A model of intermediate filament assembly as it proceeds by longitudinal annealing of unit-length filaments (top) through their loose ends (middle), followed by compactions of the segmented immature filaments (bottom). The model is overlaid on a transmission electron micrograph of a corresponding assembly mixture of recombinant Xenopus vimentin. Image and caption: Daniel Stoffler, Scripps Research Institute.
An intermediate filament is one of three major components of the cytoskeletons of eukaryotic cells. Intermediate filaments (IFs), are so called because, at 10 nanometers in diameter, they are typically intermediate in size between microfilaments and microtubules.
IFs, however, are different to microfilaments and microtubules in a number of fundamental respects. First, they tend to be more or less permanent structures in tissues such as skin and hair; in fact, in these non-living tissues IF proteins are almost the only protein. (Therefore, it is true, if somewhat prosaic, to say that beauty is only IF thick). In other cell types, IFs are modified by phosphorylation when they are required to be disassembled for example during cell division. Unlike the highly conserved actins and tubulins, more than 40 distinct IF proteins are encoded by a number of genes in mammalian cells. All IF proteins have a similar structure with a central helical rod domain and more variable head and tail domains.
The IFs can be divided into five major classes as follows:
|glial||glial cells and astrocytes|