Intermediate filaments

Unlike actin and tubulin, which are globular proteins, the intermediate filament proteins are fibrous and are a much more heterogeneous group of proteins. They form cytoplasmic filamentous structures that are intermediate in diameter (10 nm) between microfilaments and microtubules (Fig. 3.35.1). Each type of filament is composed of a defined protein or combination of proteins. Each intermediate filament protein has a homologous α-helical core structure with subunit-specific N- and C-termini of variable length. Filament formation is initiated by the formation of a parallel, coiled-coil structure composed of the homologous core structure of two subunits. Intermediate filaments are then formed by polymerization of these dimeric structures. Being based on a homologous core structure, copolymerization of mixed intermediate filament oligomers is possible when certain subunits are coexpressed in the same tissue. Not all subunit combinations are stable. Where coexpression of incompatible subunits occurs, homo-oligomers of each protein are formed (e.g. keratins and vimentin in epithelial cells).

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