We have previously identified the opium alkaloid noscapine as a microtubule interacting
agent that binds stoichiometrically to tubulin and alters its conformation. Here we show that,
unlike many other microtubule inhibitors, noscapine does not significantly promote or inhibit
microtubule polymerization. Instead, it alters the steady-state dynamics of microtubule
assembly, primarily by increasing the amount of time that the microtubules spend in an
attenuated (pause) state. Further studies reveal that even at high concentrations, noscapine …