AN ALPHA-TUBULIN MUTATION SUPPRESSES NUCLEAR MIGRATION MUTATIONS IN ASPERGILLUS-NIDULANS

Microtubules and cytoplasmic dynein, a microtubule-dependent motor, ar e required for nuclei to move along the hyphae of filamentous fungi. N uclear migration in Aspergillus nidulans is blocked by heat-sensitive (hs(-)) mutations in the nudA gene, which encodes dynein heavy chain, and the nudF gene, which encodes a G protein beta-subunit-like protein . Hs(-) mutations in the nudC and nudG genes also prevent nuclear migr ation. We have isolated extragenic suppressor mutations that reverse t he hs(-) phenotypes caused by these mutations. Here we show that one n udF suppressor also suppresses hs(-) mutations in nudA, nudC, and nudG and deletions in nudA and nudF. This suppressor mutation is in the tu bA alpha tubulin gene, and its characteristics suggest that it destabi lizes microtubules. The mutation alters microtubule staining and confe rs sensitivity to cold and benomyl, two treatments that destabilize mi crotubules. Treatment with low concentrations of benomyl also suppress es the hs(-) nudA, nudC, nudF, and nudG mutations and the nudA and nud F deletions. Suppression of the hs(-) nudA mutation and the nudA delet ion is especially interesting because these strains lack active dynein heavy chain. Together, these results suggest that microtubule destabi lization allows nuclei to migrate even in the absence of cyto plasmic dynein motor function.