Pseudomonas pili. Studies on antigenic determinants and mammalian cell receptors.

P. aeruginosa PAK pili are thin 5.2 nm diameter filaments containing a single 15-kd polypeptide subunit which is 144 amino acid residues in length. Studies on pili binding to a variety of synthetic sugars representing many di- tri- and tetra-saccharide structures found in mammalian glycoproteins and glycolipids failed to reveal any significant binding activity. On the other hand, a wide spectrum of binding activities was observed when a variety of structural proteins and enzymes were used as binding substrates. Of 30 proteins tested, phosphorylase b, pyruvate kinase and aldolase showed highest pilus binding activity. It was concluded that the PAK pilus receptor is probably a polypeptide rather than an oligosaccharide. Using arginine-specific cleavage to produce four large peptides, several proteases to produce subfragments of the large peptides, and antipilus rabbit antiserum, PAK pilin was found to contain four antigenic determinants. Epitopes near the NH2- and COOH-termini were only weakly immunogenic, whereas two epitopes near the center of the pilus protein titrated about 85% of the antipilus antibodies. Cleavage of the pilus protein into smaller peptides resulted in marked decreases in the affinity of antigenic peptides for their specific antibodies, suggesting that the immunodominant epitopes of PAK pilin are conformation-specific.