Microtubule dynamics.

Microtubules are highly dynamic and switch stochastically between growing and shrinking phases both in vivo and in vitro. This non-equilibrium behavior, known as dynamic instability, is based on the binding and hydrolysis of GTP at the nucleotide exchangeable site (E-site) in β-tubulin. Only dimers that have GTP in their E-site can polymerize (red tubulin subunits), but following polymerization this nucleotide is hydrolyzed and becomes nonexchangeable. The GTP-cap model proposes that the body of the microtubule, which comprises GDPtubulin subunits (pale red), is unstable. The microtubule structure is stabilized by a layer of GTP tubulin subunits at the end that may act to maintain association between protofilaments. When this cap is stochastically lost, the protofilaments peel outward and the microtubule rapidly depolymerizes.