Vimentin

Vimentin is a member of the intermediate filament family of proteins. Intermediate filaments are an important structural feature of eukaryotic cells. They, along with microtubules and actin microfilaments, make up the cytoskeleton.

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Structure

A vimentin monomer, like all other intermediate filaments, has a central α-helical domain, capped on each end by non-helical amino (head) and carboxy (tail) end domains.^[1] Two monomers will twist around each other to form a coiled-coil dimer. Two dimers then form a tetramer, which in turn form a sheet by interacting with other tetramers. Figure 1 shows the step-by-step process in which the filament is assembled.

The α-helical sequences contain a pattern of hydrophobic amino acids that contribute to forming a "hydrophobic seal" on the surface of the helix.^[1] This seal allows the two helices to come together and coil. Additionally, there is a periodic distribution of acidic and basic amino acids that seems to play an important role in stabilizing coiled-coil dimers.^[1] The spacing of the charged residues is optimal for ionic salt bridges, which allows for the stabilization of the α-helix structure. While this type of stabilization is intuitive for intrachain interactions, rather than interchain interactions, scientists have proposed that perhaps the switch from intrachain salt bridges formed by acidic and basic residues to the interchain ionic associations contributes to the assembly of the filament.^[1]

Function

Scientists have found that vimentin is attached to the nucleus, endoplasmic reticulum and mitochondria, either laterally or terminally.^[2] They concluded that vimentin plays a significant role in supporting and anchoring the position of the organelles in the cytosol.

Vimentin Clips offers three different clips that beautifully show vimentin movement inside the cell.

The dynamic nature of vimentin is important when offering flexibility to the cell. Scientists found that vimentin provided cells with a resilience absent from the microtubule or actin filament networks, when under mechanical stress in vivo. Therefore, it is generally accepted that vimentin is the cytoskeletal component responsible for maintaining cell integrity. (It was found that cells without vimentin were extremely delicate when disturbed with a micropuncture.) ^[3]

A study was done involving transgenic mice that lacked vimentin.^[3] Results showed that the mice were functionally normal. While the outcome is a bit surprising, it is possible that the microtubule network may have compensated for the absence of the intermediate network. This strengthens the suggestion of intimate interactions between microtubules and vimentin. Moreover, when microtubule depolymerizers were present, vimentin reorganization occurred, once again implying a relationship between the two systems.^[3]

Vimentin Images offers a gallery of images in which vimentin and other cytoskeletal structures are labeled. These images allow the visualization of interactions between vimentin and other cytoskeletal components.

Essentially, vimentin is responsible for maintaining cell shape, integrity of the cytoplasm, and stabilizing cytoskeletal interactions.

Lastly, vimentin is found to control the transport of low density lipoprotein, LDL,-derived cholesterol from a lysosome to the site of esterification.^[4] With the blocking of transport of LDL-derived cholesterol inside the cell, cells were found to store a much lower percentage of the lipoprotein than normal cells with vimentin. This dependence seems to be the first process of a biochemical function in any cell that depends on a cellular intermediate filament network. This type of dependence has ramifications on the adrenal cells, which rely on cholesteryl esters derived from LDL. ^[4]

References

Further reading