Microtubule nucleation in mitosis by a RanGTP-dependent protein complex

Curr Biol. 2015 Jan 19;25(2):131-140. doi: 10.1016/j.cub.2014.11.025. Epub 2014 Dec 18.


Background: The γ-tubulin ring complex (γTuRC) is a multisubunit complex responsible for microtubule (MT) nucleation in eukaryotic cells. During mitosis, its spatial and temporal regulation promotes MT nucleation through different pathways. One of them is triggered around the chromosomes by RanGTP. Chromosomal MTs are essential for functional spindle assembly, but the mechanism by which RanGTP activates MT nucleation has not yet been resolved.

Results: We used a combination of Xenopus egg extracts and in vitro experiments to dissect the mechanism by which RanGTP triggers MT nucleation. In egg extracts, NEDD1-coated beads promote MT nucleation only in the presence of RanGTP. We show that RanGTP promotes a direct interaction between one of its targets, TPX2, and XRHAMM that defines a specific γTuRC subcomplex. Through depletion/add-back experiments using mutant forms of TPX2 and NEDD1, we show that the activation of MT nucleation by RanGTP requires both NEDD1 phosphorylation on S405 by the TPX2-activated Aurora A and the recruitment of the complex through a TPX2-dependent mechanism.

Conclusions: The XRHAMM-γTuRC complex is the target for activation by RanGTP that promotes an interaction between TPX2 and XRHAMM. The resulting TPX2-RHAMM-γTuRC supracomplex fulfills the two essential requirements for the activation of MT nucleation by RanGTP: NEDD1 phosphorylation on S405 by the TPX2-activated Aurora A and the recruitment of the complex onto a TPX2-dependent scaffold. Our data identify TPX2 as the only direct RanGTP target and NEDD1 as the only Aurora A substrate essential for the activation of the RanGTP-dependent MT nucleation pathway.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Cell Cycle Proteins / genetics*
  • Cell Cycle Proteins / metabolism
  • Microtubule-Associated Proteins / genetics*
  • Microtubule-Associated Proteins / metabolism
  • Microtubule-Organizing Center / metabolism*
  • Microtubules / metabolism*
  • Mitosis
  • Nuclear Proteins / genetics*
  • Nuclear Proteins / metabolism
  • Ovum / metabolism
  • Phosphoproteins / genetics*
  • Phosphoproteins / metabolism
  • Xenopus Proteins / genetics*
  • Xenopus Proteins / metabolism
  • Xenopus laevis / genetics*
  • Xenopus laevis / metabolism
  • ran GTP-Binding Protein / genetics*
  • ran GTP-Binding Protein / metabolism


  • Cell Cycle Proteins
  • Microtubule-Associated Proteins
  • NEDD1 protein, Xenopus
  • Nuclear Proteins
  • Phosphoproteins
  • TPX2 protein, Xenopus
  • Xenopus Proteins
  • ran GTP-Binding Protein