Active, motor-driven mechanics in a DNA gel

Abstract

Cells are capable of a variety of dramatic stimuli-responsive mechanical behaviors. These capabilities are enabled by the pervading cytoskeletal network, an active gel composed of structural filaments (e.g., actin) that are acted upon by motor proteins (e.g., myosin). Here, we describe the synthesis and characterization of an active gel using noncytoskeletal components. We use methods of base-pair-templated DNA self assembly to create a hybrid DNA gel containing stiff tubes and flexible linkers. We then activate the gel by adding the motor FtsK50C, a construct derived from the bacterial protein FtsK that, in vitro, has a strong and processive DNA contraction activity. The motors stiffen the gel and create stochastic contractile events that affect the positions of attached beads. We quantify the fluctuations of the beads and show that they are comparable both to measurements of cytoskeletal systems and to theoretical predictions for active gels. Thus, we present a DNA-based active gel whose behavior highlights the universal aspects of nonequilibrium, motor-driven networks.L iving cells display a range of interesting mechanical activities, including the ability to change shape, to translocate, and to apply stress to their surroundings. All of these activities can be stimulated by external mechanical or chemical signals. Some do not rely on a genetic response, but rather a response at the level of the cytoskeleton, the primary mechanical system of the cell (1, 2). This suggests that some of the cell's mechanical activities can be reproduced by a relatively simple artificial soft-matter system that lacks genetic control. Such a system would be useful both as a model for testing the physical principles underlying cytoskeletal behavior and as a material with potential technological applications.To design such a system, we must consider the key components of the cytoskeleton itself: Many cellular mechanical activities are due to a network of actin filaments that gains an active, nonequilibrium character through internal stresses generated by myosin motor proteins. These motors transduce chemical energy (through ATP hydrolysis) into mechanical work. In both muscle and nonmuscle cells, the main mechanical role of myosin is to contract the filaments, creating stress that propagates through the actin gel (3). This leads to a stiffening of the cell (4) along with enhanced mechanical fluctuations (5). These two behaviors have also been seen in reconstituted networks that only contain myosin and cross-linked actin (6-9), supporting the notion that these two components are the minimum needed to recapitulate active cell mechanics.What other motors and filaments can be used to form a gel with active mechanics? There are many biomolecular and synthetic fibers that can match the structural stiffness and cross-linking capabilities of actin (if not its active polymerization traits). The motor proteins are not so easily replaced, as can be judged by the limited capabilities of synthetic molecular motors in comparison to their biologi...

Inherently unstable networks collapse to a critical point

“…Taken together, these cellular proteins form an elastic network with contractility induced by nonequilibrium motor stresses [16,18]. The motors generate forces that propagate through the network, changing the viscoelastic properties, structure, and fluctuations in living systems [16,17,[19][20][21][22][23][24][25][26][27][28][29][30].…”

Elastic response of filamentous networks with compliant crosslinks

“…However, the presented approach can be generalized to other systems with rigid filaments connected by compliant crosslinks. For instance, networks of stiff DNA nanotubes crosslinked by single-stranded flexible DNA [20]. In this case the WLC approximation has to be replaced by freely-jointed-chain approximation [34].…”

“…Both the linear and nonlinear elastic properties of actinfilamin gels appear to be dramatically affected by the flexible nature of the crosslinks, resulting in novel behavior as compared to actin-networks with noncompliant crosslinks, and to synthetic polymer gels. Similar composites of rigid filaments and compliant crosslinks can be found in other systems, such as stiff DNA nanotubes crosslinked by flexible DNA linkers [20], although much less is known about the nonlinear elastic properties of such systems.…”

“…Similar composites of rigid filaments and compliant crosslinks can be found in other systems, such as stiff DNA nanotubes * Author to whom correspondence should be addressed. Electronic mail: fcm@nat.vu.nl crosslinked by flexible DNA linkers [20], although much less is known about the nonlinear elastic properties of such systems.Recently, a model for the elastic behavior of such networks has been proposed [17,21]. The model is based on the assumption that the soft stretching modes of the crosslinks govern the elasticity of a dense network of stiff polymers.…”

“…Similar composites of rigid filaments and compliant crosslinks can be found in other systems, such as stiff DNA nanotubes * Author to whom correspondence should be addressed. Electronic mail: fcm@nat.vu.nl crosslinked by flexible DNA linkers [20], although much less is known about the nonlinear elastic properties of such systems.…”