Abiomimetic motility assay is used to analyze the mechanism of force production by site-directed polymerization of actin. Polystyrene microspheres, functionalized in a controlled fashion by the N-WASP protein, the ubiquitous activator of Arp2/3 complex, undergo actin-based propulsion in a medium that consists of five pure proteins. We have analyzed the dependence of velocity on N-WASP surface density, on the concentration of capping protein, and on external force. Movement was not slowed down by increasing the diameter of the beads (0.2 to 3 μm) nor by increasing the viscosity of the medium by 105-fold. This important result shows that forces due to actin polymerization are balanced by internal forces due to transient attachment of filament ends at the surface. These forces are greater than the viscous drag. Using Alexa®488-labeled Arp2/3, we show that Arp2/3 is incorporated in the actin tail like G-actin by barbed end branching of filaments at the bead surface, not by side branching, and that filaments are more densely branched upon increasing gelsolin concentration. These data support models in which the rates of filament branching and capping control velocity, and autocatalytic branching of filament ends, rather than filament nucleation, occurs at the particle surface.

We create tailored microstructures, consisting of complexes of lipid membranes with self-assembled biopolymer shells, to study the fundamental properties and interactions of these basic components of living cells. We measure the mechanical response of these artificial structures at the micrometer scale, using optical tweezers and single-particle tracking. These systems exhibit rich dynamics that illustrate the viscoelastic character of the quasi-two-dimensional biopolymer network. We present a theoretical model relating the rheological properties of these membranes to the observed dynamics.

The authors propose a new mechanism for actin-based force generation based on results using chains of actin-grafted magnetic colloids.

This novel approach of endogenous WASH displacement confirms previous suggestions that the retromer is the receptor of the WASH complex at the surface of endosomes and identify key residues that mediate this interaction. The interaction between these two endosomal machineries, the WASH complex and the retromer, is likely to play a critical role in forming platforms at the surface of endosomes for efficient sorting of cargoes.