Role of GTP/GDP in the assembly/disassembly of microtubules
The animation starts with 3 tubulin dimers, each of which has a bound GTP molecule. Note that in the beginning, these GTP dimers associate tightly together to form a strong oligomer (short protofilament). The tight association is illustrated with a very close fit between the GTP dimers. With time, however, the GTP is hydrolyzed to GDP, liberating a free inorganic phosphate and inducing a conformational change in the dimer. This rate of GTP --> GDP hydrolysis occurs at a constant rate. The conformational change induced by the GDP weakens the hydrogen bonds between the dimers and potentiates the disassociation of the dimers from the oligomer. The weaker association between the dimers is illustrated with an increasing gap between the GDP dimers and the rest of the oligomer. At the + end of the microtubule, dimers with bound GTP are constantly being added. At the - end of the microtubule, dimers with bound GDP disassociate. By regulating the amount of soluble cytoplasmic GTP dimer, a cell can either increase (high concentrations of GTP dimer) or decrease (low concentrations of GTP dimer) the length of their microtubules.