Stable fiber-forming and nonfiber-forming chaperone-subunit complexes in pilus biogenesis

The P pilus is a composite fiber consisting of a thin adhesive tip fibrillum joined to the pilus rod that mediates specific adherence of uropathogenic Escherichia coli to human uroepithelial cells via the PapG tip adhesin. P pilus assembly depends upon the periplasmic chaperone PapD. The interaction of PapD with different pilus subunits was investigated to gain further insight into pilus assembly. PapA, the major subunit of the pilus rod, formed two periplasmic complexes (DA₂ and DA) with PapD. PapK, an adaptor protein that joins the tip fibrillum to the pilus rod, formed only one complex with PapD (DK). Only 'fiber forming' or homopolymeric subunits, PapA in the rod and PapE in the tip fibrillum, were able to form subunit- subunit interactions in the periplasm. Subunits that are present in single or low copy in the pilus (PapK and PapG) did not form periplasmic intersubunit interactions. A pulse-chase analysis revealed that a chaperone-PapA complex is a true periplasmic intermediate in pilus assembly.