Hydrolysis and polymerisation

GTP bound b-tubulin (T form) undergoes slow hydrolysis to GDP so that GDP bound b-tubulin (D form) is the predominant form in the lattice. At the plus end since polymerisation is much faster than hydrolysis polymerising microtubules have a GTP cap. By contrast polymerisation is slower than hydrolysis at the minus end so the minus end is always in the D form.