Internal Motility in Stiffening Actin-Myosin Networks

Abstract

We present a study on filamentous actin solutions containing heavy meromyosin subfragments of myosin II motor molecules. We focus on the viscoelastic phase behavior and internal dynamics of such networks during ATP depletion. Upon simultaneously using micro-rheology and fluorescence microscopy as complementary experimental tools, we find a sol-gel transition accompanied by a sudden onset of directed filament motion. We interpret the sol-gel transition in terms of myosin II enzymology, and suggest a "zipping" mechanism to explain the filament motion in the vicinity of the sol-gel transition.PACS numbers: 87.16. Ka, 87.15.La, 87.15.Nn, 87.16.Nn Eucaryotic cells show an amazing versatility in their mechanical properties. Not only can they sustain stresses ranging from some tenths to hundreds of Pascals, but they can equally well perform such complex processes as cytokinesis and cell locomotion. A vital role for these and other cellular functions is played by the cytoskeleton, the structural framework of the cell composed of a network of protein filaments. A major component is filamentous actin (F-Actin), whose physical properties are by now well characterized [1]. The length distribution, spatial arrangement and connectivity of these filaments is controlled by a great variety of regulatory proteins.An important family of these regulatory proteins are cross-linkers, which can be further classified as passive or active. The function of passive cross-linkers, e.g. α-actinin, is mainly determined by their molecular structure and the on-off kinetics of their binding sites to actin. Upon changing the association-dissociation equilibrium and hence the degree of cross-linking by varying the temperature the network can be driven from a sol into a gel state [2]. Depending on both the concentration and the affinity of these cross-linkers for F-actin there is a tendency to either form random networks or bundles [3]. Motor proteins of the myosin family can also act as active cross-linkers. When both of its functional head groups are bound to two different filaments they can use the energy of adenosine-triphosphate (ATP) hydrolysis to exert relative forces and motion between them. However, such an event is very unlikely under physiological conditions and ATP saturation, because then myosin II spends only a short fraction of its chemomechanical cycle attached to the filament (duty ratio: r 0.02 [4]). Active relative transport yet becomes possible due to the concerted action of several motors if in vitro myosin II proteins assemble into multimeric minifilaments [5].In this letter we study actin networks containing the heavy meromyosin (HMM) subfragment lacking the light meromyosin domain responsible for myosin II assembly. Our focus is on the viscoelastic phase behavior and internal dynamics of such networks during ATP depletion. We use an experimental setup combining microrheology with fluorescent microscopy of labeled filaments. This allows us to identify a sol-gel transition accompanied by a sudden onset of directed filament...

Generic Phase Diagram of Active Polar Films

“…This seems to be observed experimentally (zipping effect in Ref. [18]). We consider here only ordered polar phases with a unitary polarization vector p, and we ignore this active coupling (setting λ = 0) for simplicity [19].…”

Steady-state hydrodynamic instabilities of active liquid crystals: Hybrid lattice Boltzmann simulations

“…In Ref. [7] it was pointed out that, instead, λ > 0 when describing concentrated actomyosin gels and other systems which display zipping or other self-alignment effects (this is relevant for the cases considered in [37]). …”

Brownian motion: a paradigm of soft matter and biological physics

“…Rather than continuing this list ad infinitum, we close this section by pointing out an ironic twist in the recent history of Brownian motion: with the physics of fluctuating polymers and membranes reasonably well understood, physicists have at last turned their attention back to the possible animate sources of Brownian fluctuations. The idea is to study the additional effect of "active noise", such as the undulations induced by molecular pumps, molecular motors, and switching channels embedded into a membrane [224,225], a theme that can be iterated in many directions; e. g. towards so-called active gels [226][227][228][229][230], solutions of biopolymers mixed with molecular motors, in which rich structure formation (asters and spirals, etc.) has been observed in vivo and in vitro.…”