The cytoskeleton of a cell is the basic structural framework of the cell. Cytoskeletal proteins provide internal transport pathways, participate in cell division, and perform functional tasks, including motion. Eukaryotic cells have long been known to have a cytoskeleton and there is evidence that prokaryotes also express cytoskeleton proteins. The connections between the proteins within the cell constantly change based on the cell’s needs and functions.
The cytoskeleton proteins in eukaryotes include microfilaments, intermediate filaments, and microtubules. A fourth category are the catenins, which are not classically considered cytoskeletal proteins, but they extend out from the cytoskeleton of one cell to the cytoskeleton of an adjacent cell by attaching to other cellular proteins called cadherins, forming cell adhesion molecule complexes. The name catenin comes from the Latin “catena”, for “chain”.
Microfilaments
Microfilaments are also known as actin filaments and are most often present directly under the cell membrane. Microfilaments are approximately 5–9 nanometers in diameter and made of two actin chains. The chains intertwine to form a double helix.
The actin filaments are responsible for the shape and membrane projections of the cell, as well as forming the cleavage furrow during replication, the plane where the cell splits in two. Actin filaments act with myosin, another cellular protein, to contract muscles. The microfilament also plays a large role in signal transduction.
Intermediate Filaments
The intermediate filaments are made of two parallel helices, so they are referred to as being tetrameric. The intermediate filaments are approximately 10 nm in diameter. These proteins are stable components of the cytoskeleton that bear tension to maintain the cell’s shape along with microfilaments. However, these cytoskeleton proteins also organize the interior of the cell; they anchor organelles, achieve the structure of the nuclear envelope, and participate in cell junctions.
Different types of cells contain different types of intermediate filaments:
- Keratins – found in skin, nails, horn, claws, and hair. Keratins offer protection for the cells and are known to contain high quantities of the amino acid glycine.
- Lamin – provides structural support to the nuclear envelope, specifically in the inner lining.
- Neurofilaments – specific to neurons and referred to as either class III or class IV intermediate filaments. Peripherin, a class III intermediate filament subunit, is common in the peripheral nervous system. Vimentins are class III intermediate filament subunits in developing neurons and involved in cellular structural support. Vimentins also position and anchor the nucleus, endoplasmic reticulum, and mitochondria in the cytoplasm. Class IV filaments are made of three different types of subunits: neurofilament low (NF-L), neurofilament middle (NF-M), and neurofilament heavy (NF-H).
Microtubules
As the name implies, microtubules are hollow tubes or cylinders. Microtubules are approximately 25 nm in diameter and comprised of protofilaments. The protofilaments are usually arranged in groups of 13 and made of alpha and beta-type tubulin, a globular protein. By binding GTP, microtubules polymerize and organize into star-shaped sets, forming the centrioles and mitotic spindle in cell division.
Microtubules are involved in cellular movement as well, forming the cellular projections cilia and flagella. The proteins also play a role in intracellular transport, particularly of vesicles and mitochondria; and they take part in the synthesis of the cell wall in plant cells.
Reference:
Alberts et al. Molecular Biology of the Cell, 4th Ed. 2002
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