The Structure of Intermediate Filaments

Intermediate filaments are named for their diameter of approximately ten nanometers, which falls between microfilaments at seven nanometers and microtubules at twenty-five nanometers. This intermediate size distinguishes them as a separate class of cytoskeletal filaments with unique structural and functional properties.

All intermediate filament tetramers share a conserved tripartite fibrous protein core consisting of a central alpha-helical rod domain flanked by variable N-terminal head and C-terminal tail domains. The rod domain contains 310 amino acids rich in hydrophobic residues like leucine and isoleucine, enabling lateral associations that drive formation of intermediate filaments.

Intermediate filaments form through multi-step association beginning with monomers that associate into dimers and tetramers, the basic soluble units. These tetramers then assemble into unit-length filaments through lateral association, which subsequently coil and associate to form the complete rope-like intermediate filament structure.

Intermediate filaments provide mechanical stability and support to cells while maintaining tissue integrity. They also facilitate cell adhesion through interactions with integrins and cytoskeletal linkers. Additionally, nesprin proteins link the nuclear matrix to the cytoskeleton via intermediate filaments at the nuclear membrane.

Unlike microfilaments and microtubules composed of globular proteins, intermediate filaments are made of rigid, fibrous rope-like monomers. Intermediate filaments are also non-polar with no defined plus or minus ends, so molecular motor proteins cannot associate with them, unlike the other two cytoskeletal filament types.

Intermediate filaments remain intact when exposed to high salt concentrations and detergents due to their hydrophobic rod domain structure. The non-polar core of hydrophobic amino acids creates strong lateral associations and coiling interactions that stabilize the filament structure against chemical disruption.

The variable head and tail domains flanking the central rod domain interact with regulatory and linking proteins including integrins, cytoskeletal linkers, and nesprin. These interactions allow intermediate filaments to connect with other cellular structures and coordinate mechanical support and cell adhesion functions.