Figure 2. Actin architecture transitions.

( A) In the convergent elongation model, the transition from lamellipodium to filopodium involves the formation of parallel actin filaments from a branched network created by the Arp2/3 complex. Elongation factors, like Ena/VASP or formin, protect barbed ends from capping protein and induce the rapid polymerization of parallel bundles. ( B) The transition from lamellipodium to contractile structures is triggered by the disassembly of the branched network at the rear of the lamellipodium by ADF/cofilin and myosin. Myosin induces actin filament alignment and the formation of fibers stabilized by crosslinkers, such as α-actinin. ( C) The fusion of contractile structures (the transverse arcs) and non-contractile structures (the radial fibers) can lead to the formation of ventral stress fibers. In this scheme, myosins connect a transverse arc and two radial fibers and, after contraction, align the radial fibers with the transverse arc, creating a ventral stress fiber. This contractile antiparallel fiber is anchored at its two ends to focal adhesions.