HURP facilitates spindle assembly by stabilizing microtubules and working synergistically with TPX2
Venecia A. Valdez, Meisheng Ma, Bernardo Gouveia, Rui Zhang, Sabine Petry
Abstract
In vertebrate spindles, most microtubules are formed via branching microtubule nucleation, whereby microtubules nucleate along the side of pre-existing microtubules. Hepatoma up-regulated protein (HURP) is a microtubule-associated protein that has been implicated in spindle assembly, but its mode of action is yet to be defined. In this study, we show that HURP is necessary for RanGTP-induced branching microtubule nucleation in Xenopus egg extract. Specifically, HURP stabilizes the microtubule lattice to promote microtubule formation from γ-TuRC. This function is shifted to promote branching microtubule nucleation through enhanced localization to TPX2 condensates, which form the core of the branch site on microtubules. Lastly, we provide a high-resolution cryo-EM structure of HURP on the microtubule, revealing how HURP binding stabilizes the microtubule lattice. We propose a model in which HURP stabilizes microtubules during their formation, and TPX2 preferentially enriches HURP to microtubules to promote branching microtubule nucleation and thus spindle assembly. Many proteins work together to promote spindle microtubule formation during cell division. Here, the authors find the protein HURP to be essential for microtubule formation by stabilizing the microtubule lattice and working synergistically with the spindle assembly factor TPX2.