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]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapter 16, Intermediate Filaments: structure, families and mechanical role
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]
Viscoelastic properties of vimentin compared with other filamentous biopolymer networks by P. A. Janmey and others. Abstract and pp. 155–160, comparison of purified networks
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