A “Primer”-Based Mechanism Underlies Branched Actin Filament Network Formation and Motility

• DOI: 10.1016/j.cub.2009.12.056
• Publication Date: 2010-02-26T10:36:53Z
• Authors: Vérane Achard; Jean-Louis Martiel; Alphée Michelot; Christophe Guérin; Anne-Cécile Reymann; Laurent Blanchoin; Rajaa Boujemaa-Pa...
• Keywords: 0301 basic medicine; 570; capping protein; Agricultural and Biological Sciences(all); Biochemistry, Genetics and Molecular Biology(all); nucleation; evanescent wave microscopy; cytoskeleton; filament network dynamics; [SDV.BC]Life Sciences [q-bio]/Cellular Biology; actin binding protein; Actin-Related Protein 2-3 Complex; Actins; Biomechanical Phenomena; 03 medical and health sciences; Microscopy, Fluorescence; Cell Movement; microscopy; CELLBIO; Arp2/3; actin

Cells use actin assembly to generate forces for membrane protrusions during movement [1] or, in the case of pathogens, to propel themselves in the host cells, in crude extracts [2], or in mixtures of actin and other purified proteins [3]. Significant progress has been made in understanding the mechanism of actin-based motility at a macroscopic level by using biomimetic systems in vitro [4-6]. Here, we combined such a system with evanescent wave microscopy to visualize Arp2/3-mediated actin network formation at single-actin-filament resolution. We found that actin filaments that we call "primers" determine the origin of the autocatalytic and propagative formation of the actin network. In the presence of capping protein, multiple "primers" generate independent networks that merge around the object to form an outer "shell" made of entangled and capped filaments. Simultaneously, newly created filaments on the surface of the particle initiate mechanical stress, which develops until symmetry breaking. Our results and extensive modeling support that the stress, which releases into propulsive forces [7], is controlled not by any specific orientation of actin filaments toward the nucleation sites but only by new monomers added near the load surface.