Stathmin Strongly Increases the Minus End Catastrophe Frequency and Induces Rapid Treadmilling of Bovine Brain Microtubules at Steady State in Vitro*□S

Stathmin is a ubiquitousmicrotubule destabilizing protein that is believed to play an important role linking cell signaling to the reg-ulation of microtubule dynamics. Here we show that stathmin strongly destabilizes microtubule minus ends in vitro at steady state, conditions in which the soluble tubulin and microtubule lev-els remain constant. Stathmin increased theminus end catastrophe frequency13-fold at a stathmin:tubulin molar ratio of 1:5. Stath-min steady-state catastrophe-promoting activity was considerably stronger at theminus ends than at the plus ends. Consistent with its ability to destabilize minus ends, stathmin strongly increased the treadmilling rate of bovine brainmicrotubules. By immunofluores-cence microscopy, we also found that stathmin binds to purified microtubules along their lengths in vitro. Co-sedimentation of puri-fied microtubules polymerized in the presence of a 1:5 initial molar ratio of stathmin to tubulin yielded a binding stoichiometry of 1mo