The animal cell cytoskeleton consists of three interconnected filament systems: actin containing microfilaments (MFs), microtubules (MTs), and intermediate filaments (IFs). Among these three filaments systems, IFs are the only one that show high extensibility at both the single filament and network levels. In this work, I am presenting a simple model of IFs extensibility based on the current structural knowledge of the filaments. The only extra information added to this model compared to previous ones is the fact that the unfolded N- and C-termini of IF proteins are sandwiched between adjacent coiled-coil rod domains within the filaments. Since we know the contour length and typical persistence length of these unfolded termini, it is possible to predict the persistence length of a single filament, its maximal extensibility and the onset of coiled-coil unfolding. The predictions of the model are in good agreement with experiments on single desmin IFs stretched on a surface by AFM and on vimentin and desmin networks probed by rheology.
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