Fig. 1.

Microtubule polymerization phases. Microtubules grow by addition of GTP-tubulin subunits. After their incorporation, subunits are hydrolyzed to become GDP-tubulin. The straight tubulin conformation within the microtubule lattice is stabilized by a ‘cap’ of tubulin-GTP subunits. Closure of the terminal sheet structure generates a metastable, blunt-ended microtubule intermediate, which might pause, undergo further growth, or switch to the depolymerization phase possibly by loss of the GTP-tubulin at the end. GDP-tubulin is free to splay out and the microtubule rapidly shrinks. Microtubules can transition from growth to shrinking phases called catastrophes. Protection of microtubules from shrinking by adding GTP-tubulin at the tip of microtubule is referred to as rescue