NMR structure of the Escherichia coli type 1 pilus subunit FimF and its interactions with other pilus subunits

Type 1 pili from uropathogenic Escherichia coli strains mediate bacterial attachment to target receptors on the host tissue. They are composed of up to 3000 copies of the subunit FimA, which form the stiff, helical pilus rod, and the subunits FimF, FimG, and FimH, which form the linear tip fibrillum. All subunits in the pilus interact via donor strand complementation, in which the incomplete immunoglobulin-like fold of each subunit is complemented by insertion of an N-terminal extension from the following subunit. We determined the NMR structure of a monomeric, self-complemented variant of FimF, FimF_F, which has a second FimF donor strand segment fused to its C-terminus that enables intramolecular complementation of the FimF fold. NMR studies on bimolecular complexes between FimF_F and donor strand-depleted variants of FimF and FimG revealed that the relative orientations of neighboring domains in the tip fibrillum cover a wide range. The data provide strong support for the intrinsic flexibility of the tip fibrillum. They lend further support to the hypothesis that this flexibility would significantly increase the probability that the adhesin at the distal end of the fibrillum successfully targets host cell receptors.