Op18/stathmin caps a kinked protofilament-like tubulin tetramer

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Abstract
Oncoprotein 18/stathmin (Op18), a regulator of microtubule dynamics, was recombinantly expressed and its structure and function analysed. We report that Op18 by itself can fold into a flexible and extended α‐helix, which is in equilibrium with a less ordered structure. In complex with tubulin, however, all except the last seven C‐terminal residues of Op18 are tightly bound to tubulin. Digital image analysis of Op18:tubulin electron micrographs revealed that the complex consists of two longitudinally aligned α/β‐tubulin heterodimers. The appearance of the complex was that of a kinked protofilament‐like structure with a flat and a ribbed side. Deletion mapping of Op18 further demonstrated that (i) the function of the N‐terminal part of the molecule is to ‘cap’ tubulin subunits to ensure the specificity of the complex and (ii) the complete C‐terminal α‐helical domain of Op18 is necessary and sufficient for stable Op18:tubulin complex formation. Together, our results suggest that besides sequestering tubulin, the structural features of Op18 enable the protein specifically to recognize microtubule ends to trigger catastrophes.