Unusual antimicrotubule activity of the antifungal agent spongistatin 1

Spongistatin 1, a macrocyclic lactone from the marine sponge Hyrtios erecta , has broad-spectrum antifungal activity. Since this compound is a potent a ntimicrotubule agent in mammalian cells, we examined its effects on the fil amentous fungus Aspergillus nidulans to determine if its antifungal effects are due to antimicrotubule activity, At 25 mu g/ml (twice the MIC), spongi statin 1 caused a greater-than-twofold elevation of the chromosome and spin dle mitotic indices. Immunofluorescence microscopy revealed that mitotic sp indles were smaller and shorter than in control germlings, However, late-an aphase and telophase nuclei were seen occasionally, and this suggests that the spindles are capable of segregating chromosomes. Spongistatin 1 had mor e dramatic effects on cytoplasmic microtubules, At 30 min after initiation of treatment, 83% of germlings contained fragmented microtubules and after 2 h of treatment, microtubules had disappeared completely from 82% of germl ings. In contrast, microtubules disappeared rapidly and completely from ger mlings treated with benomyl, We conclude that spongistatin 1 has antimicrot ubule activity in A. nidulans and that its mechanism of action may involve a novel microtubule-severing activity.