Pilus assembly is a complex process that requires many proteins. We are trying to understand how the various components of the system fit together in 3D so that we can figure out how assembly occurs. In our current model, there are three subcomplexes required for assembly:
an inner membrane polymerization and depolymerization machinery
an inner membrane alignment subcomplex
an outer membrane portal subcomplex
The polymerization-depolymerization subcomplex consists of the “platform” protein PilC, the pre-pilin peptidase PilD, the pilins and minor pilins, and the ATPases PilB, PilT and PilU that power the process. The alignment subcomplex consists of the PilMNOP and FimV proteins, and serves to ensure that the growing pilus fibre is lined up correctly with the outer membrane subcomplex, consisting of PilQ and PilF, so that it can exit the cell. The outer membrane subcomplex is a gated pore that is thought to remain closed until contacted by the growing fibre, which induces a conformational change to allow the fibre to pass to the outside.