Adhesive pili of the chaperone-usher family - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 364

FIGURE 12.5 Model of pilus assembly. Upon translocation of subunits into the periplasm via the SEC machinery, chaperone proteins bind the subunits to maintain
their proper fold and stability. At the outer membrane, the usher lies in a gated state, with the plug domain (blue) residing in the lumen of the usher pore (orange).
Once the chaperone-adhesin complex binds the usher NTD (cyan), the plug domain swings away from the pore and docks onto the NTD, thus preparing the usher
for pilus secretion. Soon thereafter, CTD1 (yellow) and CTD2 (purple) may swing over to bind and carry the chaperone-adhesin complex, freeing the NTD-plug
complex in the process. The freed NTD-plug complex recruits the incoming chaperone-subunit complex, orienting it properly so that the Nte can participate in DSE
with the CTD-docked chaperone-adhesin complex and the newly exchanged chaperone-subunit complex can transfer to the CTDs. The process repeats until the
chaperone-terminator complex, bound at the NTD-plug complex, cannot transfer to the CTDs or engage in DSE, signaling the end of pilus assembly.