Tension causes structural unfolding of intracellular intermediate filaments

Intermediate filament (IF) proteins are a class of proteins that constitute different filamentous structures in mammalian cells. As such, IF proteins are part of the load-bearing cytoskeleton and support the nuclear envelope. Molecular dynamics simulations have shown that IF proteins undergo secondary structural changes to compensate mechanical loads, which has been confirmed by experimental in vitro studies on IF hydrogels. However, the structural response of intracellular IF to mechanical load has yet to be elucidated in cellulo. Here, we use in situ nonlinear Raman imaging combined with multivariate data analysis to quantify the intracellular secondary structure of the IF cytoskeletal protein vimentin under different states of cellular tension. We find that cells under native cellular tension contain more unfolded vimentin than chemically or physically relaxed specimens. This indicates that unfolding of IF proteins occurs intracellularly when sufficient forces are applied, suggesting that IF structures act as local force sensors in the cell to mark locations under large mechanical tension.