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Information about Intermediate Filament

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Intermediate filaments (IFs) are cytoskeletal structures formed by members of a family of related proteins. Intermediate filaments have a diameter between that of actin (microfilaments) and microtubules. Most types of intermediate filaments are located in the cytosol between the nuclear envelope and the cell surface membrane. Nuclear lamins are localized to the cell nucleus.

Structure

The domain structure of IF molecules is conserved. Each protein has a non-alpha-helical (globular) domain at the N and C-termini which surrounds the alpha-helical rod domain. The basic building block for IFs is a parallel and in register dimer. The dimer is formed through the interaction of the rod domain to form a coiled coil. Cytoplasmic IF assemble into non-polar unit-length filaments which then assemble into longer structures.

The anti-parallel orientation of tetramers means that, unlike microtubules and microfilaments which have a plus end and a minus end, IFs lack polarity.

Cytoplasmic IF do not undergo treadmilling like microtubules and actin fibers, but they are dynamic. For a review see: [1].

Types

There are about 70 different genes coding for various intermediate filament proteins. However, different kinds of IFs share basic characteristics: they are all polymers that generally measure between 9-11 nm in diameter when fully assembled.

IF are subcatagorized into six types based on similarities in amino acid sequence and protein structure.

Types I and II - Acidic and Basic Keratins

Enlarge picture
keratin intermediate filaments (stained red)
For more details on this topic, see cytokeratin.
These proteins are the most diverse among IFs and constitute type I (acidic) and type II (basic) IF proteins. The many isoforms are divided in two groups: Regardless of the group, keratins are either acidic or basic. Acidic and basic keratins bind each other to form acidic-basic heterodimers and these heterodimers then associate to make a keratin filament.

Type III

There are four proteins classed as type III IF proteins which may form homo- or heteropolymeric proteins.

Type IV

Type V - Nuclear Lamins

Lamins are fibrous proteins having structural function in the cell nucleus.

In metazoan cells there are A and B type lamins which differ in their length and pI. Human cells have three differentially regulated genes. B-type lamins are present in every cell. B type lamins, B1 and B2, are expressed from the LMNB1 and LMNB2 genes on 5q23 and 19q13, respectively. A-type lamins are only expressed following gastrulation. Lamin A and C are the most common A-type lamins and are splice variants of the LMNA gene found at 1q21.

These proteins localize to two regions of the nuclear compartment, the nuclear lamina -- a proteinaceous structure layer subjacent to the inner surface of the nuclear envelope and throughout the nucleoplasm in the nucleoplasmic "veil".

Comparison of the lamins to vertebrate cytoskeletal IFs shows that lamins have an extra 42 residues (six heptads) within coil 1b. The c-terminal tail domain contains a nuclear localization signal (NLS), an Ig-fold like domain, and in most cases a carboxy-terminal CaaX box that is isoprenylated and carboxymethylated (lamin C does not have a CAAX box). Lamin A is further processed to remove the last 15 amino acids and its farnesylated cysteine.

During mitosis, lamins are phosphorylated by MPF which drives the disassembly of the lamina and the nuclear envleope.

Type VI

Cell adhesion

At the plasma membrane, some keratins interact with desmosomes (cell-cell adhesion) and hemidesmosomes (cell-matrix adhesion) via adapter proteins.

Associated proteins

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; desmin filaments are connected in a similar way in heart muscle cells.

Diseases arising from mutations in IF genes

External links

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Proteins of the cytoskeleton
MicrofilamentsActins - Actin-binding proteins - Actinin - Arp2/3 complex - Cofilin - Destrin - Gelsolin - Myosins - Profilin - Tropomodulin - Troponin (T, C, I) - Tropomyosin - Wiskott-Aldrich syndrome protein
Intermediate filamentstype 1 and 2 (Cytokeratin, type I, type II) - type 3 (Desmin, GFAP, Peripherin, Vimentin) - type 4 (Internexin, Nestin, Neurofilament, Synemin, Syncoilin) - type 5 (Lamin A, B)
MicrotubulesDyneins - Kinesins - MAPs (Tau protein, Dynamin) - Tubulins - Stathmin
CateninsAlpha catenin - Beta catenin - Plakoglobin (gamma catenin) - Delta catenin
NonhumanMajor sperm proteins - Prokaryotic cytoskeleton (Crescentin, FtsZ, MreB)
OtherAPC - Dystrophin (Dystroglycan) - plakin (Desmoplakin, Plectin) - Spectrin - Talin - Utrophin - Vinculin
cytoskeleton is a cellular "scaffolding" or "skeleton" contained, as all other organelles, within the cytoplasm. It is contained in all eukaryotic cells and recent research has shown it can be present in prokaryotic cells too.
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Actin is a globular structural, 42-47 kDa protein found in many eukaryotic cells, with concentrations of over 100 μM. It is also one of the most highly conserved proteins, differing by no more than 5% in species as diverse as algae and humans.
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Microfilaments are the thinnest filaments of the cytoskeleton found in the cytoplasm of all eukaryotic cells. These linear polymers of actin subunits are flexible and relatively strong, resisting buckling by multi-piconewton compressive forces and filament fracture by nanonewton
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Microtubules are one of the components of the cytoskeleton. They have diameter of ~ 24 nm and length varying from several micrometers to possibly millimeters in axons of nerve cells.
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The cytosol (cf. cytoplasm, which also includes the organelles) is the internal fluid of the cell, and a portion of cell metabolism occurs here. Proteins within the cytosol play an important role in signal transduction pathways and glycolysis.
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The nuclear envelope (also known as the perinuclear envelope, nuclear membrane, nucleolemma or karyotheca) is the double membrane of the nucleus that encloses genetic material in eukaryotic cells.
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alpha helix (α-helix) is a right-handed coiled conformation, resembling a spring, in which every backbone N-H group donates a hydrogen bond to the backbone C=O group of the amino acid four residues earlier ( hydrogen bonding). (See also helix.
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dimer refers to a molecule composed of two identical subunits or monomers linked together.

