Intermediate Filaments
Intermediate filaments are a very broad class of fibrous proteins that play an important role as both structural and functional elements of the cytoskeleton. Ranging in size from 8 to 12 nanometers (in diameter; see Figure 1), intermediate filaments function as tension-bearing elements to help maintain cell shape and rigidity, and serve to anchor in place several organelles, including the nucleus and desmosomes. Intermediate filaments are also involved in formation of the nuclear lamina, a net-like meshwork array that lines the inner nuclear membrane and governs the shape of the nucleus. Although all eukaryotes contain the common cytoskeletal elements actin and tubulin (both free in the cytoplasm and polymerized in the form of microfilaments and microtubules), intermediate filaments are found only in some metazoan species, including vertebrates, nematodes, and molluscs. In vertebrates, intermediate filament presence and composition is not only species-dependent, it also varies with the tissue type. For example, most animal epithelial cells contain keratins, a diverse family of intermediate filaments consisting of more than 50 members, while mesenchymal and muscle cells are rich in the fibrous proteins, vimentin and desmin, respectively. Intermediate filaments found in neurons and glial cells include peripherin, neurofilaments, and glial fibrillary acidic protein (GFAP). A variety of associated proteins bind to intermediate filaments, either to improve stability (through crosslinking) or to provide attachment sites for other protein assemblies, such as actin filaments and microtubules.