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 . Purified ATP-Alp7A forms dynamically unstable polymers with a high critical concentration for net assembly (cc = 10.