Augmin is a Ran-regulated spindle assembly factor

J Biol Chem. 2023 Jun;299(6):104736. doi: 10.1016/j.jbc.2023.104736. Epub 2023 Apr 21.


Mitotic spindles are composed of microtubules (MTs) that must nucleate at the right place and time. Ran regulates this process by directly controlling the release of spindle assembly factors (SAFs) from nucleocytoplasmic shuttle proteins importin-αβ and subsequently forms a biochemical gradient of SAFs localized around chromosomes. The majority of spindle MTs are generated by branching MT nucleation, which has been shown to require an eight-subunit protein complex known as augmin. In Xenopus laevis, Ran can control branching through a canonical SAF, TPX2, which is nonessential in Drosophila melanogaster embryos and HeLa cells. Thus, how Ran regulates branching MT nucleation when TPX2 is not required remains unknown. Here, we use in vitro pulldowns and total internal reflection fluorescence microscopy to show that augmin is a Ran-regulated SAF. We demonstrate that augmin directly interacts with both importin-α and importin-β through two nuclear localization sequences on the Haus8 subunit, which overlap with the MT-binding site. Moreover, we show that Ran controls localization of augmin to MTs in both Xenopus egg extract and in vitro. Our results demonstrate that RanGTP directly regulates augmin, which establishes a new way by which Ran controls branching MT nucleation and spindle assembly both in the absence and presence of TPX2.

Keywords: Ran pathway; TPX2; augmin; branching microtubule nucleation; importins; spindle assembly factor.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Cell Cycle Proteins / genetics
  • Cell Cycle Proteins / metabolism
  • Drosophila melanogaster
  • HeLa Cells
  • Humans
  • Microtubule-Associated Proteins* / genetics
  • Microtubule-Associated Proteins* / metabolism
  • Microtubules / metabolism
  • Multiprotein Complexes* / genetics
  • Multiprotein Complexes* / metabolism
  • Spindle Apparatus / metabolism
  • Xenopus Proteins* / genetics
  • Xenopus Proteins* / metabolism
  • Xenopus laevis / metabolism
  • alpha Karyopherins
  • beta Karyopherins
  • ran GTP-Binding Protein* / genetics
  • ran GTP-Binding Protein* / metabolism


  • Cell Cycle Proteins
  • Microtubule-Associated Proteins
  • ran GTP-Binding Protein
  • Xenopus Proteins
  • augmin protein complex, Xenopus
  • Multiprotein Complexes
  • TPX2 protein, Xenopus
  • alpha Karyopherins
  • beta Karyopherins