The cytoskeleton is a composite network of three types of protein filaments, among which intermediate filaments (IFs) are the most extensible ones. Two very important IFs are keratin and vimentin, which have similar molecular architectures, but different mechanical behaviors. Here we compare the mechanical response of single keratin and vimentin filaments using optical tweezers. We show that the mechanics of vimentin strongly depends on the ionic strength of the buffer and that its force-strain curve suggests a high degree of cooperativity between subunits. Indeed, a computational model indicates that in contrast to keratin, vimentin is characterized by strong lateral subunit coupling of its charged monomers during unfolding of α-helices. We conclude that cells can tune their mechanics by differential use of keratin versus vimentin.

Intermediate filaments (IFs) are part of the cytoskeleton of eukaryotic cells and are thus largely responsible for the cell's mechanical properties. IFs are characterized by a pronounced extensibility and remarkable resilience that enable them to support cells in extreme situations. Previous experiments showed that under strain, α-helices in vimentin IFs might unfold to β-sheets. Upon repeated stretching, the filaments soften, however, the remaining plastic strain is negligible. Here we observe that vimentin IFs do not recover their original stiffness on reasonable time scales, and we explain these seemingly contradicting results by introducing a third, less well-defined conformational state. Reversibility on the nanoscale can be fully rescued by introducing crosslinkers that prevent transition to the β-sheet. Our results classify IFs as a nano-1 material with intriguing mechanical properties, which is likely to play a major role for the cell's local adaption to external stimuli.Keywords: cell mechanics, cytoskeleton, intermediate filaments, force-strain behavior, 3-state system, optical tweezers.The mechanical properties of biological cells are defined by the cytoskeleton, a composite network of microtubules, actin filaments and intermediate filaments (IF). 1,2 Although the exact division of labour among the three filament types is still not fully resolved, 1,2 there is ample evidence that IFs are the load bearing elements when cells are subjected to external tensile 3,4 or compressive 5 stress. During embryogenesis and tissue formation, in particular, cells undergo dramatic changes in shape and size. The force scales expected for cellular processes lie between single motor protein forces of a few pN, which can be measured by FRET sensors, 6 and the collective forces of several nN measured for whole cells, as determined, e.g., by traction force microscopy. 7 In order to withstand strong transitions, cells show reversible superelasticity, which is linked to their IF network. 3 In order to achieve the required material properties for IFs, nature applies design principles on the nanoscale distinct from human engineering solutions and instead relies on self-organization and structural hierarchy. As a consequence, IFs stand out among the cytoskeletal filaments by their high flexibility 8,9 and enormous extensibility. [10][11][12][13] Within the IF family, vimentin is typical for cells of mesenchymal origin. 14 Like all cytoskeletal IFs, vimentin monomers comprise an α-helical rod domain with intrinsically unstructured head and tail domains. 15 The monomers assemble following a hierarchical pathway resulting in filaments with laterally and longitudinally arranged monomers ( Fig. 1a). 16,17 Theoretical considerations, 12,13 molecular dynamics simulations 18,19 and Xray diffraction studies 20 have shown that the intriguing tensile properties of IFs originate from conformational changes on different levels of the hierarchical filament structure. no. 654148). Further financial support was received from the Deutsche Forsch...

Intermediate filaments (IFs) are part of the cytoskeleton of eukaryotic cells and are thus largely responsible for the cell's mechanical properties. IFs are characterized by a pronounced extensibility and remarkable resilience that enable them to support cells in extreme situations. Previous experiments showed that under strain, α-helices in vimentin IFs might unfold to β-sheets. Upon repeated stretching, the filaments soften, however, the remaining plastic strain is negligible. Here we observe that vimentin IFs do not recover their original stiffness on reasonable time scales, and we explain these seemingly contradicting results by introducing a third, less well-defined conformational state. Reversibility on the nanoscale can be fully rescued by introducing crosslinkers that prevent transition to the β-sheet. Our results classify IFs as a nano-1 . CC-BY-NC-ND 4.

The cytoskeleton is a composite network of three types of protein filaments, among which intermediate filaments (IFs) are the most extensible ones. Two very important IFs are keratin and vimentin, which have similar molecular architectures, but different mechanical behaviors. Here we compare the mechanical response of single keratin and vimentin filaments using optical tweezers. We show that the mechanics of vimentin strongly depends on the ionic strength of the buffer and that its force-strain curve suggests a high degree of cooperativity between subunits. Indeed, a computational model indicates that in contrast to keratin, vimentin is characterized by strong lateral subunit coupling of its charged monomers during unfolding of α-helices. We conclude that cells can tune their mechanics by differential use of keratin versus vimentin.

Cell mechanics is 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 the epithelial-to-mesenchymal transition is associated with a switch from keratin to vimentin. The transition coincides with a change in cellular mechanics, and thus 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 subunits sliding within keratin filaments and non-equilibrium α helix unfolding in vimentin filaments.