The actin cytoskeleton is an active gel which constantly remodels during cellular processes such as motility and division. Myosin II molecular motors are involved in this active remodeling process and therefore control the dynamic self-organization of cytoskeletal structures. Due to the complexity of in vivo systems, it is hard to investigate the role of myosin II in the reorganization process which determines the resulting cytoskeletal structures. Here we use an in vitro model system to show that myosin II actively reorganizes actin into a variety of mesoscopic patterns, but only in the presence of bundling proteins. We find that the nature of the reorganization process is complex, exhibiting patterns and dynamical phenomena not predicted by current theoretical models and not observed in corresponding passive systems (excluding motors). This system generates active networks, asters and even rings depending on motor and bundling protein concentrations. Furthermore, the motors generate the formation of the patterns, but above a critical concentration they can also disassemble them and even totally prevent the polymerization and bundling of actin filaments. These results may suggest that tuning the assembly and disassembly of cytoskeletal structures can be obtained by tuning the local myosin II concentration/activity.

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The cellular cytoskeleton is a complex dynamical network that constantly remodels as cells divide and move. This reorganization process occurs not only at the cell membrane, but also in the cell interior (bulk). During locomotion, regulated actin assembly near the plasma membrane produces lamellipodia and filopodia. Therefore, most in vitro experiments explore phenomena taking place in the vicinity of a surface. To understand how the molecular machinery of a cell self-organizes in a more general way, we studied bulk polymerization of actin in the presence of actin-related protein 2͞3 complex and a nucleation promoting factor as a model for actin assembly in the cell interior separate from membranes. Bulk polymerization of actin in the presence of the verprolin homology, cofilin homology, and acidic region, domain of WiskottAldrich syndrome protein, and actin-related protein 2͞3 complex results in spontaneous formation of diffuse aster-like structures. In the presence of fascin these asters transition into stars with bundles of actin filaments growing from the surface, similar to star-like structures recently observed in vivo. The transition from asters to stars depends on the ratio [fascin]͞[G actin]. The polarity of the actin filaments during the transition is preserved, as in the transition from lamellipodia to filopodia. Capping protein inhibits star formation. Based on these experiments and kinetic Monte Carlo simulations, we propose a model for the spontaneous selfassembly of asters and their transition into stars. This mechanism may apply to the transition from lamellipodia to filopodia in vivo.actin self-assembly ͉ asters ͉ cellular protrusions ͉ Monte Carlo simulations ͉ stars D uring cellular migration, regulated actin assembly takes place at the plasma membrane (1-3), with continuous disassembly deeper in the cell interior. The site-directed actin polymerization at the plasma membrane results in the extension of cellular protrusions in the form of lamellipodia and filopodia. Although most cultured animal cells assemble both lamellipodia and filopodia, some cells, like keratocytes, emphasize lamellipodia, whereas dendritic cells are dominated by filopodia. One open question is how does a cell ''choose'' between the formation of lamellipodia and filopodia and what dictates the preference of one structure over the other. For that, it is essential to understand the self-assembly of these actin-based structures and the factors controlling these processes.Although protrusions of lamellipodia and filopodia are tightly coupled to actin polymerization, the distinct organization and generation of filaments in each structure uses a different mechanism to produce mechanical force. In the lamellipodia, the actin filaments organize into a flat 2D branched network (2, 4), whereas bundles of parallel actin filaments support filopodia (5). In the lamellipodia, the branched nucleation is driven by activation of the actin-related protein (Arp) 2͞3 complex (6) by the Wiskott-Aldrich syndrome protein family (7-8), followed...