Intermediate filaments are one component of the cytoskeleton - important structural components of living cells. Their size is intermediate between that of microfilaments and microtubules. They are assembled from several different proteins. IFs crisscross the cytosol from the nuclear envelope to the cell membrane.
The different kinds of IFs share some basic characteristics: they are from 9 to 11 nm. in diameter and are very stable; their main function being a structural one. Different types of IFs are distinguished by the protein each is made of.
These form a network, the nuclear lamina, that supports the nuclear envelope. There are lamin A, B, and C filaments. Lamin A and C form the lamin network and are attached to the nuclear membrane by lamin B.
(See also: nuclear envelope)
These proteins are the most diverse among IFs. The many isoforms are divided in two groups: "soft" keratins (cytokeratins) in epithelial cells (image to right), and "hard" keratins (hair keratins) wich make up hair, nails, horns and reptilian scales. Ragardless of the group, keratin can be acidic or basic. Acidic and basic keratins can bind each other to form acidic-basic heterodimers, these heterodimers can then associate to make a keratin filament.
Type III IFs
These are found in nerve cells and are implicated in the radial growth of the axon.
- Neurofilament-L (NF-L)
- Neurofilament-M (NF-M)
- Neurofilament-H (NF-H)
Intermediate filament type VI. It is found in neural stem cells.
At the plasma membrane, IFs are attached by adapter proteins forming desmosomes (cell-cell adhesion) and hemidesmosomes (cell-matrix adhesion).