Intermediate filaments (IFs) are often viewed as forming a static cytoskeletal network that allows cells to resist mechanical stress and deformation. But they are in fact remarkably dynamic. In a Commentary on p. 133, Robert Goldman and co-workers review recent work that has revealed the complex motile behaviour of IFs in cells, which is coupled to their assembly and disassembly. Experiments using vimentin-GFP fusion proteins, for example, have shown that IFs exist in at least three different structural forms: particles, short `squiggles' and long IFs. All three forms of vimentin can associate with cytoskeletal motors such as kinesin and cytoplasmic dynein, which transport them bidirectionally along microtubules at speeds of up to 2 μm/s. Other studies have shown that keratin IFs, by contrast, probably move along microfilaments, employing actin-based myosin motors. In neurons, defective IF transport could be responsible for the accumulations of neural IFs that characterize neurodegenerative diseases...

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