Tubulin‐Dolastatin rings undergo rouleau aggregation while cryptophycin‐tubulin rings are non‐interacting

We combine fluorescence correlation spectroscopy, small‐angle neutron scattering, and modeling to study interactions of tubulin and either of two cytotoxic peptides –dolastatin 10 and cryptophycin 1‐ in solution. These peptides inhibit tubulin polymerization into microtubules, and instead induce the formation of predominantly single‐walled tubulin rings. We find that the cryptophycin‐tubulin rings appear rigid, composed of 8 tubulin dimers, and stable upon dilution to nanomolar concentration. In contrast, the dolastatin‐tubulin rings are composed of 14 dimers and appear to disassemble upon to dilution. Further, at micromolar concentrations, dolastatin‐tubulin samples show additional aggregation of the primary rings, resulting in large structures that settle to the bottom of their container under the effect of gravity. Further detailed analysis of the SANS profiles indicate, remarkably, that the dolastatin‐tubulin aggregates are ordered structures, most likely columns of rings. These results show the advantages of combining SANS and FCS for studying closed ring polymers and their assemblies.