Drosophila dTBCE recruits tubulin around chromatin to promote mitotic spindle assembly

Proper assembly of mitotic spindles requires microtubule nucleation at centrosomes but also around chromatin. In this study, we reveal a novel mechanism by which an enrichment of tubulin in the nuclear space following nuclear envelope breakdown promotes nucleation of spindle microtubules. This event mediated by the tubulin-specific chaperone dTBCE, depends on its tubulin binding CAP-Gly motif and is regulated by Ran. Live imaging, proteomic and biochemical analyses suggest that dTBCE is enriched in the nucleus at nuclear envelope breakdown and interacts with nuclear pore proteins and the Ran machinery to create an environment that facilitates subsequent tubulin enrichment. We propose that dTBCE-dependent increase in tubulin concentration in the nuclear space is an important mechanism for microtubule nucleation in organisms where compartmentalization prevents free diffusion of tubulin.