These pili are assembled at usher proteins (above) that are embedded in the cell walls of bacteria; chaperone proteins are also involved in the assembly process. The usher protein recruits chaperone-subunit complexes, catalyzes the polymerization of the subunits, and allows passage of the pili to the cell surface. The usher protein must be activated in order to act as a catalyst; activation involves moving a ‘plug’ (purple) that blocks the channel in the protein (left) through an angle of ∼150° to open the channel (right). The movement of the plug also orients the domains (NTD, CTD1 and CTD2) that recruit the chaperone-subunit complexes (not shown). These models are based on X-ray structures of the PapC usher protein (PDB:2vqi; Remaut et al., 2008) and the FimD usher protein bound to a chaperone-subunit complex (PDB:3rfz; Phan et al., 2011; Geibel et al., 2013). Farabella et al. explored the roles of the alpha-helix (yellow) and the beta-hairpin (blue/grey) in the activation process.