The animal cell cytoskeleton consists of three interconnected filament systems: actin-containing microfilaments (MFs), microtubules (MTs), and the lesser known intermediate filaments (IFs). All IF proteins share a common tripartite domain structure and the ability to assemble into 8-12 nm wide filaments. Electron microscopy data suggest that lFs are built according to a completely different plan from that of MFs and MTs. lFs are known to impart mechanical stability to cells and tissues but, until recently, the biomechanical properties of single IFs were unknown. However, with the discovery of naturally occurring micrometer-wide IF bundles and the development of new methodologies to mechanically probe single filaments, it is now possible to propose a more unified view of IF biomechanics. Unlike IVIFs and MTs, single IFs can now be described as flexible, extensible and tough, which has important implications for our understanding of cell and tissue mechanics. Furthermore, the molecular mechanisms at play when lFs are deformed point toward a pivotal role for them in mechanotransduction
