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Last modified May 2004

Intermediate filaments (IF) are the most diverse of the three major cytoskeletal systems in animal cells. The proteins comprising IF are encoded by approximately 70 different genes. This large family of proteins is subdivided into five types, four of which are located in the cytoplasm (cytoskeletal IF) and one in the nucleus (nucleoskeletal IF). In the cytoplasm, one, two or even more types of IF protein chains can polymerize into cytoskeletal IF of 10 nm diameter. Types I and II IF proteins are the acidic and basic keratins. Proteins of both types are required to polymerize (obligatory heteropolymers) into IF. Type III proteins such as vimentin, peripherin, desmin and glial fibrillary acidic protein (GFAP) assemble into homopolymer IF. Type IV IF proteins include the classical neurofilaments containing 3 protein chains, NF-L, NF-M, and NF-H; as well as -internexins and nestin. The type V proteins are the nuclear lamins and are found exclusively in the nucleus in a variety of polymeric states.

In most cells, IF assemble into complex networks that course through the cytoplasm between the nucleus and the cell surface. Towards the cell center, IF appear to attach to the nuclear envelope, and in the region of the plasma membrane, they are associated with various adhesion structures such as the desmosomes and hemidesmosomes of epithelial cells and the focal adhesions of fibroblasts. Cytoskeletal IF play important roles in a wide range of cellular functions. These include the formation and maintenance of cell shape, cellular mechanical integrity, signal transduction, and the overall stability and integration of other cytoskeletal systems including microtubules and microfilaments.

All IF proteins have three subdomains : an -helical central "rod" domain flanked on either side by non--helical "head" and "tail" domains (see above image). The rod domain is mainly -helical with a heptad repeat pattern in segments 1A and 1B forming coil 1, as well as in segments 2A and 2B constituting coil 2. These segments are joined by short, non-helical linkers (L1, L12, L2). The most conserved regions are found at the N-terminal end of the rod (1A) and the C-terminal end of the rod domain (2B). These conserved regions play vital roles in IF assembly. There is also a four-residue insertion near the middle of segment 2B. This so-called stutter is highly conserved in all IF.

During their assembly into 10nm filaments, individual IF protein chains interact in a hierarchical fashion. The basic and essential building block of IF is the dimer. This is formed through the coiled-coil interactions of two parallel and in register IF protein chains. Two dimers associate in a staggered and antiparallel fashion to form tetramers. On average eight such tetramers (a total of 32 monomers) combine laterally to form cylindrical structures known as unit length filaments (ULFs), with an approximate diameter of 16nm. These ULFs link up end-to-end, and through a process of compaction they are finally assembled into mature 10 nm diameter IF.

Picture courtesy of H. Herrmann (German Cancer Research Center, Heidelberg, Germany) and U. Aebi (Maurice E. Müller Institute for Structural Biology, Biozentrum Basel, Switzerland).

For many years textbooks described IF as very stable and rigid structures, only recognized for their maintenance of the mechanical stability of cells. However, the results of live cell imaging studies demonstrate the opposite. These studies have shown that IF networks are active and dynamic components of the cytoskeleton.

The first studies revealing the dynamic properties of IF employed the use of fluorescence recovery after photobleaching (FRAP) (Vikstrom et al. 1992, Yoon et al. 1998). Motile properties of vimentin intermediate filament networks in living cells. These studies revealed that there was constant subunit exchange between subunits and polymerized IF. They also revealed that IF were not polarized with respect to subunit exchange as recovery took place at equal rates all along the length of the photobleached zones. Fast IF movements have been described in spreading fibroblasts which are enriched in non-filamentous precursors to mature IF, known as IF particles (Link to movies).

These IF particles move bidirectionally along microtubule tracks at speeds up to ~1-2 um/min through interactions with the molecular motors, kinesin and dynein. Many of these particles assemble into short IF termed “squiggles” which in turn form the longer “filaments” found in established IF networks. Even the extensively polymerized IF networks of spread cells move, albeit more slowly, and many individual filaments appear to be constantly altering their shapes. Unlike faster particle and squiggle movements, these changes in shape, which frequently appear like propagated waves, do not require interactions with microtubules and/or microfilaments, suggesting that there are intrinsic, albeit unknown, regulatory factors responsible for some aspects of IF motility.