Invadopodia are matrix-degrading membrane protrusions in invasive carcinoma 
cells. The mechanisms regulating invadopodium assembly and maturation are not 
understood. We have dissected the stages of invadopodium assembly and maturation 
and show that invadopodia use cortactin phosphorylation as a master switch 
during these processes. In particular, cortactin phosphorylation was found to 
regulate cofilin and Arp2/3 complex-dependent actin polymerization. Cortactin 
directly binds cofilin and inhibits its severing activity. Cortactin 
phosphorylation is required to release this inhibition so cofilin can sever 
actin filaments to create barbed ends at invadopodia to support Arp2/3-dependent 
actin polymerization. After barbed end formation, cortactin is dephosphorylated, 
which blocks cofilin severing activity thereby stabilizing invadopodia. These 
findings identify novel mechanisms for actin polymerization in the invadopodia 
of metastatic carcinoma cells and define four distinct stages of invadopodium 
assembly and maturation consisting of invadopodium precursor formation, actin 
polymerization, stabilization, and matrix degradation.