Stathmin: A Tubulin-Sequestering Protein Which Forms a Ternary T2S Complex with Two Tubulin Molecules

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Abstract
Stathmin is an important regulatory protein thought to control the dynamics of microtubules through the cell cycle in a phosphorylation-dependent manner. Here we show that stathmin interacts with two molecules of dimeric αβ−tubulin to form a tight ternary T2S complex, sedimenting at 7.7 S. This complex appears in slow association−dissociation equilibrium in the analytical ultracentrifuge. The T2S complex is formed under a variety of ionic conditions, either from GTP− or GDP−tubulin 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 formation of the T2S complex. It does not act directly on microtubule ends to promote catastrophe nor enhance microtubule dynamics.