Chemistry

The molecules in a dimer are connected by covalent bonds or weaker interactions such as hydrogen bonds.
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coiled coil is a structural motif in proteins, in which 2-7[1] alpha-helices are coiled together like the strands of a rope (Dimers and trimers are the most common types).
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In reference to biological molecules, particularly microtubules and microfilaments, treadmilling refers to the constant removal of the protein subunits from these filaments at one end while protein subunits are constantly added at the other end.
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Proteins are large organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid residues.
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Cytokeratins are intermediate filament keratins found in the intracytoplasmic cytoskeleton of epithelial tissue. There are two types of cytokeratins: the low weight, acidic type I cytokeratins and the high weight, basic or neutral type II cytokeratins.
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Type I keratins (or Type I cytokeratins) constitutes the Type I intermediate filaments (IFs) of the intracytoplasmatic cytoskeleton, which is present in all mammalian epithelial cells.
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Type II keratins (or Type II cytokeratins) constitutes the Type II intermediate filaments (IFs) of the intracytoplasmatic cytoskeleton, which is present in all mammalian epithelial cells.
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A protein isoform is a version of a protein with only small differences to another isoform of the same protein. Different forms of a protein may be produced from different but related genes, or may arise from the same gene by alternative splicing.
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epithelium is a tissue composed of a layer of cells. Epithelium lines both the outside (skin) and the inside cavities and lumen of bodies. The outermost layer of our skin is composed of dead stratified squamous, keratinized epithelial cells.
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Hair keratin is a type of keratin found in hair and nails. There are two types of hair keratin:
  • the acidic type I hair keratin
  • type I hair keratin 1, KRT31

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Hair is a filamentous outgrowth of protein, found only on mammals. It projects from the epidermis, though it grows from hair follicles deep in the dermis. Although many other organisms, especially insects, show filamentous outgrowths, these are not considered "hair".
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nail is a horn-like structure at the end of a human's or an animal's finger or toe. See also claw.

Parts of the nail

Anatomically fingernails and toenails
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horn is a living, vein and artery filled, pointed projection of the skin of various animals, consisting mainly of keratin as well as other proteins. True horns are found only among the ruminant artiodactyls, in the families Antilocapridae (pronghorn) and Bovidae (cows, buffalo,
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Sauropsida*
Goodrich, 1916

Subclasses
  • Anapsida
  • Diapsida
Synonyms
  • Reptilia Laurenti, 1768
Reptiles are tetrapods and amniotes, animals whose embryos are surrounded by an amniotic membrane, and members of the class
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scale (Greek lepid, Latin squama) is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration.
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polymer is a substance composed of molecules with large molecular mass composed of repeating structural units, or monomers, connected by covalent chemical bonds. The word is derived from the Greek, πολυ, polu, "many"; and μέρος, meros,
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A heteropolymer or copolymer is a polymer derived from two (or more) monomeric species, as opposed to a homopolymer where only one monomer is used.[1] Copolymerization refers to methods used to chemically synthesize a copolymer.
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Desmin is a type III intermediate filament found near the Z line in sarcomeres. It was first purified in 1977, the gene was characterized in 1989, and the first knock-out mouse was created in 1996.
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A sarcomere is the basic unit of a muscle's cross-striated myofibril. Sarcomeres are multi-protein complexes composed of three different filament systems.
  • The thick filament system is composed of myosin protein which is connected from the M-line to the Z-disc by Titin

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Glial fibrillary acidic protein (GFAP) is an intermediate filament (IF) protein that is found in glial cells such as astrocytes. First described in 1971[1], GFAP is a type III IF protein that maps, in humans, to 17q21.
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Astrocytes (also known collectively as astroglia) are characteristic star-shaped glial cells in the brain. They perform many functions, including biochemical support of endothelial cells which form the blood-brain barrier, the provision of nutrients to the nervous tissue,
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Glial cells, commonly called neuroglia or simply glia (greek for "glue"), are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system.
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