Collective unfolding of α-helices in vimentin intermediate filaments
The Janshoff and Köster groups (projects A17 and A12) studied vimentin intermediate filaments by optical traps and atomic force microscopy and recorded precise force-strain curves of these extremely extensible cytoskeletal filaments. By combining the experimental data with a stochastic model and numerical simulations, they linked their experimental observations to the peculiar molecular architecture of intermediate filaments. The high extensibility is explained by collective, non-equilibrium unfolding of α-helices into β-sheets in the vimentin monomers constituting the filaments. By the same mechanism, the filaments are able to dissipate more than 70% of the input energy. When extending and relaxing the filaments repeatedly, they display softening and tensile memory (Block et al., Phys. Rev. Lett. (2017) 118:048101, Block et al., Sci. Adv. (2018) 4:eaat1161).
Tensile memory and softening of single vimentin intermediate filaments. Left: Schematic of vimentin filament structure in relaxed and extended state. Right top: measurement protocol (distance against time) for repeated stretch-relax cycles. The filament is stretched to increasing strains with every cycle. Right bottom: resulting force-strain curves (black: early cycles, red: late cycles). The filament becomes softer with each repeated cycle. Inset: fluorescence image of the optical trap geometry consisting of two microspheres (4.5 μm diameter) and a single vimentin filament stretched in-between.