to host target cells. Each pilus is composed of thousands o f major pilus subunits, pilins, pilus-associated protein PilC, and possibly other components. Piliated and nonpiliated gonococcal clones may secrete a soluble smaller pilin (S-pilin) that is cleaved after amino acid 39 of the mature pilin protein. Here, purified S-pilin was found to migrate as a 61- to 64-kDa double band on nondenaturing gels, suggesting the formation of t etrameric S-pilin proteins with two isomeric forms. In situ studies of bind ing to formalin-fixed tissue sections demonstrated the binding of S-pilin t o human tissue but not to tissue from mouse or rat organs, showing the pres ence of a human-specific receptor-binding domain within the pilin polypepti de. Pretreatment of the target tissues with proteinase K decreased gonococc al binding dramatically, whereas pretreatment with neuraminidase and meta-p eriodate, which cleave carbon-carbon linkages between vicinal hydroxyl grou ps in carbohydrates, did not affect gonococcal binding. In overlay assays, purified S-pilin bound to a band with a migration pattern and size similar to those of CD46, a cellular pilus receptor. Further, binding of N. gonorrh oeae to target cells and tissues could be blocked by both CD46 antibodies a nd purified S-pilin. These data argue that S-pilin interacts with a protein domain(s) of the CD46 receptor on human cells.