Pironetin reacts covalently with cysteine-316 of α-tubulin to destabilize microtubule

Molecules that alter the normal dynamics of microtubule assembly and disassembly include many anticancer drugs in clinical use. So far all such therapeutics target beta-tubulin, and structural biology has explained the basis of their action and permitted design of new drugs. However, by shifting the profile of beta-tubulin isoforms, cancer cells become resistant to treatment. Compounds that bind to alpha-tubulin are less well characterized and unexploited. The natural product pironetin is known to bind to alpha-tubulin and is a potent inhibitor of microtubule polymerization. Previous reports had identified that pironetin reacts with lysine-352 residue however analogues designed on this model had much lower potency, which was difficult to explain, hindering further development. We report crystallographic and mass spectrometric data that reveal that pironetin forms a covalent bond to cysteine-316 in alpha-tubulin via a Michael addition reaction. These data provide a basis for the rational design of alpha-tubulin targeting chemotherapeutics.