Actin network assembly and force generation: The mechanism of filament nucleation by the Arp2/3 complex and the emergent properties of viscoelastic filament networks

The Arp2/3 complex nucleates actin filaments and crosslinks them into networks that exert the force that drives cell, vesicle and bacterial motility. The activation of Arp2/3 complex is by the WASP family protein VCA region which binds both the Arp2/3 complex and an actin monomer. We show here that the Arp2 and Arp3 subunits of the Arp2/3 complex bind ATP and Arp2 hydrolyzes ATP rapidly upon nucleation of a new actin filament, stimulated by interaction with a single actin monomer co-ordinated by VCA. Our observation that capping of filament pointed ends by the Arp2/3 complex also stimulates rapid ATP hydrolysis on Arp2 identifies this actin monomer as the first monomer at the pointed end of the daughter filament. WASP-family VCA domains therefore activate the Arp2/3 complex by driving its interaction with a single conventional actin monomer to form an Arp2-Arp3-actin nucleus and the actin monomer becomes the first monomer of the polymerizing the daughter filament.;Actin networks built by Arp2/3 complex behave as viscoelastic gels. It has been proposed that energy can be built up, stored and released by these gels to produce propulsive motion. I create a theoretical framework for computer modeling of such networks and implement the model in C++. The model recreates in silico the in vitro behavior of bead and Listeria motility and validates the viscoelastic energy buildup and release as a simple mechanism for actin-based motility leading to a detailed 'steady-state symmetry-breaking' model. This model makes several experimentally testable predictions, including that pulsatile motion is caused by an imbalance in the rate of energy build up and energy release, and that symmetrically coated Listeria will move sideways not lengthways.