Transformation of taxol-stabilized microtubules into inverted tubulin tubules triggered by a tubulin conformation switch

Bundles of taxol-stabilized microtubules (MTs) - hollow tubules comprised of assembled αβ-tubulin heterodimers - spontaneously assemble above a critical concentration of tetravalent spermine and are stable over long times at room temperature. Here we report that at concentrations of spermine several-fold higher the MT bundles (B MT) quickly become unstable and undergo a shape transformation to bundles of inverted tubulin tubules (B ITT), the outside surface of which corresponds to the inner surface of the B MT tubules. Using transmission electron microscopy and synchrotron small-angle X-ray scattering, we quantitatively determined both the nature of the B MT -to-B ITT transformation pathway, which results from a spermine-triggered conformation switch from straight to curved in the constituent taxol-stabilized tubulin oligomers, and the structure of the B ITT phase, which is formed of tubules of helical tubulin oligomers. Inverted tubulin tubules provide a platform for studies requiring exposure and availability of the inside, luminal surface of MTs to MT-targeted drugs and MT-associated proteins.