Molecular dissection of PapD interaction with PapG reveals two chaperone-binding sites

P pili are composite adhesive fibres that allow uropathogenic Escherichia coli to gain a foothold in the host by binding to receptors present on the uroepithelium via the adhesin PapG. The assembly of P pili requires a periplasmic chaperone, PapD, that has an immunoglobulin-like three-dimensional structure. PapD-subunit complex formation involves a conserved anchoring mechanism in the chaperone cleft and a 'molecular zippering' to the extreme C-terminus of pilus subunits. A chaperone-binding assay was developed using fusions of the C-terminus of PapG to maltose-binding protein (MBP/G fusions) to investigate whether chaperone-subunit complex formation requires additional interactions. PapD bound strongly to an MBP/G fusion containing the C-terminal 140 amino acids of PapG (MBP/G175-314) but only weakly to the MBP/G234-314 fusion containing 81 C-terminal residues, arguing that the region between residues 175-234 contains additional information that is required for strong PapD-PapG interactions. PapD was shown to interact with a PapG C-terminal truncate containing residues 1-198 but not a truncate containing residues 1-145, suggesting the presence of a second, independent PapD interactive site. Four peptides overlapping the second site region were tested for binding to PapD in vitro to further delineate this motif. Only one of the peptides synthesized was recognized by PapD. The MBP/G fusion containing both binding sites formed a tight complex with PapD in vivo and inhibited pilus assembly by preventing chaperone-subunit complex formation.