Destabilization of Microtubules

Microtubules destabilize when the rate of catastrophe, or shortening, exceeds the rate of rescue, or elongation. This imbalance can be triggered by microtubule-associated proteins like stathmin and kinesin-13, which actively promote tubulin subunit detachment. Temperature and critical concentration of free tubulin heterodimers also influence destabilization rates.

Stathmin destabilizes microtubules through two mechanisms. In the cytoplasmic pool, it sequesters tubulin heterodimers by binding to them and altering their conformation, preventing assembly onto microtubules. Additionally, stathmin binds to microtubule tips and bends the filaments, sequentially removing tubulin subunits and shifting the balance from elongation to shrinkage.

Kinesin-13 is a non-motile ATP-dependent destabilizer that binds at microtubule tips and promotes conversion of GTP-beta-tubulin to GDP-beta-tubulin. This conversion lowers the heterodimer's affinity for adjacent subunits, causing easy release of tubulin dimers. ATP hydrolysis then separates kinesin-13 from the detached subunit.

Temperature significantly influences microtubule destabilization rates. In vitro studies show that microtubules disassemble faster at 4°C while rapidly reassembling at 37°C. This temperature sensitivity reflects the dynamic nature of tubulin polymerization and the energy-dependent processes controlling microtubule stability and assembly.

Critical concentration (Cc) is the concentration of free αβ-tubulin heterodimers at which net polymerization equals zero. Microtubule destabilization occurs when heterodimer concentration falls below the Cc. During the cell cycle, cells transition between catastrophe and rescue depending on whether tubulin concentration is above or below this critical threshold.

Microtubule lifespan varies significantly by cell type and cell cycle stage. During interphase, microtubules typically persist for about 30 minutes, while during cell division they last approximately 15 minutes. In specialized structures like axonal microtubules in neurons or axonemes in cilia and flagella, microtubules have considerably longer lifespans.

Katanin is a microtubule destabilizer with two subunits that sever longitudinal bonds between protofilaments, causing rapid destabilization. During cell division, katanin detaches microtubules from centrosomes, destabilizing spindle fibers. Its destabilizing activity also occurs during interphase in proliferating cells, contributing to dynamic microtubule remodeling and the movement of organelles and vesicles.