<i>Non-equilibrium fluctuations in cells report on driving forces and organelle mechanics</i>

Cells are complex active materials. Dispersed motor proteins drive in more or less organized manners transport, organelle dynamics, shape changes and cell movements. Forces and local material properties are difficult to measure. We have here developed a method that takes advantage of motor-generated forces deforming rod shaped elastic objects, to measure both, the driving forces and the elastic properties of the rods. Examples of such rods are microtubules, intermediate filaments or externally introduced probes, such as carbon nanotubes. Their shapes are determined by the active forces, the response characteristics of the cytoplasm and the material properties of the rods. If the response of the cytoplasm is measured independently by microrheology, the other two quantities can be determined. We present the theory describing the non-equilibrium dynamics of a probe filament embedded in the cytoplasm and show how the method can be applied in HeLa cells.