Cytoskeleton

The cytoskeleton is a structural "scaffold" or "skeleton" that is present inside all eukaryotic cells. This dynamic structure maintains the shape of cells, and enables cells to move, using structures such as flagella and cilia. It also has important roles in intracellular transport (for example, the movement of vesicles and organelles) and in cell division.

Eukaryotic cells

Actin cytoskeleton of mouse embryo fibroblasts, stained with phalloidin

Eukaryotic cells contain three kinds of cytoskeletal filaments.

Actin filaments, about 7 nm in diameter, are composed of two actin chains oriented in an helicoidal shape. They are mostly concentrated just beneath the plasma membrane, as they keep cellular shape, form cytoplasmatic protuberances (like pseudopodia and microvilli), and participate in some cell-to-cell or cell-to-matrix junctions and in the transduction of signals. They are also important for cytokinesis and, along with myosin, muscular contraction.

Intermediate filaments, 8-11 nanometers in diameter, are the more stable (strongly bound) and heterogeneous constituents of the cytoskeleton. They organize the internal structure of the cell (they are structural components of the nuclear envelope or the sarcomeres for example). They also participate in some cell-cell and cell-matrix junctions.

Different intermediate filaments are:

• made of vimentins, being the common structural support of many cells.
• made of keratin, found in skin cells, hair and nails.
• neurofilaments of neural cells.
• made of lamin, giving structural support to the nuclear envelope.

Microtubules are hollow cylinders of about 25 nm in diameter, formed by 13 protofilaments which are polymers of alpha and beta tubulin. They have a very dynamic behaviour, binding GTP for polymerization. They are organized by the centrosome.

They play key roles in:

• intracellular transport (associated with dyneins and kinesins they transport organelles like mitochondria and vesicles).
• the axoneme of cilia and flagella.
• the mitotic spindle.
• synthesis of the cell wall in plants.

The cytoskeleton in prokaryote cells

The cytoskeleton was thought to be a feature only of eukaryotic cells, but homologues of the major proteins of the eukaryotic cytoskeleton have been found in prokaryotes..

FtsZ was the first of these to be identified. Like tubulin, FtsZ forms filaments in the presence of GTP, but these filaments do not group into tubules. During cell division, FtsZ is the first protein to move to the division site, and is essential for recruiting other proteins that produce a new cell wall between the dividing cells.

Prokaryotic actin-like proteins, such as MreB, are involved in maintaining cell shape. All non-spherical bacteria have genes that encode actin-like proteins; these proteins form a helical network beneath the cell membrane that guides the proteins involved in cell wall biosynthesis.

Some plasmids encode a partitioning system that involves an actin-like protein ParM. Filaments of ParM exhibit dynamic instability, and may partition plasmid DNA into the dividing daughter cells by a mechanism like that used by microtubules during eukaryotic mitosis.

The bacterium Caulobacter crescentus expresses a protein, crescentin, that is related to the intermediate filaments of eukaryotic cells. Crescentin is also involved in maintaining cell shape, but how it does this is unclear.