Journal
NATURE
Volume 404, Issue 6781, Pages 1007-1011Publisher
MACMILLAN PUBLISHERS LTD
DOI: 10.1038/35010008
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Most nucleated cells crawl about by extending a pseudopod that is driven by the polymerization of actin filaments in the cytoplasm behind the leading edge of the plasma membrane(1,2). These actin filaments are linked into a network by Y-branches, with the pointed end of each filament attached to the side of another filament and the rapidly growing barbed end facing forward(3). Because Arp2/3 complex nucleates actin polymerization and links the pointed end to the side of another filament in vitro, a dendritic nucleation model has been proposed 4 in which Arp2/3 complex initiates filaments from the sides of older filaments. Here we report, by using a light microscopy assay, many new features of the mechanism. Branching occurs during, rather than after, nucleation by Arp2/3 complex activated by the Wiskott-Aldrich syndrome protein (WASP) or Scar protein; capping protein and profilin act synergistically with Arp2/3 complex to favour branched nucleation; phosphate release from aged actin filaments favours dissociation of Arp2/3 complex from the pointed ends of filaments; and branches created by Arp2/3 complex are relatively rigid. These properties result in the automatic assembly of the branched actin network after activation by proteins of the WASP/ Scar family and favour the selective disassembly of proximal regions of the network.
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