Microtubule Instability

Dynamic instability is the unpredictable alternation between microtubule growth and shrinkage phases. Individual microtubules rapidly switch between elongation and rapid shortening, with some actively assembling while others simultaneously disassemble. This dynamic behavior occurs both in living cells and in laboratory conditions, enabling microtubules to respond quickly to cellular needs during different phases of the cell cycle.

Catastrophe occurs when a growing microtubule rapidly shifts to shortening, while rescue is when a shrinking microtubule begins elongating again. These transitions are determined by the rate of GTP hydrolysis on beta-tubulin subunits. GTP-bound tubulin promotes elongation and stability, whereas GDP-bound tubulin favors disassembly. The balance between catastrophe and rescue rates determines whether a microtubule net grows or shrinks at any given moment.

When GTP bound to beta-tubulin is hydrolyzed to GDP, the tubulin subunit's conformation slightly changes, causing protofilaments to curve outward. This structural distortion destabilizes the microtubule framework, causing the structure to splay at the tip and triggering depolymerization down the length of the filament. This mechanism allows cells to rapidly disassemble microtubules when needed.

Microtubule-associated proteins (MAPs) are critical regulators classified as stabilizers or destabilizers. Stabilizer MAPs reduce catastrophe events and promote elongation, dominating during interphase to maintain stable microtubule arrays. Destabilizer MAPs promote catastrophe and are more common during mitosis, facilitating the dynamic microtubule rearrangements needed for spindle assembly dynein kinesin and microtubules during chromosome segregation.

During interphase, most animal cells maintain long, stable microtubules radiating from the centrosome through dominant stabilizer MAPs. As cells transition to mitosis, microtubule instability increases as destabilizer MAPs become more prevalent. This increased instability facilitates rapid formation of a dense, dynamic array of mitotic microtubules, which is essential for proper spindle formation and chromosome segregation.

Microtubule polymerization initiates when GTP-bound alpha-beta tubulin heterodimers exceed a threshold concentration called the critical concentration. Below this threshold, free tubulin subunits remain unpolymerized in the cytoplasmic pool. Once the critical concentration is reached, tubulin heterodimers polymerize end-to-end, with GTP-bound beta-tubulin acting as a stabilizing cap at the growing end to prevent protofilament curvature.

Destabilizing proteins like stathmin and kinesin-13 bind to microtubules after GTP hydrolysis converts beta-tubulin to its GDP-bound form. The conformational change in GDP-bound tubulin causes protofilaments to curve, creating binding sites for these destabilizing proteins. Once bound, they facilitate removal of alpha-beta tubulin heterodimers, accelerating microtubule depolymerization during mitosis.