Microtubules, composed of αβ-tubulin heterodimers, are crucial targets for chemotherapeutic agents and possess eight binding sites. Our previous study identified cevipabulin as the only one agent capable of simultaneously binding to two different sites (Vinblastine site and The Seventh site). Binding to The Seventh site by cevipabulin induces tubulin degradation. This study aimed to investigate whether it is binding to the Vinblastine site and The Seventh site exhibited an interactive cellular effect. Surprisingly, we discovered that cevipabulin induced abnormal tubulin protofilaments polymerization, a previously undefined tubulin morphology, and we proved it was an interactive effect of Cevipabulin's binding to both Vinblastine site and The Seventh site. Immunofluorescence and transmission electron microscopy confirmed cevipabulin induced the formation of linear tubulin protofilaments and their subsequent aggregation into irregular tubulin aggregates. Competition binding assays and the αY224G mutation revealed that binding of cevipabulin to both sites was necessary for the tubulin protofilaments polymerization effect. Moreover, we found that co-treatment with a microtubule stabilization agent binding the Vinblastine site and a microtubule destabilization agent binding at the intra-dimer interface of tubulin could also induce similar tubulin protofilaments polymerization. We proposed a mechanism where a microtubule stabilization agent on the Vinblastine site enhances longitudinal interactions between tubulin dimers, while, a microtubule destabilization agent binding at the intra-dimer interface prevents the adoption of a straight conformation of the tubulin dimer and disrupts lateral interactions between tubulins, consequently leading to tubulin protofilaments polymerization. This study reported a new inhibitor-induced-tubulin-morphology-change and would provide insight into tubulin dynamic instability and also guide further study of cevipabulin.
Keywords: The Seventh site; Vinblastine site; cevipbulin; microtubule; tubulin protofilaments polymerization.
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