Making mature attachments

During mitosis, the absence of Ran-GTPase pathway components at kinetochores leads to aberrant attachment of kinetochore fibers, disordered mitotic spindle assembly, and haphazard chromosome segregation at anaphase. To ensure that Ran complex proteins get where they need to be, Crm1—the Ran-GTP–binding nuclear export receptor—goes moonlighting at kinetochores after nuclear envelope breakdown, according to Alexei Arnaoutov, Mary Dasso (National Institute of Child Health and Human Development, Bethesda, Maryland), and colleagues.

Crm1 binds to its export cargo in complexes that also contain Ran-GTP. Blocking the formation of these Crm1-cargo-Ran-GTP complexes, either by preventing cargo binding or by starving cells of Ran-GTP, resulted in no Ran-GAP1/Ran-BP2 kinetochore recruitment, which was bad news for dividing cells. Crm1-blocked cells rarely established mature kinetochore fiber attachments between kinetochores and spindle poles. The microtubules within kinetochore...