Guanasone 5'-(alpha,beta-methylene)triphosphate enhances specifically microtubule nucleation and stops the treadmill of tubulin protomers.

Substitution of pp(CH2)pT for GTP in the polymerization of microtubular protein results in a marked enhancement of both the rate and the extent of microtubule nucleation. Comparison of the kinetics of microtubule polymerization and pp(CH2)pG hydrolysis reveals that massive microtubule nucleation occurs in the absence of pp(CH2)pG hydrolysis. The shortest microtubule nuclei formed in the presence of pp(CH2)pG are curled ribbons on three protofilaments 0.15 to 0.2 micrometer long. No specific effect of pp(CH2)pG on microtubule propagation is observed. Nucleotide chase experiments suggest that the rings of microtubular protein present at 4 degrees C are not incorporated directly into the microtubule. Microtubules polymerized by pp(CH2)pG do not show the treadmill of tubulin protomers characteristic of the microtubules polymerized by GTP. Nucleotide analysis of microtubules polymerized by pp(CH2)pG and GTP reveals that 95% of the exchangeable nucleotide contained in the microtubules is p(CH2)pG and GDP, respectively. pp(CH2)pG blocks the treadmill of tubulin protomers from microtubules assembled by GTP by suppressing depolymerization at the depolymerization end of the microtubule. On the other hand, GTP promotes the treadmill of tubulin from microtubules assembled by pp(CH2)pG by reactivating the depolymerization end of the microtubule.