The ability of pili from Pseudomonas aeruginosa K (PAK) to act as an adhesin to human respiratory epithelial cells was examined using an in vitro adhesion assay. Equilibrium analysis of PAK binding to human buccal epithelial cells (BECs) and tracheal epithelial cells (TECs) by means of a Langmuir adsorption isotherm revealed that the maximum numbers of binding sites per epithelial cell (N) were 255 for BECs and 236 for TECs, with apparent association constants (Ka) of 2.8 x 10(-9) and 5.8 x 10(-9) ml/CFU, respectively. Trypsinization of the BECs before the binding assay increased N to 605 and decreased the Ka to 1.7 x 10(-9) ml/CFU. Addition of homologous pili to the binding assay with BECs or TECs or the addition of anti-pilus Fab fragments inhibited PAK adherence. Binding of purified pili to BECs was shown to reach saturation. Purified pili and PAK competed for the same receptor on the BEC surface. Further, by using peptide fragments of PAK pilin (derived from the native pili or produced synthetically) in the binding assay for PAK to BECs, we have presumptively identified the pilus binding domain in the C-terminal region of the pilin and shown that the C-terminal disulfide bridge is important in maintaining the functionality of the binding domain
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