Crystal structure of the ternary FimC–FimFt–FimDN complex indicates conserved pilus chaperone–subunit complex recognition by the usher FimD

Type 1 pili, anchored to the outer membrane protein FimD, enable uropathogenic Escherichia coli to attach to host cells. During pilus biogenesis, the N-terminal periplasmic domain of FimD (FimDN) binds complexes between the chaperone FimC and pilus subunits via its partly disordered N-terminal segment, as recently shown for the FimC–FimHP–FimDN ternary complex. We report the structure of a new ternary complex (FimC–FimFt–FimDN) with the subunit FimFt instead of FimHp. FimDN recognizes FimC–FimFt and FimC–FimHP very similarly, predominantly through hydrophobic interactions. The conserved binding mode at a “hot spot” on the chaperone surface could guide the design of pilus assembly inhibitors.