Abstract
Dynamic networks designed to model the cell cytoskeleton can be reconstituted from filamentous actin, the motor protein myosin and a permanent cross-linker. They are driven out of equilibrium when the molecular motors are active. This gives rise to athermal fluctuations that can be recorded by tracking probe particles that are dispersed in the network. We have here probed athermal fluctuations in such “active gels” using video microrheology. We have measured the full distribution of probe displacements, also known as the van Hove correlation function. The dominant influence of thermal or athermal fluctuations can be detected by varying the lag time over which the displacements are measured. We argue that the exponential tails of the distribution derive from single motors close to the probes, and we extract an estimate of the velocity of motor heads along the actin filaments. The distribution exhibits a central Gaussian region which we assume derives from the action of many independent motor proteins far from the probe particles when athermal fluctuations dominate. Recording the whole distribution rather than just the typically measured second moment of probe fluctuations (mean-squared displacement) thus allowed us to differentiate between the effect of individual motors and the collective action of many motors.
Highlights
Living cells and organisms maintain their activity by continually harvesting external sources of energy
P(Dx(s)) of the displacement of colloidal particles dispersed in an active gel, which addresses both the spatial and temporal characteristics of athermal fluctuations
HDx(s)2i or C(u) is sufficient to deduce the viscoelastic properties of the surrounding material, if it is in equilibrium
Summary
Living cells and organisms maintain their activity by continually harvesting external sources of energy. Athermal fluctuations violate the fluctuation–dissipation theorem,[1] which links the thermal fluctuation of a given observable to the response of the same observable to an externally applied stimulus.[2,3] Such fluctuations have been recently characterized both in living cells and reconstituted model cytoskeletons using microrheology techniques. The athermal fluctuations of a probe particle were estimated from a comparison of the material’s viscoelastic response measured with active microrheology[4,5,6] and the total fluctuations of a probe particle measured with passive microrheology.[7,8,9] Any difference between the results from these two experiments, which were observed at low (i.e., biologically relevant) frequencies, is attributed to athermal fluctuations.[3,5,10]
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