PapD is the periplasmic chaperone required for the assembly of P pili in pyelonephritic strains of Escherichia coli. It consists of two immunoglobulin‐like domains bisected by a subunit binding cleft. PapD is the prototype member of a superfamily of immunoglobulin‐like chaperones that work in concert with their respective ushers to assemble a plethora of adhesive organelles including pilus‐ and non‐pilus‐associated adhesins. Three highly conserved residue clusters have been shown to play critical roles in the structure and function of PapD, as determined by site‐directed mutagenesis. The in vivo stability of the chaperone depended on the formation of a buried salt bridge within the cleft. Residues along the G1 beta strand were required for efficient binding of subunits consistent with the crystal structure of PapD–peptide complexes. Finally, Thr‐53, a residue that is part of a conserved band of residues located on the amino‐terminal domain surface opposite the subunit binding cleft, was also found to be critical for pilus assembly, but mutations at Thr‐53 did not interfere with chaperone–subunit complex formation.
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