Fig. 6. A polymerization-associated subunit switching mechanism, the cytomotive switch is required for robust single-stranded filament dynamics.

Two models for protein filament polymerization are shown. In (A), subunits are rigid. In (B), subunits perform a cytomotive switch, i.e., they have two conformations: one compatible with being in a filament but that is unstable when unpolymerized (blue), and a second that is incompatible with polymerization but is stable when unpolymerized (orange). Arrows of the same appearance indicate rates that are identical; their widths are proportional to the rates they represent. Three black dots denote NTP-loaded subunits; two black dots mean NDP. A gray dot means exchange of NDP/NTP. The filament in (A) faces two problems, which render it not usefully cytomotive; the filament in (B) uses a cytomotive switch to solve both problems. End asymmetry/coupling of kinetic and structural polarities is explained further in fig. S12.