Intermediate filament

From Wikipedia, the free encyclopedia

1 Structure
2 Types
3 Cell adhesion
4 Associated proteins
5 Diseases arising from mutations in IF genes
6 External links

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.

[edit] 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].

[edit] 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.

[edit] Types I and II - Acidic and Basic Keratins

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:

epithelial keratins (about 20) in epithelial cells (image to right)
trichocytic keratins (about 13) (hair keratins) which make up hair, nails, horns and reptilian scales.

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.

[edit] Type III

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

Desmin IFs are structural components of the sarcomeres in muscle cells.
GFAP (glial fibrillary acidic protein) is found in astrocytes and other glia.
Peripherin found in peripheral neurons.
Vimentin, the most widely distributed of all IF proteins, can be found in fibroblasts, leukocytes, and blood vessel endothelial cells. They support the cellular membranes and keep some organelles in a fixed place within the cytoplasm.

[edit] Type IV

α-Internexin
Nestin
Neurofilaments - the type IV family of intermediate filaments that is found in high concentrations along the axons of vertebrate neurons.
Synemin
Syncoilin

[edit] Type V - Nuclear Lamins

Lamins

[edit] Type VI - Nestin

[edit] Cell adhesion

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

[edit] 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.

[edit] Diseases arising from mutations in IF genes

Epidermolysis bullosa simplex; K5 or K14 mutation
Laminopathies are a family of diseases caused by mutations in nuclear lamins and include Hutchinson Gilford Progeria Syndrome and various lipodystrophies and cardiomyopathies among others.
Human Intermediate Filament Database(HIFD), a comprehensive database of human intermediate filament proteins, their associated variations and diseases.