Amino acid substitutions at proline 220 of beta-tubulin confer resistance to paclitaxel and colcemid.

Chinese hamster ovary cells selected for resistance to paclitaxel have a high incidence of mutations affecting L215, L217, and L228 in the H6/H7 loop region of beta1-tubulin. To determine whether other mutations in this loop are also capable of conferring resistance to drugs that affect microtubule assembly, saturation mutagenesis of the highly conserved P220 codon in beta1-tubulin cDNA was carried out. Transfection of a mixed pool of plasmids encoding all possible amino acid substitutions at P220 followed by selection in paclitaxel produced cell lines containing P220L and P220V substitutions. Similar selections in colcemid, on the other hand, yielded cell lines with P220C, P220S, and P220T substitutions. Site-directed mutagenesis and retransfection confirmed that these mutations were responsible for drug resistance. Expression of tubulin containing the P220L and P220V mutations reduced microtubule assembly, conferred resistance to paclitaxel and epothilone A, but increased sensitivity to colcemid and vinblastine. In contrast, tubulin with the P220C, P220S, and P220T mutations increased microtubule assembly, conferred resistance to colcemid and vinblastine, but increased sensitivity to paclitaxel and epothilone A. The results are consistent with molecular modeling studies and support a drug resistance mechanism based on changes in microtubule assembly that counteract the effects of drug treatment. These studies show for the first time that different substitutions at the same amino acid residue in beta1-tubulin can confer cellular resistance to either microtubule-stabilizing or microtubule-destabilizing drugs.