Intermediate filament

all about MSc zoology

1. Intermediate filament -By zoomania

2. Intermediate filament Intermediate filaments (IFs) are cytoskeletal structural components found in the cells of vertebrates, and many invertebrates.  Homologues of the IF protein have been noted in an invertebrate, the cephalochordate Branchiostoma. Intermediate filaments are composed of a family of related proteins sharing common structural and sequence features. Initially designated 'intermediate' because their average diameter (10 nm) is between those of narrower microfilaments (actin) and wider myosin filaments found in muscle cells, the diameter of intermediate filaments is now commonly compared to actin microfilaments (7 nm) and microtubules (25 nm).

3. Initially designated 'intermediate' because their average diameter (10 nm) is between those of narrower microfilaments (actin) and wider  myosin  filaments found in muscle cells, the diameter of intermediate filaments is now commonly compared to actin microfilaments (7 nm) and microtubules (25 nm).

4. Animal intermediate filaments are subcategorized into six types based on similarities in amino acid sequence and protein structure .  Most types are cytoplasmic, but one type, Type V is a nuclear lamin. Unlike microtubules, IF distribution in cells show no good correlation with the distribution of either mitochondria or endoplasmic reticulum.

5. M icrofiament Structure The structure of proteins that form intermediate filaments (IF) was first predicted by computerized analysis of the amino acid sequence of a human epidermal keratin derived from cloned cDNAs .

6. Analysis of a second keratin sequence revealed that the two types of keratins share only about 30% amino acid sequence homology but share similar patterns of secondary structure domains.  As suggested by the first model, all IF proteins appear to have a central alpha-helical rod domain that is composed of four alpha-helical segments (named as 1A, 1B, 2A and 2B) separated by three linker regions.

7. The central building block of an intermediate filament is a pair of two intertwined proteins that is called a coiled-coil structure. Structural analysis of a pair of keratins shows that the two proteins that form the coiled-coil bind by hydrophobic.

8. Types There are about 70 different human genes coding for various intermediate filament proteins. Animal IFs are subcategorized into six types based on similarities in amino acid sequence and protein structure: Types I and II – acidic and basic keratins Type III Type IV Type V - nuclear lamins Type VI

9. Types I and II – acidic and basic keratins 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. Keratin intermediate filaments (stained red) around epithelial cells .

10. 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, keep some organelles in a fixed place within the cytoplasm, and transmit membrane receptor signals to the nucleus. Vimentin fibers in fibroblasts .

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

12. Type V - nuclear lamins L amins Lamins are fibrous proteins having structural function in the cell nucleus. Type VI Beaded filaments:  Filensin, Phakinin. Nestin  (was once proposed for reclassification but due to differences, remains as a type VI IF protein) .

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