Cytoskeleton

=Cytoskeleton=

The **cytoskeleton** is a protein framework found within the cell. It has roles in cell structure, resistance to shear and mechanical stress, cell migration and the trafficking of organelles and vesicles between destinations. The cytoskeleton is composed primarily of three fibre types - actin (sometimes microfilaments), microtubules and intermediate filaments - as well as various motor proteins that interact with these fibres. These constituents are described in greater detail below.


 * Actin (microfilaments)**


 * Microtubules**

//Microtubules// are the tracks for intracellular movement of organelles, vesicles and chromosomes (the latter during mitosis). They are hollow tubes (diameter = 25nm), made of adjacent dimers of alpha-tubulin and beta-tubulin, each encoded for by 8-9 genes. The dimers assemble to form protofilaments, which in turn assemble to form the hollow tubes. Microtubules grow from specialised structures called //microtubule organising centres// (or //MTOCs//), such as centrosomes and basal bodies. They form specialised structures in cilia and flagella.

Microtubules have polarity - a **+** and a **-** end - which is important when considering its associated motor proteins, **kinesin** and **dyenin.** Because microtubules are formed of alpha dimers connected to adjacent beta dimers, polarity can be expressed as an exposed alpha end (**-**) and an exposed beta end (**+**). Microtubules polymerise at their **+** end by the addition of GTP-tubulin; they depolymerise at their **-** end where GDP-tubulin falls off (furthermore, the **-** end is capped so that polymerisation is only directed at the **+** end).

//Kinesin// is a plus-end directed motor