Abstract
Actomyosin networks give cells the ability to move and divide. These networks contract and expand while being driven by active energy-consuming processes such as motor protein walking and actin polymerization. Actin dynamics is also regulated by actin-binding proteins, such as the actin-related protein 2/3 (Arp2/3) complex. This complex generates branched filaments, thereby changing the overall organization of the network. In this work, the spatiotemporal patterns of dynamical actin assembly accompanying the branching-induced reorganization caused by Arp2/3 were studied using a computational model (mechanochemical dynamics of active networks [MEDYAN]); this model simulates actomyosin network dynamics as a result of chemical reactions whose rates are modulated by rapid mechanical equilibration. We show that branched actomyosin networks relax significantly more slowly than do unbranched networks. Also, branched networks undergo rare convulsive movements, "avalanches," that release strain in the network. These avalanches are associated with the more heterogeneous distribution of mechanically linked filaments displayed by branched networks. These far-from-equilibrium events arising from the marginal stability of growing actomyosin networks provide a possible mechanism of the "cytoquakes" recently seen in experiments.
Highlights
Actomyosin networks give cells the ability to move and divide
We employ a powerful computational software for flexibly modeling the complexity of cytoskeletons, mechanochemical dynamics of active networks (MEDYAN), that was developed by Papoian and his group [20,21,22,23,24]
The mechanochemical aspects of MEDYAN make it possible to investigate how the nucleation and branching initiated by the actin-related protein 2/3 (Arp2/3) complex change the dynamics and structures of actomyosin systems
Summary
James Limana,b , Carlos Buenob,c , Yossi Eliazb,d, Nicholas P. These results semiquantitively agree with the experimental data from the Weitz laboratory [3], which studied these unbranched systems. (B and C) Two snapshots of actin filaments, motors, and linkers where the tension is indicated by color; these show the morphology of the network before (B) and after (C) the abrupt drop of Δ(Rg=Rig) when an avalanche occurs. The branched networks with low concentrations of motor globally contract more than do unbranched networks with the same low motor concentrations These contractions apparently are caused by the Arp2/3 branchers inhibiting the polymerization and depolymerization of the actin fibers, which would otherwise favor the expansion of the network. Avalanches rarely occur in networks containing only small and isolated actin clusters
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