Polymer dynamics of Alp7A reveals how two critical concentrations govern assembly of dynamically unstable actin-like proteins.

Dynamically unstable polymers capture and move cellular cargos in bacteria and eukaryotes, but regulation of their assembly remains poorly understood. Here we describe polymerization of Alp7A, a bacterial actin-like protein (ALP) that distributes copies of plasmid pLS20 among daughter cells in Bacillus subtilis. Purified ATP-Alp7A forms dynamically unstable polymers with a high critical concentration for net assembly (cc_N = 10.3 µM), but a much lower critical concentration for filament elongation (cc_E = 0.6 µM). Rapid nucleation and stabilization of Alp7A polymers by the accessory factor, Alp7R, decrease cc_N into the physiological range. Stable populations of Alp7A filaments appear under two conditions: (i) when Alp7R slows catastrophe rates or (ii) when Alp7A concentrations are high enough to promote filament bundling. These results reveal how dynamic instability maintains high steady-state concentrations of monomeric Alp7A, and how accessory factors regulate Alp7A assembly by modulating cc_N independently of cc_E.