Microtubules are tubes of about 25 nm in diameter that are critically involved in a variety of cellular functions including motility, compartmentalization, and division. They are considered as pseudo-helical polymers whose constituent ab-tubulin heterodimers share lateral homotypic interactions, except at one unique region called the seam. Here, we used a segmented sub-tomogram averaging strategy to reassess this paradigm and analyze the organization of the ab-tubulin heterodimers in microtubules assembled from purified porcine brain tubulin in the presence of GTP and GMPCPP, and in Xenopus egg cytoplasmic extracts. We find that in almost all conditions, microtubules incorporate variable protofilament and/or tubulin subunit helical-start numbers, as well as variable numbers of seams. Strikingly, the seam number and location vary along individual microtubules, generating holes of one to a few subunits in size within their lattices. Together, our results reveal that the formation of mixed and discontinuous microtubule lattices is an intrinsic property of tubulin that requires the formation of unique lateral interactions without longitudinal ones. They further suggest that microtubule assembly is tightly regulated in a cytoplasmic environment.

Paclitaxel (Taxol®) is a taxane and a first-line chemotherapeutic drug that stabilizes microtubules. While the interaction of paclitaxel with microtubules is well described, the current lack of high-resolution structural information on a tubulin-taxane complex precludes a comprehensive description of the binding determinants that affect the drug's mechanism of action. Here, we solved the crystal structure of the core baccatin III moiety of paclitaxel lacking the C13 side chain in complex with tubulin at 1.9 Å resolution. Based on this information, we engineered two tailor-made taxanes with modified C13 side chains, solved their crystal structures in complex with tubulin, and analyzed their effects along with those of paclitaxel, docetaxel, and baccatin III on the microtubule lattice by X-ray fiber diffraction. We then compared high-resolution structures of ligand-bound tubulin and microtubule complexes with apo forms and used molecular dynamics simulations to understand the consequences of taxane binding to tubulin as well as to simplified protofilament and microtubule-lattice models. Our combined approach sheds light on three mechanistic questions. Firstly, taxanes bind better to microtubules as compared to unassembled tubulin due to a dual structural mechanism: Tubulin assembly is linked to a conformational reorganization of the bM loop, which otherwise occludes ligand access to the taxane site, while the bulky C13 side chains preferentially recognize the microtubule-assembled over the unassembled conformational state of tubulin. Second, the occupancy of the taxane site by a ligand has no influence on the straightness of tubulin protofilaments. Finally, the longitudinal expansion of the microtubule lattices arises from the accommodation of the taxane core within the site, a process that is, however, not related to the microtubule stabilization mechanism of taxanes, as all analogs tested expand the microtubule lattice, despite the fact that one of them, Baccatin III, is biochemically inactive. In conclusion, our combined experimental and computational approach allowed us to describe the tubulin-taxane interaction in atomic detail and assess the structural determinants for binding.

Infectious diseases caused by apicomplexan parasites remain a global public health threat. The presence of multiple ligand‐binding sites in tubulin makes this protein an attractive target for anti‐parasite drug discovery. However, despite remarkable successes as anti‐cancer agents, the rational development of protozoan parasite‐specific tubulin drugs has been hindered by a lack of structural and biochemical information on protozoan tubulins. Here, we present atomic structures for a protozoan tubulin and microtubule and delineate the architectures of apicomplexan tubulin drug‐binding sites. Based on this information, we rationally designed the parasite‐specific tubulin inhibitor parabulin and show that it inhibits growth of parasites while displaying no effects on human cells. Our work presents for the first time the rational design of a species‐specific tubulin drug providing a framework to exploit structural differences between human and protozoa tubulin variants enabling the development of much‐needed, novel parasite inhibitors.

The binding and release of ligands from their protein targets is central to fundamental biological processes as well as to drug discovery. Photopharmacology introduces chemical triggers that allow the changing of ligand affinities and thus biological activity by light. Insight into the molecular mechanisms of photopharmacology is largely missing because the relevant transitions during the light-triggered reaction cannot be resolved by conventional structural biology. Using time-resolved serial crystallography at a synchrotron and X-ray free-electron laser, we capture the release of the anti-cancer compound azo-combretastatin A4 and the resulting conformational changes in tubulin. Nine structural snapshots from 1 ns to 100 ms complemented by simulations show how cis-to-trans isomerization of the azobenzene bond leads to a switch in ligand affinity, opening of an exit channel, and collapse of the binding pocket upon ligand release. The resulting global backbone rearrangements are related to the action mechanism of microtubule-destabilizing drugs.

Microtubules are polymers assembled from tubulin α-β-heterodimers. They typically display lateral α-α and β-β-homotypic interactions, except at one region, called the seam, where heterotypic α-β and β-α interactions occur. Here, we decorated microtubules assembled in vitro or in cytoplasmic Xenopus egg extracts with kinesin-motor domains, and analyzed their lattice organization using dual axis cryo-electron tomography followed by segmented sub-tomogram averaging. In both conditions, microtubules incorporated variable protofilament and/or tubulin subunit helix start numbers. While microtubules assembled in vitro displayed variable numbers of seams, those assembled in extracts displayed preferentially one seam. The seam location varied within individual microtubules implying the presence of lattice holes. Thus, the formation of discontinuous microtubule lattices is an intrinsic property of tubulin assembly, a process that is controlled in cells.