26.14 : The Structure of Intermediate Filaments

The intermediate filaments are one of three widely studied cytoskeletal filaments. They are so named as their diameter (10 nm) is in between that of microfilaments (7 nm) and the microtubules (25nm). These filaments are highly stable and can remain intact when exposed to high salt concentrations and detergents. These filaments are responsible for providing stability and mechanical support to the cells. They also help in cell adhesion and maintaining tissue integrity.

Intermediate filaments are found in almost all eukaryotic cells except lower eukaryotes like fungi and invertebrates like arthropods. In humans, around 70 genes code for different types of intermediate filament with cell type-specific expression depending on the function they perform. The mutation in these genes can lead to different diseases or disorders in humans, including Werner's syndrome, Alexander's disease, and Charcot-Marie tooth disease.

Intermediate filaments are non-polar with no defined plus or minus ends like microfilaments and microtubules; thus, no molecular motor proteins are known to be associated with them. Unlike microfilaments (F-actins) and microtubules made up of globular proteins, the monomeric units of intermediate filaments are rigid, fibrous rope-like proteins. These monomers vary among the cell types, forming a specific type of intermediate filament depending on their function. However, all monomeric units comprise a conserved alpha-helical central coiled-coil rod domain flanked by the head and tail domains. The conserved alpha-helical rod domain has 310 amino acids divided into three conserved segments with hydrophobic amino acid sequences separated by linkers. The rod domain helps in the lateral association of monomers to form the dimers and subsequently the tetramers, the basic soluble units of the intermediate filaments. The tetramers then assemble into unit-length filaments, which further associate to form the intermediate filaments.