Importin β, once thought to be exclusively a nuclear transport receptor, is emerging as a global regulator of diverse cellular functions. Importin β acts positively in multiple interphase roles: in nuclear import, as a chaperone for highly charged nuclear proteins, and as a potential motor adaptor for movement along microtubules. In contrast, importin β plays a negative regulatory role in mitotic spindle assembly, centrosome dynamics, nuclear membrane formation, and nuclear pore assembly. In most of these, importin β is counteracted by its regulator, Ran-GTP. In light of this, the recent discovery of Ran's involvement in spindle checkpoint control suggested a potential new arena for importin β action, although it is also possible that one of importin β's relatives, the karyopherin family of proteins, manages this checkpoint. Lastly, importin β plays a role in transducing damage signals from the axons of injured neurons back to the cell body.
Bibliographical noteFunding Information:
The current papers concerning importin β and the karyopherins number close to a thousand; the authors regret that only a fraction could be cited. The authors were supported by National Institutes of Health grant R01-GM033279 to D.J.F. and a United States-Israel Bi-national Sciences Foundation Grant 200295 to D.F. and Michael Elbaum (Weizmann Institute, Rehovot, Israel). The authors thank Rene Chan, Valerie Delmar, and Corine Lau for help in preparing the manuscript.