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Revertants of a Chinese Hamster Ovary Cell Mutant with an Altered β-Tubulin: Evidence that the Altered Tubulin Confers Both Colcemid Resistance and Temperature Sensitivity on the Cell

We recently described the isolation of a mutant Chinese hamster ovary cell (Cmd 4) resistant to the cytotoxic effects of colcemid (Cabral et al., Cell 20:29-36, 1980). This mutant carries an altered β-tubulin but still grows normally at 37°C. In the present study we found that Cmd 4 is temperature sensitive for growth at 40.3°C. A class of revertants selected for temperature resistance had simultaneously lost colcemid resistance and the altered β-tubulin. In addition, we isolated a temperature-resistant revertant which carries a further alteration in the mutant β-tubulin polypeptide. This second alteration appears to make the mutant β-tubulin incompetent to assemble into microtubules, resulting in a strain which is again colcemid sensitive. These revertant cell lines provide strong evidence that a mutation in β-tubulin can confer both colcemid resistance and temperature sensitivity on a mammalian cell line. Cellular microtubules studied by indirect immunofluorescence in both mutant and revertant cell lines had an apparently normal distribution at permissive and nonpermissive temperatures, yet mitosis appears to be abnormal in the mutant cell line. We conclude from these studies that incorporation of the altered β-tubulin into microtubules does not affect their distribution but may affect their function during mitosis.