1 The cytoskeleton
1.1 Microtubules and microfilaments
The elements of the cytoskeleton each have their own distribution within the cell. Microtubules extend from the microtubule organising centre (MTOC), which in animal cells is the centrosome, usually located close to the nucleus. The centrosome consists of two centrioles, short cylinders of microtubules arranged at 90° to each other, which are embedded in a matrix of protein (Figure 1). The filamentous network of microtubules extends throughout the cell and has an important role in determining cell shape. As we shall see later, microtubules have plus and minus ends and they tend to extend and shrink from the plus end. The MTOC stabilises the minus ends of microtubules.
The microfilament network formed from polymerised or fibrous actin (F-actin) extends throughout the cytoplasm, but its organisation, the length of the filaments and their degree of branching depend very much on the type of cell, its shape, and how it is interacting with the extracellular matrix. The great majority of cell adhesion molecules, which allow a cell to bind to the extracellular matrix, interact inside the cell with microfilaments – usually via adaptor proteins. Indeed, one can view the microfilament network as an intracellular matrix, which links to the extracellular matrix by adhesion molecules. Reorganisation of the microfilament network can take place both locally (affecting only part of the cell) and globally (across the whole cell). In particular, cells that are moving rearrange their entire cytoskeleton as the cell becomes polarised, with engagement of microfilaments required for movement of the cell, and microtubules involved in moving the organelles.
Microfilaments are formed from pools of actin monomers, which associate non-covalently and can grow or shrink or remain stable, and do so continuously. Microtubules are formed from units consisting of a dimer of α-tubulin and β-tubulin. Direct observation of cells shows that microtubules in particular are highly dynamic structures and microfilaments somewhat less so. The dynamic rearrangement of the cytoskeleton requires energy from GTP in the case of microtubules and ATP for microfilaments. Proteins other than actin and tubulin associated with the microtubules and microfilaments are critically important, and determine their overall stability and whether they will grow or shrink. In the case of microfilaments, the proteins can also determine to what extent the network will branch.