Fig. 7.

Model for motor-independent retraction of T4P. Pilus polymerization involves docking of the major pilin at the pilus base followed by ATP hydrolysis by the assembly motor and outward extrusion of the growing filament. When pilus assembly stalls, the terminal pilin subunit may enter a state of dynamic instability (white pilin with dashes). In retraction motor–dependent pili, pilin–pilin interactions may be strong (yellow pilin), with the terminal pilin being more stably associated with the filament and preventing its diffusion back into the membrane in the absence of retraction motors. In retraction motor–independent pili, pilin–pilin interactions may be sufficiently weak (red pilin) whereby the terminal pilin is less stably associated with the base of the filament, which may favor its diffusion into the membrane. Loss of this terminal pilin may cause the pilus to collapse into the inner membrane, positioning the next subunit into this unstable state. We propose that this iterative diffusion of the terminal pilin from the base of the filament results in motor-independent retraction via spontaneous depolymerization. One factor involved in destabilizing pilin–pilin interactions may be bulkiness of residues in α1C that are oriented into the pilus core. If residues in this region are small (yellow pilin), the resulting pilin–pilin interactions may be strong, and the ability of the filament to retract in the absence of motors is reduced. Conversely, if residues in α1C are large (red pilin), the resulting pilin–pilin interactions may be weaker, and the filaments are more likely to undergo motor-independent retraction.