Published online
doi:10.1083/jcb.200709092
The Journal of Cell Biology, Vol. 180, No. 5, 887-895
The Rockefeller University Press, 0021-9525 $30.00
© Rouiller et al.
The structural basis of actin filament branching by the Arp2/3 complex
Isabelle Rouiller1,
Xiao-Ping Xu1,
Kurt J. Amann2,
Coumaran Egile3,
Stephan Nickell4,
Daniela Nicastro5,
Rong Li3,6,
Thomas D. Pollard7,8,9,
Niels Volkmann1, and
Dorit Hanein1
1 Burnham Institute for Medical Research, La Jolla, CA 92037
2 Cellular and Molecular Biology Program, University of Wisconsin-Madison, Madison, WI 53706
3 Department of Cell Biology, Harvard Medical School, Boston, MA 02115
4 Max Planck Institute für Biochemie, D-82152 Martinsried, Germany
5 Laboratory for 3D Electron Microscopy of Cells, University of Colorado, Boulder, CO 80309
6 Stowers Institute for Medical Research, Kansas City, MO 64110
7 Department of Molecular, Cellular, and Developmental Biology, 8 Department of Cell Biology, and 9 Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, CT 06520
Correspondence to Niels Volkmann: niels{at}burnham.org; or Dorit Hanein: dorit{at}burnham.org
The actin-related protein 2/3 (Arp2/3) complex mediates the formation of branched actin filaments at the leading edge of motile cells and in the comet tails moving certain intracellular pathogens. Crystal structures of the Arp2/3 complex are available, but the architecture of the junction formed by the Arp2/3 complex at the base of the branch was not known. In this study, we use electron tomography to reconstruct the branch junction with sufficient resolution to show how the Arp2/3 complex interacts with the mother filament. Our analysis reveals conformational changes in both the mother filament and Arp2/3 complex upon branch formation. The Arp2 and Arp3 subunits reorganize into a dimer, providing a short-pitch template for elongation of the daughter filament. Two subunits of the mother filament undergo conformational changes that increase stability of the branch. These data provide a rationale for why branch formation requires cooperative interactions among the Arp2/3 complex, nucleation-promoting factors, an actin monomer, and the mother filament.
I. Rouiller's present address is Department of Anatomy and Cell Biology, McGill University, Montreal, Quebec H3A 2B2, Canada.
C. Egile's present address is Experimental Therapeutics and Translational Research Oncology, 94403 Vitry Sur Seine, France.
D. Nicastro's present address is Brandeis University, Rosenstiel Center, Waltham, MA 02454.
Abbreviation used in this paper: Arp, actin-related protein.
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