Tubulin mutations and Taxol/epothilone dependence. Three mutations (βGln²⁹²Glu, βVal⁶⁰Phe and αLeu¹⁹⁵Met) in tubulin are present in a highly epothilone B-resistant/dependent A549 cell line. βVal60 has a critical role in the drug dependence phenotype of these cells. The structural model of a tubulin heterodimer is viewed from the inside ( left ) and from the outside ( right ) with β-tubulin at the top. The locations of the mutations are indicated with backbone atoms ( yellow spheres ). A molecule of GTP ( red ) and a molecule of GDP ( red ) bind to α-tubulin and β-tubulin, respectively. Epothilone A ( green ), a potentially important antitumor drug, interacts with β-tubulin. For details, see 987 in this issue.