STATHMIN - A TUBULIN-SEQUESTERING PROTEIN WHICH FORMS A TERNARY T2S COMPLEX WITH 2 TUBULIN MOLECULES

Stathmin is an important regulatory protein thought to control the dyn amics of microtubules through the cell cycle in a phosphorylation-depe ndent manner. Here we show that stathmin interacts with two molecules of dimeric alpha beta-tubulin to form a tight ternary T2S complex, sed imenting at 7.7 S. This complex appears in slow association-dissociati on equilibrium in the analytical ultracentrifuge. The T2S complex is f ormed under a variety of ionic conditions, either from GTP- or GDP-tub ulin or from the tubulin-colchicine complex. The S16/25/38/63E mutated stathmin in contrast is in rapid equilibrium with tubulin in the T2S complex. The T2S complex cannot polymerize in microtubules nor in ring oligomers. Stathmin acts as a pure tubulin-sequestering protein via f ormation of the T2S complex. It does not act directly on microtubule e nds to promote catastrophe nor enhance microtubule dynamics.