Taxol binds differentially to flagellar outer doublets and their reassembled microtubules

Taxol induces the in vitro assembly of calcium stable microtubules from flagellar tubulin solubilized from sea urchin (Strongylocentrotus purpuratus) sperm tail outer doublets by sonication. Assembly occurs in the presence or absence of exogenous GTP. The drug (10 microM) reduces the critical concentration of protein required for assembly to less than or equal to 0.04 mg/ml. 3H-Taxol binds specifically to both isolated flagellar outer doublets and to reassembled microtubules with calculated maximal binding ratios of 0.25 and 1.32 moles taxol/mole polymerized flagellar tubulin dimer, respectively. We suggest that the discrepancy in maximal binding ratios may result from the presence of an endogenous molecule(s) along the surface of outer doublet microtubules that restricts taxol binding to that structure.