[3H]Diazonamide A ([3H]DZA), prepared from the natural product isolated from Diazona angulata, bound to tubulin in larger aberrant assembly products (>500 kDa by sizing HPLC) but not to the αβ-tubulin heterodimer. The binding reaction was rapid, but stoichiometry was low. Stoichiometry was enhanced up to 8-fold by preincubating the tubulin in the reaction mixture prior to adding the [3H]DZA. Although Mg2+ did not affect binding stoichiometry, the cation markedly increased the number of tubulin rings (diameter about 50 nm) observed by negative stain electron microscopy. Bound [3H]DZA did not dissociate from the tubulin oligomers despite extensive column chromatography but did dissociate in the presence of 8 M urea. With preincubated tubulin, a superstoichiometric amount of [3H]DZA appeared to bind to the tubulin oligomeric structures, consistent with observations that neither nonradiolabeled DZA nor DZA analogues inhibited binding of [3H]DZA to the tubulin oligomers. Only weak inhibition of binding was observed with multiple antimitotic compounds. In particular, no inhibition occurred with vinblastine, and the best inhibitors of those examined were dolastatin 10 and cryptophycin 1. We compared the aberrant assembly reaction induced by DZA to those induced by other antimitotic peptides and depsipeptides, in particular dolastatin 10, cryptophycin 1, and hemiasterlin, but the results obtained varied considerably in terms of requirements for maximal reactions, polymer morphology, and inhibitory effects observed with antimitotic compounds.
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