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Attachment Conditions Control Actin Filament Buckling and the Production of Forces

Julien Berro ^*, Alphée Michelot , Laurent Blanchoin , David R. Kovar  and Jean-Louis Martiel ^*

^* Laboratoire Techniques de l'Imagerie, de la Modélisation et de la Complexité, Institut National de la Santé et de la Recherche Médicale and Université Joseph Fourier, F38706, La Tronche, France; Institut de Recherches en Technologie et Sciences pour le Vivant, Laboratoire de Physiologie Cellulaire Végétale, Commissariat à l'Energie Atomique, Centre National de la Recherche Scientifique, Institut National de la Recherche Agronomique and Université Joseph Fourier, F38054, Grenoble, France; and Departments of Molecular Genetics and Cell Biology, and Biochemistry and Molecular Biology, The University of Chicago, Chicago, Illinois

Correspondence: Address reprint requests to Jean-Louis Martiel, Tel.: 33-4-56-52-00-69; E-mail: jean-louis.martiel{at}imag.fr.

Actin polymerization is the driving force for a large number of cellular processes. Formation of lamellipodia and filopodia at the leading edge of motile cells requires actin polymerization induced mechanical deformation of the plasma membrane. To generate different types of membrane protrusions, the mechanical properties of actin filaments can be constrained by interacting proteins. A striking example of such constraint is the buckling of actin filaments generated in vitro by the cooperative effect of a processive actin nucleating factor (formin) and a molecular motor (myosin II). We developed a physical model based on equations for an elastic rod that accounts for actin filament buckling. Both ends of the rod were maintained in a fixed position in space and we considered three sets of boundary conditions. The model qualitatively and quantitatively reproduces the shape distribution of actin filaments. We found that actin polymerization counterpoises a force in the range 0.4–1.6 pN for moderate end-to-end distance (1 µm) and could be as large as 10 pN for shorter distances. If the actin rod attachment includes a spring, we discovered that the stiffness must be in the range 0.1–1.2 pN/nm to account for the observed buckling.