Non-equilibrium shape fluctuations in living cells report driving forces and organelle mechanics

The cytoplasm of cells is an active composite material containing a high concentration of macromolecules, including motor proteins, as well as cytoskeletal filaments and a multitude of organelles. The mechanical properties of cytoskeletal filaments are important for their dynamics and functions. Microtubules (MTs) are relatively rigid and form extended networks. Their mechanical properties in the cell are believed to be regulated by post-translational modifications, but in vivo measurements of MT mechanics are rather difficult. We here report a method using motor-generated forces that deform MTs in living cells (which can also be applied to other rod-shaped organelles), to measure both, the driving forces and the elastic properties of the MTs. MT bending dynamics are governed by their material properties, the active forces, and the response characteristics of the cytoplasm. We probe the cytoplasm by active magnetic-bead microrheology, and can thus determine the other two quantities. Reversing this logic, we show that the cytoplasmic response can alternatively be measured from localized large-amplitude bends of MTs of known stiffness. We present a theoretical description and measure the bending stiffness of MTs in vivo, finding general agreement with published in vitro data. We then study the effect of polyglutamylation, a post-translational modification, of MTs on their stiffness. We find that polyglutamylation significantly stiffens microtubules.