Molecular mechanism of P pilus termination in uropathogenic Escherichia coli

Abstract

P pili are important adhesive fibres that are assembled by the conserved chaperone–usher pathway. During pilus assembly, the subunits are incorporated into the growing fibre by the donor‐strand exchange mechanism, whereby the β‐strand of the chaperone, which complements the incomplete immunoglobulin fold of each subunit, is displaced by the amino‐terminal extension of an incoming subunit in a zip‐in‐zip‐out exchange process that is initiated at the P5 pocket, an exposed hydrophobic pocket in the groove of the subunit. In vivo , termination of P pilus growth requires a specialized subunit, PapH. Here, we show that PapH is incorporated at the base of the growing pilus, where it is unable to undergo donor‐strand exchange. This inability is not due to a stronger PapD–PapH interaction, but to a lack of a P5 initiator pocket in the PapH structure, suggesting that PapH terminates pilus growth because it is lacking the initiation point by which donor‐strand exchange proceeds.