Structure and antigenic properties of the tip-located P pilus proteins of uropathogenic Escherichia coli.

Pyelonephritogenic Escherichia coli frequently expresses pili which bind to Gal alpha (1-4)Gal receptors present on the uroepithelium. Binding of these pili is mediated by a pilus-associated adhesin, PapG, and not by the major subunit which constitutes the bulk of the pilus structure. The adhesin and two pilinlike proteins, PapE and PapF, are present in only a few copies each at the pilus tip. Surface exposure of both PapF and PapG is required to achieve receptor-specific binding. The nucleotide sequences for the genes encoding the tip-associated proteins PapE, PapF, and PapG were determined for two E. coli clones expressing P pili of serotypes F11 and F7(2) and compared with the corresponding sequences established for proteins of F13 pili. Specific antisera were used to study the cross-reactivity between the F13 tip proteins and the equivalent proteins in F11 and F7(2) pili. We present data showing that, like the major pilus subunit, PapE varies its structure and antigenic properties among pili of different serotypes. In contrast, the PapF protein was highly conserved, and PapF-specific antisera raised against serotype F13 cross-reacted with the PapF proteins of both F11 and F7(2) serotypes. The PapG adhesin protein from F11 and F7(2) pili differed by only five amino acids out of 316 residues. However, the F13 adhesin showed only 45% amino acid homology with the other two variants.