IDENTIFICATION OF A DISTINCT CLASS OF VINBLASTINE BINDING-SITES ON MICROTUBULES

Vinblastine, at concentrations above approximately 1 to 2 .mu.M, causes depolymerization of steady-state bovine brain microtubules in vitro by a fraying of microtubule ends into protofilament-like spirals. Microtubule depolymerization is associated with the binding of vinblastine in approximately mola stoiochiometry to tubulin in mcirotubules with apparent low affinity, as determiend by binding experiments with radiolabeled vinblastine and by the ability of vinblastine to inhibit DEAE-dextran decoration of microtubule surfaces. Our data suggest that depolymerization occurs by a propagated mechanism, initially involving binding of vinblastine to a limited number of available sites on microtubule surfaces. This appears to cause lossening of protofilament associations which results in the exposure of new vinblastine-binding sites. Additional vinblastine binding in turn results in further loosening of protofilament associations. Such loosening, when it occurs at microtubule ends, results in protofilament-like splaying and end-wisedepolymerization. Microtubule depolymerization appears mechanistically distinct from inhibiton of microtubule polymerization by the drug, which is associated with the binding of vinblastine to small numbers of high-affinity binding sites on tubulin at one or both microtubule ends.