LAUSR

Summary Microtubules are cytoskeletal filaments of eukaryotic cells made of αβ-tubulin heterodimers. Structural studies of non-microtubular tubulin rely mainly on molecules that prevent its self-assembly and are used as crystallization chaperones. Here we identified artificial proteins from an αRep library that are specific to α-tubulin. Turbidity experiments indicate that these αReps impede microtubule assembly in a dose-dependent manner and total internal reflection fluorescence microscopy further shows that they specifically block growth at the microtubule (−) end. Structural data indicate that they do so by targeting the α-tubulin longitudinal surface. Interestingly, in one of the complexes studied, the α subunit is in a conformation that is intermediate between the ones most commonly observed in X-ray structures of tubulin and those seen in the microtubule, emphasizing the plasticity of tubulin. These α-tubulin-specific αReps broaden the range of tools available for the mechanistic study of microtubule dynamics and its regulation.