tubulin molecule C with bound GTP GTP tubulin molecules a protofilaments containing GDP add to end of microtubule tubulin are unstable and peel away from the microtubule wall addition proceeds faster GDP tubulin is released than GTP hydrolysis to the cytosol 翻鞲、` tubulin molecule with bound GDP GTP cap GROWING MICROTUBULE SHRINKING MICROTUBULE