Why do microtubules require GTP and Ca ions for their assembly and disassmebly ?

GTP hydrolysis has been shown to be a key regulator of microtubule polymerization dynamics. It was demonstrated that a stoichiometric amount of GTP was sufficient to promote microtubule assembly and that the amount of inorganic phosphate liberated from GTP was equal to that of tubulin incorporated into the microtubules. The role of GTP in microtubule assembly is to give tubulin a conformation favorable for polymerization, and then to shift the equilibrium irreversibly toward polymerization through the hydrolysis of tubulin-bound GTP. The hydrolysis of GTP appears to be necessary for subsequent disassembly of microtubules, since GTP-assembled microtubules are sensitive to calcium-induced depolymerization.