Microtubule - Dynamic Instability

Dynamic Instability

Dynamic instability refers to the coexistence of assembly and disassembly at the (+) end of a microtubule. The microtubule can switch between the growing and shrinking phases dynamically at this region. During polymerization, both the α- and β-subunits of the tubulin dimer are bound to a molecule of GTP. While the GTP bound to α-tubulin is stable, the GTP bound to β-tubulin may be hydrolyzed to GDP shortly after assembly. The kinetics of GDP-tubulin are different from those of GTP-tubulin; GDP-tubulin is prone to depolymerization. A GDP-bound tubulin subunit at the tip of a microtubule will fall off, though a GDP-bound tubulin in the middle of a microtubule cannot spontaneously pop out. Since tubulin adds onto the end of the microtubule only in the GTP-bound state, there is a cap of GTP-bound tubulin at the tip of the microtubule, protecting it from disassembly. When hydrolysis catches up to the tip of the microtubule, it begins a rapid depolymerization and shrinkage. This switch from growth to shrinking is called a catastrophe. GTP-bound tubulin can begin adding to the tip of the microtubule again, providing a new cap and protecting the microtubule from shrinking. This is referred to as "rescue".

In vivo microtubule dynamics vary considerably. Assembly, disassembly, and catastrophe rates depend on which microtubule-associated proteins (MAPs) are present.