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An intermediate filament is a rope-like protein assembly of the cytoskeleton, typically about 10 nanometres in diameter, between an actin filament and a microtubule in thickness. Different cells use different constituent proteins: keratins reinforce many epithelial cells, vimentin forms networks in many connective-tissue cells, and lamins support the cell nucleus. Their assembled subunits do not establish the uniform head-to-tail polarity characteristic of an actin filament.^[ref]

The mechanical role can be tested outside a biological cell. In 1991, P. A. Janmey and colleagues compared purified vimentin, actin and microtubule networks under deformation. The vimentin networks tolerated larger deformations before disruption, illustrating how an intermediate filament network can combine compliance with resistance to damage. This helps explain why load-bearing tissues need a network that can stretch rather than simply a rigid internal frame.^[ref]

Keratin is a family of structural intermediate filament proteins abundant in epithelial cells. Compatible type I and type II keratins assemble together into filaments, which distribute mechanical stresses across a cell and connect to junctional structures such as desmosomes. Keratin is therefore part of a cellular load-bearing system rather than a molecular motor. ^[ref]

The connection to desmosomes explains how force can pass between neighbouring cells while each cell retains its own boundary. Hair and the outer layers of skin are familiar keratin-rich materials, but living epithelial cells also use keratin networks internally. Different keratin expression patterns can identify cellular states without proving identical mechanical behaviour in every tissue.

A lamin is a type of intermediate filament protein forming part of the nuclear lamina, a network associated with the inner surface of the nuclear envelope. Lamins contribute to nuclear shape and mechanical organization and interact with components connecting the envelope to chromatin. They differ from laminin, an extracellular-matrix protein, despite the similar names. ^[ref]

Regulated assembly is useful during cell division. Phosphorylation of lamins helps promote lamina disassembly as the nuclear envelope breaks down in open mitosis; subsequent reassembly supports daughter nuclei. Thus a structural protein can form a load-bearing network while retaining a chemically regulated route for dismantling it.