1.4 Specialised intermediate filaments
Compared with other cytoskeletal elements, intermediate filaments are more like a fixed scaffolding for the cell. They have a higher tensile strength than microtubules and microfilaments. Consequently they contribute greatly to the overall integrity of the cell and preservation of its shape. Not all eukaryotic cells have cytoskeletal intermediate filaments, and of those that do, each cell type has its own distinct set of intermediate filaments. The intermediate filaments, being cell-type specific, are more related to maintaining the characteristic architecture of each cell type. Intermediate filaments appear to have arisen by gene duplication and diversification on a number of separate occasions from the nuclear lamina. One of the largest families is the keratins with more than 20 different members in epithelial cells, and about 10 of them are specific to cells that form hair and nails (Table 1).
Table 1 Examples of intermediate filament proteins.
|Intermediate filament||Polypeptides||Expressed in|
|nuclear||lamins A, B and C||nuclear lamina|
|vimentin-like||vimentin||many mesenchymal cells|
|glial fibrillary acidic protein (GFAP)||astrocytes, some Schwann cells|
Intermediate filaments are elongated molecules with a central section of α helix (Figure 8a). Two molecules associate to form a coiled-coil dimer (Figure 8b), and the dimers associate in a staggered head-to-tail arrangement as shown in Figure 8c and d, which further pack together to yield a rope-like filament (Figure 8e). This arrangement gives the intermediate filaments their tensile strength. Although intermediate filaments are dynamic structures, much less is known about how they are controlled than microfilaments or microtubules. Cells can change their expression of intermediate filaments in response to activation or a change in requirements. For example, astrocytes are cells in the brain that express the vimentin-like intermediate filament, glial fibrillary acidic protein (GFAP); in resting astrocytes the level of expression is quite low, but this increases greatly when the cells are activated, as occurs in individuals with multiple sclerosis. Activation is associated with an increase in the size and mobility of the astrocytes, which clearly places additional requirements on the cytoskeleton to maintain the structural integrity of the cell.