Crystal structure of the FimD usher bound to its cognate FimC–FimH substrate

Type 1 pili are the archetypal representative of a widespread class of adhesive multisubunit fibres in Gram-negative bacteria. During pilus assembly, subunits dock as chaperone-bound complexes to an usher, which catalyses their polymerization and mediates pilus translocation across the outer membrane. Here we report the crystal structure of the full-length FimD usher bound to the FimC–FimH chaperone–adhesin complex and that of the unbound form of the FimD translocation domain. The FimD–FimC–FimH structure shows FimH inserted inside the FimD 24-stranded β-barrel translocation channel. FimC–FimH is held in place through interactions with the two carboxy-terminal periplasmic domains of FimD, a binding mode confirmed in solution by electron paramagnetic resonance spectroscopy. To accommodate FimH, the usher plug domain is displaced from the barrel lumen to the periplasm, concomitant with a marked conformational change in the β-barrel. The amino-terminal domain of FimD is observed in an ideal position to catalyse incorporation of a newly recruited chaperone–subunit complex. The FimD–FimC–FimH structure provides unique insights into the pilus subunit incorporation cycle, and captures the first view of a protein transporter in the act of secreting its cognate substrate.