Crystallography and electron microscopy of chaperone/usher pilus systems

Among bacteria, the chaperone-usher (CU) pathway is a widespread conserved assembly and translocation system for adhesive protein fibres, called pili or fimbriae. Pili are large linear polymers that protrude from the outer bacterial surface and consist of several subunits. Pili contain adhesin proteins at the tip that are used by pathogenic bacteria to mediate attachment to host cells and initiate infections. Well studied examples of CU pili are P and type 1 pili of uropathogenic Escherichia coli (UPEC), which are responsible for kidney and bladder infections, respectively. Upon secretion into the periplasm, pilus subunits are stabilized by periplasmic chaperones and the resulting chaperone:subunit complexes are guided to the usher located in the outer membrane. The usher catalyzes the ordered assembly of pilus subunits while releasing the chaperones and translocating the growing pilus stepwise to the outer surface. Here we review the structural biology of the chaperone-usher pathway that has helped to understand the mechanisms by which biogenesis of an important class of bacterial organelles occurs