Abstract
Microtubule, composed of αβ-tubulin heterodimers, remains as one of the most popular anticancer targets for decades. To date, anti-microtubule drugs mainly target β-tubulin to inhibit microtubule dynamic instability (MDI) while agents binding to α-tubulin are less well characterized and also the molecular mechanism of MDI is far from being articulated. Cevipabulin, an oral microtubule-active antitumor clinical candidate, is widely accepted as a microtubule stabilizing agent (MSA) but binds to the microtubule-destabilization vinblastine site on β-tubulin and this unusual phenomenon has so far failed to be explained. Our X-ray crystallography study reveals that, in addition binding to the vinblastine site, cevipabulin also binds to a novel site on α-tubulin (named the seventh site) which located at the region spatially corresponding to the vinblastine site on β-tubulin. Interestingly, cevipabulin exhibits two unique site-dependent functions. Cevipabulin binding to the seventh site promotes tubulin degradation through interaction of the non-exchengeable GTP to reduce tubulin stability. Cevipabulin binding to the vinblastine site enhances longitudinal interactions but inhibits lateral interactions of tubulins, thus inducing tubulin protofilament polymerization (but not microtubule polymerization like MSAs), and then tangling into irregular tubulin aggregates. Importantly, the tubulin-cevipabulin structure is an intermediate between “bent” and “straight” tubulins and the involved bent-to-straight conformation change will be helpful to fully understand the molecular mechanism of tubulin assembly. Our findings confirm cevipabulin is not an MSA and shed light on the development of a new generation of anti-microtubule drugs targeting the novel site on α-tubulin and also provide new insights into MDI.
Competing Interest Statement
The authors have declared no competing interest.
