The cytoskeleton is formed of microtubules, microfilaments and intermediate filaments. Microtubules are formed by polymerisation of tubulin and microfilaments by polymerisation of actin. Assembly and disassembly are faster at the plus end of the filament. Both microtubules and microfilaments can display treadmilling and dynamic instability, in appropriate conditions.
Actin is an ATPase, and actin-ATP is less readily dissociated from the ends of microfilaments than actin-ADP. Tubulin is a GTPase and tubulin-GTP is less readily dissociated from microtubules than tubulin-GDP.
Capping proteins stabilise the ends of microfilaments and microtubules. The minus ends of microtubules are stabilised and nucleated by the MTOC. The minus ends of microfilaments are nucleated as branches on the actin network or at the plasma membrane.
Microtubule associated proteins modulate the formation, stability and disruption of microtubules: an analogous set of proteins acts on microfilaments in a similar way.
Intermediate filaments are structural elements found only in some types of cell. They are found in a wide variety of cell-specific types and they have high tensile strength, which helps to maintain cellular integrity.