Mechanics of Intermediate Filaments

The eukariotic cytoskeleton consists of three major filament types: actin filaments, microtubules and intermediate filaments. While the first two filament types are highly conserved, intermediate filaments vary between cell types. Here, we focus on the intermediate filament vimentin, which occurs mesenchymal cells. A defining feature of intermediate filaments is their assembly process. The filaments assemble in a hierarchical manner from tetrameric complexes of half-staggered antiparalell coiled-coils. These tetrameric structures then align laterally to form unit-length filaments which in turn form filaments by annealing longitudinally. Using fluorescence microscopy techniques we perform experiments on vimentin filaments in vitro: Sub-unit exchange between two differently labeled filament populations is observed to depend on the filaments cross-section polymorphism. We find that increasing polymorphism leads to a faster sub-unit exchange process. Thus, vimentin intermediate filaments are found to be dynamic structures. Notably these observations are made in the absence of cellular regulatory mechanisms. Aside from the changes in composition of vimentin filaments we observe the mechanical properties of vimentin filaments and make observations on the polymer physical aspects of the filaments. To this end the fluorescently labeled filaments are confined in microchannels with widths in the range of micrometers. We find that vimentin filaments can accurately be described by the worm-like chain model; fluctuations in perpendicular planes decouple. In addition a scaling law describing filaments confined in the Odijk regime is confirmed for polymers with different persistence lengths by the combination of data from vimentin and actin filaments