Intermediate Filaments definition
Intermediate Filaments refers to filaments of fibrous proteins that form the cytoskeleton and give strength and mechanical strength to cells.
Cytoskeleton
Cytoskeleton is present in virtually all cells and is the cellular constituent responsible for maintaining cell’s shape and assisting in movement. Cytoskeleton consists of three types of filaments: actin filaments, intermediate filaments and microtubules. However, to form a functional cytoskeleton, these three types of filaments need to bind to proteins and thereafter to bind to other cellular components and other filaments.
Assembly of Intermediate Filaments
The first step in assembling the intermediate filaments is formation of monomeric polypeptide chains by polymerization of peptides. Next, two polypeptide chains coil together in an elliptical double α-helix conformation, forming dimers. These dimers associate in an anti-parallel form, through strong hydrophobic lateral bonds, to form tetramers. Tetramers represent the soluble subunit of these filaments and bind at their ends forming protofilaments. Each intermediate filament is composed of approximately eight protofilaments joined and wound together in a rope-like conformation with a diameter of about 10nm.
Figure 1 – Assembly of intermediate filaments. A) monomeric polypeptide chain; B) dimer; C) tetramer; D) protofilament; E) intermediate filament.
These filaments, like the other two types of filaments of cytoskeleton, are highly dynamic structures, however, there is little information about their mechanism of association and dissociation. It is known, though, that phosphorylation of peptides regulates the assembly and dissociation of intermediate filaments. This mechanism is observed in lamina of nuclear envelope, in which its subunits are phosphorylated for their dissociation and dephosphorylated for their polymerization; and, also, in vimentin-like filaments, which are phosphorylated in mitosis, for their dissociation and dephosphorylated for polymerization during cell division.
Intermediate Filament Families
There are more than 50 different types of intermediate filaments identified, which are expressed in different cells: keratin’s’ family (the most diverse family), neurofilament’s family (found in high concentrations in axons) and vimentin-like filament’s family.
Intermediate Filaments’ characteristics
Intermediate filaments are only found in vertebrates, nematodes and mollusks and aren’t present in all cells of these organisms. These filaments are especially present in cells that are constantly being subjected to mechanical stress and are about 10 times more abundant than actin filaments and microtubules in the axons of neurons and in epidermal cells.
The name ‘intermediate filaments’ derives from their size, since they have an intermediate diameter in relation to the other two filaments constituting cytoskeleton. These filaments, because of their rope-like conformation, are especially resistant to stretching and bending, being extremely stable and difficult to break.
Its conformation coupled with its association with cell membrane, strengthens cell’s structure and aids the organization of cells in tissues. Association of intermediate filaments with cell membrane and the consequent cell-cell and substrate-cell interaction is made through desmosomes and hemidesmosomes.
References:
Alberts B., Johnson A., Lewis J., Raff M., Keith R., Walter P. (2007). Molecular Biology of the Cell (5th edition). Garland Science, New York.
Cooper G.M. (2000). The Cell: A Molecular Approach (2th edition). Sinauer Associates, Sunderland (MA).
Lodish H., Berk A., Zipursky S.L., Matsudaira P., Baltimore D., Darnell J. (2000). Molecular Cell Biology (4th edition). W. H. Freeman, New York.
