Motor-driven dynamics in actin-myosin networks.

The effect of myosin motor protein activity on the filamentous actin (F-actin) rheological response is studied using diffusing wave spectroscopy. Under conditions of saturating motor activity, we find an enhancement of longitudinal filament fluctuations corresponding to a scaling of the viscoelastic shear modulus G(d)(omega) approximately omega(7/8). As the adenosine tri-phosphate reservoir sustaining motor activity is depleted, we find an abrupt transient to a passive, "rigor state" and a return to dissipation dominated by transverse filament modes. Single-filament measurements of the apparent persistence length support the notion that motor activity leads to an increase in the effective temperature for tangential motion.