Effect of guanine nucleotides on the assembly of brain microtubles: ability of 5'-guanylyl imidodiphosphate to replace GTB in promoting the polymerization of microtubules in vitro.

Polymerization of microtubule requires the presence of GTP, and the tubulin-bound GTP is hydrolyzed during microtubule formation. However, it was found that an unhydrolyzable analog of GTP, 5′-guanylyl imidodiphosphate (Gpp(NH)p), was able to replace GTP. The hydrolysis of the terminal phosphate group of GTP, therefore, does not seem to be a prerequisite to $\text{in vitro}$ assembly of microtubules. The microtubules formed in the presence of Gpp(NH)p were indistinguishable from those formed in the presence of GTP under electron microscopy, but a remarkable decrease was noted in their sensitivity to depolymerization by calcium ions.