Investigating the molecular mechanisms of microtubule catastrophe

Abstract

The microtubule network in cells continuously remodels to facilitate its vital functions in cell division, cell motility and intracellular transport. Organization of the microtubule network is enabled by regulation of the dynamics of individual microtubule filaments, which switch between phases of growth and shrinkage, in a process known as dynamic instability. The growing microtubule end maintains a cap of GTP-tubulin, thought to protect the microtubule against transitions from growth to shrinkage (known as ‘microtubule catastrophe’). The GTP-cap size is thought to increase with the microtubule growth rate, presumably endowing fast-growing microtubules with enhanced stability. However, it is known that microtubules in cells simultaneously grow quickly and are highly dynamic. It is unknown what GTP-cap properties permit frequent microtubule catastrophe despite fast growth. This thesis investigates the relationship between GTP-cap size and microtubule stability, concluding that GTP-cap size is not the sole determinant of microtubule stability. In vitro, the microtubule plus ends either grow slowly, resulting in a small GTP-cap, and undergo frequent catastrophes, or grow quickly, producing a large GTP-cap, and undergoing fewer catastrophes. Interestingly, the relationship between GTP-cap size and microtubule stability is not universal, but dependent on the microtubule end and the presence of microtubule-associated proteins. The microtubule minus end has a small GTP-cap, a result of slow growth rate, but is very stable, undergoing few catastrophes. Additionally, the relationship between GTP-cap size and microtubule stability is broken at the plus end by the microtubule-associated protein XMAP215. Together, this work demonstrates that the GTP-cap size is not the sole determinant of microtubule stability, providing a novel insight into the molecular mechanisms regulating microtubule stability, essential for the life of the cell.