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Fig. 2. Microtubules are assembled from 
/ß-tubulin dimers, which associate head-to-tail to form linear protofilaments (boxed area). 12-15 protofilaments associate laterally to form the walls of the microtubule cylinder, which has a diameter of 
25 nm. The GTP-binding pocket in -tubulin is covered by ß-tubulin at all times and is therefore not exchangeable (see text). By contrast, the nucleotide on ß-tubulin can be exchanged while the dimer is not part of a microtubule. GTP hydrolysis is coupled to the incorporation of the dimer into the microtubule, and its energy facilitates rapid microtubule disassembly. What triggers the transition between growth and shrinkage of the microtubule, or between its shrinkage and growth, is not yet understood. Tubulin subunits can add to both ends of the cylinder, but the plus end grows about three times faster than the minus end in vitro. Within the cell, almost all microtubule minus ends are anchored at MTOCs, leaving the plus ends to extend into the cell periphery. During rapid microtubule disassembly, protofilaments peel back from the microtubule wall. Disassembly is facilitated by tension introduced into the protofilament by a conformational change in ß-tubulin upon GTP hydrolysis, which occurs when the subunit is incorporated into the microtubule lattice.
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