Handover mechanism of the growing pilus by the bacterial outer‐membrane usher FimD

Pathogenic bacteria such as Escherichia coli assemble surface structures termed pili, or fimbriae, to mediate binding to host‐cell receptors. Type 1 pili are assembled via the conserved chaperone usher pathway. The outer‐membrane usher FimD recruits pilus subunits bound by the chaperone FimC via the periplasmic N‐terminal domain of the usher. Subunit translocation through the β‐barrel channel of the usher occurs at the two C‐terminal domains (which we label CTD1 and CTD2) of this protein. How the chaperone–subunit complex bound to the N‐terminal domain is handed over to the C‐terminal domains, as well as the timing of subunit polymerization into the growing pilus, have previously been unclear. Here we use cryo‐electron microscopy to capture a pilus assembly intermediate (FimD–FimC–FimF–FimG–FimH) in a conformation in which FimD is in the process of handing over the chaperone‐bound end of the growing pilus to the C‐terminal domains. In this structure, FimF has already polymerized with FimG, and the N‐terminal domain of FimD swings over to bind CTD2; the N‐terminal domain maintains contact with FimC–FimF, while at the same time permitting access to the C‐terminal domains. FimD has an intrinsically disordered N‐terminal tail that precedes the N‐terminal domain. This N‐terminal tail folds into a helical motif upon recruiting the FimC‐subunit complex, but reorganizes into a loop to bind CTD2 during handover. Because both the N‐terminal and C‐terminal domains of FimD are bound to the end of the growing pilus, the structure further suggests a mechanism for stabilizing the assembly intermediate to prevent the pilus fibre diffusing away during the incorporation of thousands of subunits. Support or Funding Information R01GM062987 Van Andel Institute Graduate School This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .