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Intermediate Filaments

Intermediate filaments are a broad class of fibrous proteins that maintain cell shape, anchor organelles, and act as a structural component of the nuclear lamina and sarcomeres. The composition of intermediate filaments varies from one cell type to another as well as within a single cell. In fact, intermediate filament proteins are divided into six different types (I-VI) according to amino acid sequence and protein structure. Despite the range of proteins that form intermediate filaments, all have a central alpha-helical rod domain flanked by amino- and carboxy-terminal domains of differing sizes. The central rod domains of two intermediate filament proteins associate to form a coiled-coil structure. This dimer interacts with other dimers to form tetramers which subsequently assemble end to end to form protofilaments. Finally, the protofilaments join together to form an intermediate filament. Although intermediate filaments are generally stable structures, their assembly and disassembly can be affected by phosphorylation. This is particularly important during mitosis when phosphorylation of the intermediate filament protein Lamin causes disassembly of the nuclear lamina and subsequent breakdown of the nuclear envelope.