Tubulin

Tubulin is the protein building material of microtubules. Alpha- and beta-tubulin form a heterodimer; repeated addition of these units produces polarized protofilaments assembled into a hollow tube. The two ends of a microtubule differ in organization and growth behaviour. GTP binding and hydrolysis by beta-tubulin couple assembly history to stability. ^[ref]

This explains how the same material supports both persistent cellular architecture and rapid rearrangement of a spindle apparatus. Losing a stabilizing end region can trigger rapid shortening even while soluble tubulin remains available. Fluorescence measurements of tubulin amount alone therefore do not determine how much is assembled, the direction of growth or the lifetime of an individual microtubule.