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Stochastic Severing of Actin Filaments by Actin Depolymerizing Factor/Cofilin Controls the Emergence of a Steady Dynamical Regime

Jeremy Roland ^*, Julien Berro ^*, Alphée Michelot , Laurent Blanchoin  and Jean-Louis Martiel ^*

^* Université Joseph Fourier, TIMC-IMAG Laboratory, Grenoble, France; CNRS UMR 5525, Grenoble, France; INSERM, IRF 130, Grenoble, France; and 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

Correspondence: Address reprint requests to Jean-Louis Martiel, TIMC-IMAG Laboratory, Taillefer Building, Faculty of Medicine, F-38706 La Tronche, France. Tel.: 33-456-520-069; E-mail: jean-louis.martiel{at}imag.fr.

Actin dynamics (i.e., polymerization/depolymerization) powers a large number of cellular processes. However, a great deal remains to be learned to explain the rapid actin filament turnover observed in vivo. Here, we developed a minimal kinetic model that describes key details of actin filament dynamics in the presence of actin depolymerizing factor (ADF)/cofilin. We limited the molecular mechanism to 1), the spontaneous growth of filaments by polymerization of actin monomers, 2), the ageing of actin subunits in filaments, 3), the cooperative binding of ADF/cofilin to actin filament subunits, and 4), filament severing by ADF/cofilin. First, from numerical simulations and mathematical analysis, we found that the average filament length, L, is controlled by the concentration of actin monomers (power law: 5/6) and ADF/cofilin (power law: –2/3). We also showed that the average subunit residence time inside the filament, T, depends on the actin monomer (power law: –1/6) and ADF/cofilin (power law: –2/3) concentrations. In addition, filament length fluctuations are 20% of the average filament length. Moreover, ADF/cofilin fragmentation while modulating filament length keeps filaments in a high molar ratio of ATP- or ADP-P_i versus ADP-bound subunits. This latter property has a protective effect against a too high severing activity of ADF/cofilin. We propose that the activity of ADF/cofilin in vivo is under the control of an affinity gradient that builds up dynamically along growing actin filaments. Our analysis shows that ADF/cofilin regulation maintains actin filaments in a highly dynamical state compatible with the cytoskeleton dynamics observed in vivo.