Transportin acts to regulate mitotic assembly events by target binding rather than Ran sequestration

Mol Biol Cell. 2014 Apr;25(7):992-1009. doi: 10.1091/mbc.E13-08-0506. Epub 2014 Jan 29.


The nuclear import receptors importin β and transportin play a different role in mitosis: both act phenotypically as spatial regulators to ensure that mitotic spindle, nuclear membrane, and nuclear pore assembly occur exclusively around chromatin. Importin β is known to act by repressing assembly factors in regions distant from chromatin, whereas RanGTP produced on chromatin frees factors from importin β for localized assembly. The mechanism of transportin regulation was unknown. Diametrically opposed models for transportin action are as follows: 1) indirect action by RanGTP sequestration, thus down-regulating release of assembly factors from importin β, and 2) direct action by transportin binding and inhibiting assembly factors. Experiments in Xenopus assembly extracts with M9M, a superaffinity nuclear localization sequence that displaces cargoes bound by transportin, or TLB, a mutant transportin that can bind cargo and RanGTP simultaneously, support direct inhibition. Consistently, simple addition of M9M to mitotic cytosol induces microtubule aster assembly. ELYS and the nucleoporin 107-160 complex, components of mitotic kinetochores and nuclear pores, are blocked from binding to kinetochores in vitro by transportin, a block reversible by M9M. In vivo, 30% of M9M-transfected cells have spindle/cytokinesis defects. We conclude that the cell contains importin β and transportin "global positioning system"or "GPS" pathways that are mechanistically parallel.

Publication types

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

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Cell Extracts
  • Chromatin / metabolism
  • Cytokinesis
  • Drosophila Proteins / chemistry
  • Drosophila Proteins / metabolism*
  • HeLa Cells
  • Humans
  • Interphase
  • Karyopherins / chemistry
  • Karyopherins / metabolism*
  • Kinetochores / metabolism
  • Mitosis*
  • Models, Biological
  • Molecular Sequence Data
  • Mutant Proteins / metabolism
  • Nuclear Envelope / metabolism
  • Nuclear Localization Signals / metabolism
  • Nuclear Pore Complex Proteins / metabolism
  • Peptides / chemistry
  • Peptides / metabolism
  • Protein Binding
  • Spindle Apparatus / metabolism
  • Xenopus
  • Xenopus Proteins / metabolism*
  • beta Karyopherins / metabolism
  • ran GTP-Binding Protein / metabolism*


  • Cell Extracts
  • Chromatin
  • Drosophila Proteins
  • Karyopherins
  • Mutant Proteins
  • Nuclear Localization Signals
  • Nuclear Pore Complex Proteins
  • Peptides
  • Tnpo protein, Drosophila
  • Xenopus Proteins
  • beta Karyopherins
  • ran GTP-Binding Protein