NUCLEAR IMPORT Losing Importin Functions Under Stress

Proteins bearing a nuclear localization sequence (NLS) are transported into the nucleus in a complex with importin α and importin β. The small guanosine triphosphatase (GTPase) Ran is found in the nucleus, mostly in its GTP-bound form, and binds as Ran-GTP to importin β, causing dissociation of the complex and release of the protein cargo into the nucleus. Importin α and β then return to the cytoplasm, ready for the next load of cargo. Miyamoto et al. used confocal microscopy to show that subjecting HeLa cells to any of various stresses--including UV irradiation, treatment with H₂O₂, and heat shock--led to nuclear accumulation of enhanced green fluorescent protein (EGFP)-labeled importin α. Importin α slowly redistributed to the cytoplasm after removal of H₂O₂ or return to 37°C. Time-lapse experiments indicated that, in cells treated with H₂O₂, nuclear import of an injected protein containing an NLS was suppressed--an effect that could be mitigated by coinjection of importin α. In cells subjected to any of the three stresses, Ran localized in the cytoplasm. Further, when a GTPase-deficient glutathione S-transferase-fused RanGTP mutant was injected into the nucleus, importin α localized in the cytoplasm of stressed cells (this mutant did not interfere with nuclear import of importin α). Pharmacological analysis indicated that, under stress conditions, importin α entered the nucleus through an alternative route independent of Ran and importin β. Thus, nuclear accumulation of importin α under stress--and the consequent inability to import proteins through the classical NLS pathway--appears to depend on a lack of export resulting from Ran redistribution and continued importin α entry through a nonclassical pathway.

Y. Miyamoto, T. Saiwaki, J. Yamashita, Y. