sentences:  Thus, polymerization and branching of actin is promoted in areas of the cell where cortactin is localized.Cortactin is very active in lamellipodia, protrusions of the cell membrane formed by actin polymerization and treadmilling that propel the cell along a surface as it migrates towards some target.Cortactin acts as a link between extracellular signals and lamellipodial “steering.” When a receptor tyrosine kinase on the cell membrane binds to an adhesion site, for example, cortactin will be phosphorylated locally to the area of binding, activate and recruit Arp2/3 to the actin cortex in that region, and thus stimulate cortical actin polymerization and movement of the cell in that direction.