Abstract
Single molecule experiments have measured stall forces and procession rates of molecular motors on isolated cytoskeletal fibers in Newtonian fluids. But in the cell, these motors are transporting cargo through a highly complex cytoskeletal network. To compare these single molecule results to the forces exerted by motors within the cell, an evaluation of the response of the cytoskeletal network is needed. Using magnetic tweezers, we quantify force-velocity curves for magnetic beads moving through entangled F-actin networks [12 uM]. Below a certain critical force, we see an elastic response with a plateau indicating a shear modulus of 0.1 Pa, comparable to bulk rheological measurements. Above this critical force we find a viscous response with a viscosity of approximately 0.3 Pa.s. The exact value of the critical force ranges from roughly 6-14 pN, reflecting the spatial heterogeneity of the network. This non-Newtonian force-velocity relationship, as well as the considerable heterogeneity in the network response, suggests the local cytoskeletal environment is an important factor when considering cargo transport inside the cell.
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