FIMH ADHESIN OF TYPE-1 PILI IS ASSEMBLED INTO A FIBRILLAR TIP STRUCTURE IN THE ENTEROBACTERIACEAE

Type 1 pill are heteropolymeric mannose-binding fibers produced by all members of the Enterobacteriaceae family. The bulk of the fiber is composed of FimA. Two macromolecular complexes responsible for mediating an interaction with mannose-containing receptors were purified from fimA(-) Escherichia coli by mannose affinity chromatography and ion-exchange chromatography. One complex contained only the mannose-binding adhesin, FimH, associated with FimG, a minor component of the type 1 pilus. In the other complex the FimG-FimH moiety was loosely associated with a chaperone-minor subunit complex (FimC-FimF), possibly representing an intermediate in tip fibrilla assembly, The FimC chaperone has also been shown to form a preassembly complex with FimH that has been purified and characterized previously. Purified FimC did not bind to the FimG-FimH complex but did recognize FimH dissociated from the FimG-FimH complex, Quick-freeze deep-etch electron microscopy revealed that the FimG-FimH complex had a thin fibrillar architecture. High-resolution electron microscopy of type 1 pill revealed that a 16-nm fibrillar tip structure with an architecture identical to that of the FimG-FimH complex was joined end-to-end to the pilus rod, In a fimH(-) deletion mutant, the tip fibrillae joined to pilus rods were approximate to 3 nm in length. The full-length tip fibrilla was restored by complementation with the fimH gene in trans. The bipartite nature of the type 1 pilus was also demonstrated on pill purified from clinical isolates of members of the Enterobacteriaceae family arguing that it is a conserved feature of the type 1 pilus.