Intermediate filaments (IFs) are a component of the cytoskeleton - important structural components of living cells. Their size is intermediate between that of microfilaments and microtubules. They are assembled from several different proteins. IFs crisscross the cytosol from the nuclear envelope to the cell membrane.
Each IF molecule has a globular domain at both ends, separated by a long alpha-helical region. IFs are formed of dimers in which the two monomers are joined by the winding of their alpha-helical parts into a coiled coil, oriented in the same direction. Two dimers join side-by-side, anti-parallel, forming a tetramer. Each dimer is 48 nanometers long; because the dimers are staggered the tetramer is somewhat longer. The anti-parallel orientation of tetramers means that, unlike microtubules and microfilaments which have a plus end and a minus end, IFs lack polarity.
Although they cannot undergo treadmilling as microtubules and microfilaments, IFs are dynamic, continually disassembled into soluble tetramers and reassembled into filaments. Until 2003 IFs were thought to be static structural components.
Different kinds of IFs share basic characteristics: they are from 9 to 11 nm. in diameter and are very stable; their main function being a structural one. Different types of IFs are distinguished by the protein each is made of.
These proteins localize to two distict regions of the nuclear compartment. The first is the nuclear lamina, a proteinaceous layer located at the inner surface of the inner nuclear envelope membrane. The second is throughout the nucleoplasm in a structure termed the nucleoplasmic veil. Human cells express two types of lamin, A and B, from three differentially regulated genes. Lamin A and C are the most common A-type lamins and are splice variants of the LMNA gene found at 1q21. B type lamins, B1 and B2, are expressed from the LMNB1 and LMNB2 genes on 5q23 and 19q13, respectively.
keratin intermediate filaments
These proteins are the most diverse among IFs. The many isoforms are divided in two groups: "soft" keratins (cytokeratins) in epithelial cells (image to right), and "hard" keratins (hair keratins) which make up hair, nails, horns and reptilian scales. Regardless of the group, keratin can be acidic or basic. Acidic and basic keratins can bind each other to form acidic-basic heterodimers, these heterodimers can then associate to make a keratin filament.
• Desmin IFs are structural components of the sarcomeres in muscle cells.
• Vimentin IFs can be found in fibroblasts and endothelial cells, they support the cell membrane and keep some organelles in a fixed place within the cytoplasm.
• Peripherin found in peripheral neurons.
• GFAP (glial fibrillary acidic protein) is found in astrocytes.
• Neurofilament-L (designated NF-L for 'light')
• Neurofilament-M (designated NF-M for 'medium')
• Neurofilament-H (designated NF-H for 'heavy')
Neurofilaments are a family of intermediate filaments that is found in high concentrations along the axons of vertebrate neurons. The three types of neurofilament proteins coassemble in vivo, forming a heteropolymer that contain NF-L plus one of the others. The NF-H and NF-M proteins have lengthy C-terminal tail domains that bind to neighboring filaments, generating aligned arrays with a uniform interfilament spacing. During axonal growth, new neurofilament subunits are incorporated all along the axon in a dynamic process that involves the addition of subunits along the filament length, as well as the addition of subunits at the filament ends. After an axon has grown and connected with its target cell, the diameter of the axon may increase as much as fivefold. The level of neurofilament gene expression seems to directly control axonal diameter, which in turn controls how fast electrical signals travel down the axon.
Axonal structure also depends on microtubules as well as actin filaments. Actin filaments line the cortex of the axon, just beneath the plasma membrane, and actin-based motor proteins such as myosin V are also abundant in the axon, perhaps to help move materials, although their exact function is still unclear. The specialized neurofilaments of nerve cells provide the most important structural support in the axon. A disruption in neurofilament structure, or in the cross-linking proteins that attach the neurofilaments to the microtubules and actin filaments distributed along the axon, can result in axonal disorganization and eventually axonal degeneration.
Plectin-like cross-links between microtubules and neurofilaments are seen in micrographs of nerve cell axons. They may represent intermediate filament associated proteins whose function is to cross-link neurofilaments and microtubules into a stable cytoskeleton. Alternatively, these connections to microtubules may be the long arms of the NF-H, which is known to bind microtubules.
Intermediate filament type VI. It is found in neural stem cells.
At the plasma membrane, IFs are attached by adapter proteins forming desmosomes (cell-cell adhesion) and hemidesmosomes (cell-matrix adhesion).
Filaggrin binds to keratin fibers in epidermal cells. Plectin links vimentin to other vimentin fibers, as well as to microfilaments, microtubules, and myosin II.
Keratin filaments in epithelial cells link to desmosomes through plakoglobin, desmoplakin, desmogleins and desmocollins. Similar for desmin filaments in heart muscle cells.
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