Introduction

Vimentin is part of the intermediate filament family. Intermediate filaments are an important structural feature of the eukaryotic cell. They, like microtubules and actin, are an integral member of the cytoskeleton. They can be divided into five different classes: these classes primarily differ by their immunological and biochemical characteristics.

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Table 1. [1]

Intermediate filaments are non-polar structures and are therefore not involved in treadmilling or dynamic instability, features of the microtubule and actin networks. Still, the intermediate filaments are known to be dynamic and continually assembled/disassembled. The figure below describes the assembly process of intermediate filaments. All five classes of the intermediate filaments, including Vimentin, assemble in the same manner; the difference in structure arises at the α-helical region of the monomer.

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Figure 1. [2]

As mentioned in Table 1., Vimentin can be found in fibroblasts and endothelial cells. They support the cell membrane and also anchor some organelles in a fixed place within the cytoplasm. Some scientists have argued that microtubules are necessary for vimentin structure to be maintained: experiments in which anti-tubulin antibody was injected into the cell showed that normal distribution of vimentin was disturbed. There are still issues that require clarification as the relationship between intermediate filaments and microtubules is not completely understood. Most scientists agree, however, that vimentin, along with the other intermediate filaments, does not contribute to cellular transport. Instead, vimentin is thought to anchor the nucleus and other organelles in position.

Vimentin 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. 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. 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. The spacing of the charged residues are 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.

Vimentin Function

Scientists have found that vimentin is attached to the nucleus, endoplasmic reticulum and mitonchondria, either laterally or terminally. They concluded that vimentin plays a significant role in supporting and anchoring the position of the organelles in the cytosol. Figure 2 shows vimentin labeling in cells. Each cell is clearly distinguishable from surrounding ones. Additionally, vimentin structure surrounding the nucleus and dispersed throughout the cytosol is easily seen as well.

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Figure 2. [3]

Because vimentin does not contain a nucleotide binding spot in its structure, the monomer, and in turn, the filament, is non-polar. Therefore, no dynamic instability or treadmilling is observed. Experiments have shown, however, that vimentin is almost constantly changing shape, and thus very dynamic.

Vimentin Movie <http://cpmcnet.columbia.edu/dept/gsas/anatomy/Faculty/Gundersen/movies.html> offers three different clips that beautifully show vimentin movement inside the cell.