Actin filaments align into hollow comets for rapid VASP-mediated propulsion.
For cells, the growth of a dense array of branched actin filaments organized by the actin-related proteins 2 and 3 (Arp2/3) complex at the plasma membrane offers an explanation as to how movement is produced, and this arrangement is considered to be optimal for motility. Here, we challenged this assumption by using an in vitro system of polystyrene beads in cell extracts that contained a complex mix of actin polymerization proteins as in vivo. We employed the surface of the bead as a reactor where we mixed two different actin polymerization-activating factors, the Arp2/3 complex and the vasodilator-stimulated phosphoprotein (VASP), to examine their contribution to actin-based movement and filament organization. We varied the coating of the bead surface but left the extracts identical for all assays. We found that the degree of filament alignment in the actin comet tails depended on the surface ratio of VASP to Arp2/3. Alignment of actin filaments parallel to the direction of bead movement in the presence of VASP was accompanied by an abrupt 7-fold increase in velocity that was independent of bead size and by hollowing out of the comets. The actin filament-bundling proteins fimbrin and fascin did not appear to play a role in this transformation. Together with the idea that VASP enhances filament detachment and with the presence of pulling forces at the rear of the bead, a mesoscopic analysis of movement provides a possible explanation for our results.
Laboratoire Physico-Chimie Curie, Unité Mixte de Recherche 168, Institut Curie, Centre National de la Recherche Scientifique, 11, rue Pierre et Marie Curie, 75231 Paris Cedex 05, France. julie.plastino@curie.fr
Julie
J
Plastino
Stéphane
S
Olivier
Cécile
C
Sykes
D008841:Actin Cytoskeleton / Q000378:metabolism*; D051377:Actin-Related Protein 2; D051378:Actin-Related Protein 3; D015815:Cell Adhesion Molecules / Q000378:metabolism*; D002465:Cell Movement / Q000502:physiology*; D003598:Cytoskeletal Proteins / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D008840:Microfilament Proteins; D008854:Microscopy, Electron; D008863:Microspheres; D010750:Phosphoproteins / Q000378:metabolism*; D011137:Polystyrenes; D000098662:Vasodilator-Stimulated Phosphoprotein
C495926:ACTR2 protein, human; C495928:ACTR3 protein, human; D051377:Actin-Related Protein 2; D051378:Actin-Related Protein 3; D015815:Cell Adhesion Molecules; D003598:Cytoskeletal Proteins; D008840:Microfilament Proteins; D010750:Phosphoproteins; D011137:Polystyrenes; D000098662:Vasodilator-Stimulated Phosphoprotein