4.2 The cytoskeleton
The cytosol of eukaryotic cells contains a system of specialised protein assemblies that together form what is known as the cytoskeleton. Another term often used is cellular 'scaffolding' which is perhaps rather misleading, since the protein assemblies that make up the cytoskeleton are not fixed, but are highly dynamic and play an essential role in the transport of organelles and some molecules within the cell. Cytoskeletal proteins also have other roles; their actions produce all kinds of cell movements, including the movement of motile cells and intracellular movements, such as those of chromosomes during cell division. In animal cells, the cytoskeleton also provides mechanical strength and support, and helps to maintain cell shape. In plant cells, the cell wall fulfils this role.
It used to be thought that prokaryotic cells did not have a cytoskeleton, but recent research has revealed that they have filaments assembled from proteins similar to those found in eukaryotic cells, e.g. actin. The prokaryotic cytoskeleton has roles in cell division and maintaining cell shape and polarity.
Cytoskeletal proteins form long, filament-like assemblies. There are three types of filaments, formed from different proteins: microfilaments (also known as actin filaments), microtubules and intermediate filaments (Figure 8).