Microtubules are dynamic polymers that interconvert between phases of growth and shrinkage, yet they somehow provide lasting structural stability to cells. Growth involves hydrolysis of GTP-tubulin to GDP-tubulin, which releases energy that is stored within the microtubule lattice and destabilizes it; a GTP cap at microtubule ends is thought to prevent GDP subunits from rapidly dissociating and causing catastrophe. We show that GDP-tubulin, usually considered as an inactive tubulin species, can itself assemble into microtubules preferentially at the minus end, and promote persistent growth. We characterized by mass spectrometry-based proteomics the protein content of tubulin purified from bovine brain (Total) and of microtubules grown from stable GMPCPP seeds in the presence of either GDP-tubulin or GTP-tubulin.