Keratin filament mechanics and energy dissipation are determined by metal-like plasticity

Cell mechanics are determined by an intracellular biopolymer network, including intermediate filaments that are expressed in a cell-type-specific manner. A prominent pair of intermediate filaments are keratin and vimentin, as they are expressed by non-motile and motile cells, respectively. Therefore, the differential expression of these proteins coincides with a change in cellular mechanics and dynamic properties of the cells. This observation raises the question of how the mechanical properties already differ on the single filament level. Here, we use optical tweezers and a computational model to compare the stretching and dissipation behavior of the two filament types. We find that keratin and vimentin filaments behave in opposite ways: keratin filaments elongate but retain their stiffness, whereas vimentin filaments soften but retain their length. This finding is explained by fundamentally different ways to dissipate energy: viscous sliding of subunits within keratin filaments and non-equilibrium α helix unfolding in vimentin filaments. [Display omitted] •Keratin and vimentin filaments dissipate over 50% of supplied energy•Energy dissipation is based on very different physical mechanisms•Keratin filaments elongate but do not soften when repeatedly pulled•Vimentin filaments soften but do not elongate when repeatedly pulled Nature offers a large portfolio of high-performance materials that can serve as a blueprint for novel sustainable materials and are used, for example, in biomedicine, industrial applications, and soft robotics. In particular, their distinct adaptability and non-linear material properties are of great interest for the design of biologically inspired materials. We have identified two types of protein filaments—keratin and vimentin—that are exceptionally stretchable and can dissipate a large fraction of any supplied energy. These properties make them excellent candidates for shock-absorbing materials. Due to minor structural differences, they exhibit opposite behavior when stretched: keratin filaments stretch with each pull and maintain their stiffness (like solid metals), while vimentin filaments soften but maintain their length (like double-network gels). We mechanically characterize two different biological protein materials that exhibit softening and elongation properties reminiscent of solid metals and dual network gels, respectively. Both materials can also dissipate large amounts of energy. These very different material properties