Cellular Biology

Basic Cytoskeleton Proteins



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Cytoskeletal proteins make up the cytoskeleton of a cell, which is the basic structural framework providing the cell a shape and providing internal transport pathways, aiding in cell division, and performing functional tasks, such as motion. The cytoskeleton is traditionally thought to occur only in eukaryotes, but evidence of their presence in prokaryotes has also been observed. The eukaryotic cytoskeletal proteins include microfilaments, intermediate filaments, and microtubules and their connections constantly change based on the needs and functions of the cell.

Microfilaments, also known as actin filaments, are approximately 5-9 nm in diameter and made of two intertwined actin chains, forming a double helix. They are most often present directly beneath the cell membrane. They are responsible for the cell's shape, membrane projections, and forming the cleavage furrow during replication. They also act with the protein myosin to perform muscle contraction and play a large role in signal transduction.

Intermediate filaments are tetrameric, meaning that they are made of two parallel helices, and approximately 10 nm in diameter. They are stable components of the cytoskeleton and, along with microfilaments, bear tension to maintain the cell's shape. These filaments, though, also organize the interior of the cell by anchoring organelles, are integrated into the structure of the nuclear envelope, and participate in cell junctions. Different types of intermediate filaments are present in different types of cells and structures keratin, lamin, neurofilaments, and vimentins.

Keratins are found in the skin, nails, horn, claws, and hair. They offer a strong structure and protection for the cells. They are known to contain a lot of the amino acid glycine. Lamin provides structural support to the inner lining of the nuclear envelope within cells. Neurofilaments are specific to neurons and considered to be either class III and IV intermediate filaments.

The class III neurofilament is the subunit peripherin, common in the peripheral nervous system. Class IV filaments are found in heteropolymers, meaning that the filament is made of different types of subunits. There are three subunit types NF-L (low), NF-M (middle), and NF-H (heavy). Vimentins are involved in cellular structural support offering flexibility to a cell. They are a class III intermediate filament subunit in developing neurons. They are also found attached to the nucleus, endoplasmic reticulum, and mitochondria to position and anchor them in the cytoplasm.

Microtubules are hollow tubes or cylinders 25 nm in diameter and are comprised of protofilaments, usually 13 of them, which are made up of alpha and beta-type tubulin, a globular protein. Microtubules bind GTP for polymerization and are organized by the centrosome into star-shaped sets to form the centrioles of cell division. Microtubule sets can also form wheel shapes to produce cilia and flagella, cellular projections that assist in movement. The proteins also play a role in the transport of intracellular structures, such as vesicles and mitochondria; the formation of the mitotic spindle during cell replication; and the synthesis of the cell wall in plant cells.

Another type of protein that interacts with, and can be considered to be cytoskeletal proteins, are catenins. Catenins are found in complexes with cell adhesion molecules in animal cells. They extend out from the cytoskeleton of one cell to the cytoskeleton of an adjacent cell by attaching to other cellular proteins called cadherins. As such, the name catenin comes from the Latin, catena, for "chain".

Reference:
Alberts et al. Molecular Biology of the Cell, 4th Ed. 2002

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