Actin-based motility: cooperative symmetry-breaking and phases of motion

A microscopic model is proposed for the motility of a bead driven by the polymerization of actin filaments. The model exhibits a rich spectrum of behaviours similar to those observed in biomimetic experiments, which include spontaneous symmetry-breaking, various regimes of the bead's motion and correlations between the structure of the actin tail which propels the bead and the bead dynamics. The dependences of the dynamical properties (such as symmetry-breaking time, regimes of motion, mean velocity, and tail asymmetry) on the physical parameters (the bead radius and viscosity) agree well with the experimental observations. We find that most experimental observations can be reproduced taking into account only one type of filaments interacting with the bead: the detached filaments that push the bead. Our calculations suggest that the analysis of mean characteristics only (velocities, symmetry-breaking times, etc) does not always provide meaningful information about the mechanism of motility. The aim should be to obtain the corresponding distributions, which might be extremely broad and therefore not well represented by their mean only. Our findings suggest a simple coarse-grained description, which captures the main features obtained within the microscopic model.