Identification and Characterization of the Chaperone-Subunit Complex-binding Domain from the Type 1 Pilus Assembly Platform FimD

The outer membrane protein FimD represents the assembly platform of adhesive type 1 pili from Escherichia coli. FimD forms ring-shaped oligomers of 91.4 kDa subunits that recognize complexes between the pilus chaperone FimC and individual pilus subunits in the periplasm and mediate subunit translocation through the outer membrane. Here, we have identified a periplasmic domain of FimD (FimDN) comprising the N-terminal 139 residues of FimD. Purified FimDN is a monomeric, soluble protein that specifically recognizes complexes between FimC and individual type 1 pilus subunits, but does not bind the isolated chaperone, or isolated subunits. In addition, FimDN retains the ability of FimD to recognize different chaperone-subunit complexes with different affinities, and has the highest affinity towards the FimC–FimH complex. Overexpression of FimDN in the periplasm of wild-type E. coli cells diminished incorporation of FimH at the tip of type 1 pili, while pilus assembly itself was not affected. The identification of FimDN and its ternary complexes with FimC and individual pilus subunits opens the avenue to structural characterization of critical type 1 pilus assembly intermediates.