Microtubule growth activates Rac1 to promote lamellipodial profusion in fibroblasts

Microtubules are involved in actin-based protrusion at the leading-edge lam ellipodia of migrating fibroblasts. Here we show that the growth of microtu bules induced in fibroblasts by removal of the microtubule destabilizer noc odazole activates Rad GTPase, leading to the polymerization of actin in lam ellipodial protrusions. Lamellipodial protrusions are also activated by the rapid growth of a disorganized array of very short microtubules induced by the microtubule-stabilizing drug taxol. Thus, neither microtubule shorteni ng nor long-range microtubule-based intracellular transport is required for activating protrusion. We suggest that the growth phase of microtubule dyn amic instability at leading-edge lamellipodia locally activates Rad to driv e actin polymerization and lamellipodial protrusion required for cell migra tion.