User:Dhail/Protein filament

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A transmembrane protein with an extracellular surface faced toward the top of the image, and the ATP binding domains on the cytosolic side at the bottom.

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In biology, a protein filament is a "long chain of proteins, such as those found in hair, muscle, or in flagella." [] Protein filaments form together to make the cytoskeleton of the cell. They are often bundled together to provide support, strength, and rigidity to the cell. When the filaments are packed up together, they are able to form three different cellular parts. These three cellular parts make up the cytoskeleton, where each structure has its own size and protein composition. The three cellular examples include:

• Microfilaments
• Microtubules
• Intermediate filaments

Developing wood cells in poplar showing microfilaments (in green color) and cell nuclei (in red color)

Microfilaments

Main article: Microfilaments

Compared to the other parts of the cytoskeletons, the microfilaments contain the thinnest filaments, with a diameter of approximately 7nm. Microfilaments are part of the cytoskeleton that are composed of protein called actin. Microfilaments can either occur in the monomeric G-actin or filamentous F-actin. []

A human cell showing the tubulin component of the cytoskeleton in green and the nucleus in red. The blue staining is a single cytoplasmic protein.

Microtubules

Main article: Microtubules

Microtubules are the largest type of filament, with a diameter of 25 nm wide, in the cytoskeleton. [] A single microtubule consists of 13 linear microfilaments. Unlike microfilaments, microtubules are composed of a protein called tubulin. The tubulin consists of dimers, named either "αβ-tubulin" or "tubulin dimers", which polymerize to form the microtubules. []

Human neural stem cells stained for Sox2, in green, and vimentin, in red. Vimentin is a type III intermediate filament (IF) protein.

Intermediate filaments

Main article: Intermediate filaments

Intermediate filaments are part of the cytoskeleton structure found in most eukaryotic cells. Intermediate filaments contain an average diameter of 10 nm, which is small than that of microtubules, but larger than that of microfilaments. []