High copies of SUM1 enhance the stability of wild-type microtubules against adverse conditions in Saccharomyces cerevisiae

► Two budding yeast α-tubulin mutants show low levels of α-tubulin at 14 °C. ► Higher dosage of a transcriptional repressor Sum1p elevates their α-tubulin levels. ► Higher dosage of Sum1 stabilizes microtubules formed by improperly folded tubulin. ► Sum1's stabilizing effect extends to wild-type spindle microtubules as well. ► Sum1p is a dosage suppressor of microtubule instability against adverse conditions.