Cofilin is a ubiquitous actin-binding factor required for the reorganization of actin filaments. ADF/Cofilin family members bind G-actin monomers and depolymerize actin filaments through two mechanisms: severing and increasing the off-rate for actin monomers from the pointed end. "Older" ADP/ADP-Pi actin filaments free of tropomyosin and proper pH are required for cofilin to function effectively. In the presence of readily available ATP-G-actin cofilin speeds up actin polymerization via its actin-severing activity (providing free barbed ends for further polymerization and nucleation by the Arp2/3 complex). As a long-lasting "in vivo" effect, cofilin recycles older ADP-F-actin, helping cell to maintain ATP-G-actin pool for sustained motility. pH, phosphorylation and phosphoinositides regulate cofilin's binding and associating activity with actin