Podophyllotoxin poisoning of microtubules at steady-state: effect of substoichiometric and superstoichiometric concentrations of drug.

Depolymerization kinetics of microtubules assembled to steady-state by podophyllotoxin treatment show a dose-dependent effect of this mitotic poison on the net rate of microtubule disassembly. Pulse-chase experiments with microtubules at steady-state indicate that the depolymerization effect induced by superstoichiometric concentrations of podophyllotoxin relative to tubulin is polar and time-dependent, i.e. the rate of tubulin loss decreases along with the time of treatment in the presence of the drug. Under these conditions the rate of microtubule disassembly is much faster than one could expect from a unique effect of drug-tubulin complex on the microtubule assembly end. Podophyllotoxin-tubulin complex is not able to induce active depolymerization of microtubules, while free podophyllotoxin is. These results are consistent with the hypothesis that this drug acts on the microtubule assembly-disassembly process by two different mechanisms: 1) as a free drug, it actively promotes polar depolymerization of microtubules, and 2) as a drug-tubulin complex, it retards the addition of subunits into the microtubule ends.