Importin-β1 functions as a chromatin sensor to position the contractile ring for cytokinesis

Area of Science:

• Cell Biology
• Molecular Biology
• Cancer Research

Background:

• Cytokinesis requires precise actomyosin ring positioning, regulated by chromatin-associated Ran-GTP.
• The precise mechanism by which Ran-GTP influences ring positioning, particularly involving importins, remains unclear.
• Importins are key nuclear transport proteins, but their role during anaphase and cytokinesis is not well understood.

Purpose of the Study:

• To investigate whether importins form a gradient in response to chromatin-associated Ran-GTP during cell division.
• To determine if importins regulate actomyosin ring assembly and ingression.
• To explore the potential of importin-mediated pathways as a therapeutic target in aneuploid cancer cells.

Main Methods:

• Endogenous tagging of importin-β1 with mNeonGreen in HeLa and HCT 116 cell lines.
• Live-cell imaging to observe importin-β1 localization during anaphase.
• Optogenetic tools to rapidly disrupt importin-β1 function and FLIM-FRET imaging to assess Ran-binding status.

Main Results:

• Importin-β1 transiently enriched between segregating chromosomes in anaphase HeLa cells, but not HCT 116 cells.
• Disruption of importin-β1 function impaired actomyosin ring ingression in HeLa cells, indicating its requirement.
• Ran-free importin-β1 enrichment at the equator was observed in HeLa cells, dependent on the chromatin-to-cytosol ratio.

Conclusions:

• Importin-β1 forms a Ran-GTP-dependent gradient that is crucial for actomyosin ring positioning in certain cell types.
• The chromatin-to-cytosol ratio significantly influences importin-β1 and anillin localization, impacting ring assembly.
• Aneuploid cancer cells' reliance on importin-mediated anillin recruitment presents a potential targetable vulnerability.