The Structure of Intermediate Filaments

Intermediate filaments are cytoskeletal proteins with long rope-like structures.

They are called intermediate filaments because their width is approximately ten nanometers, which is between the widths of microfilaments and microtubules.

Intermediate filaments form through the lateral association of unit length filaments which are formed by multi-step association of the tetramers.

Structurally, all tetramers have a common tripartite fibrous protein core, consisting of a conserved alpha-helical rod domain at its center, a variable N-terminal head, and C-terminal tail domains.

The helical subunits have repeats of seven amino acids each, rich in hydrophobic amino acids like leucine, isoleucine, methionine, or valine, making the core of intermediate filaments non-polar.

The hydrophobic rod domain helps in the lateral associations and coiling of monomers for filament formation. The head and tail domains interact with proteins, including integrins, the cytoskeletal linkers in the cytoplasm, and nesprin linking the nuclear matrix and cytoskeleton in the nuclear membrane.