The F4 fimbrial chaperone FaeE is stable as a monomer that does not require self-capping of its pilin-interactive surfaces

Many Gram-negative bacteria use the chaperone-usher
pathway to express adhesive surface structures, such as
fimbriae, in order to mediate attachment to host cells. Periplasmic
chaperones are required to shuttle fimbrial subunits or
pilins through the periplasmic space in an assembly-competent
form. The chaperones cap the hydrophobic surface of the
pilins through a donor-strand complementation mechanism.
FaeE is the periplasmic chaperone required for the assembly
of the F4 fimbriae of enterotoxigenic Escherichia coli. The
FaeE crystal structure shows a dimer formed by interaction
between the pilin-binding interfaces of the two monomers.
Dimerization and tetramerization have been observed previously
in crystal structures of fimbrial chaperones and has
been suggested to serve as a self-capping mechanism that
protects the pilin-interactive surfaces in solution in the
absence of the pilins. However, thermodynamic and biochemical
data show that FaeE occurs as a stable monomer in
solution. Other lines of evidence indicate that self-capping of
the pilin-interactive interfaces is not a mechanism that is
conservedly applied by all periplasmic chaperones, but is
rather a case-specific solution to cap aggregation-prone
surfaces.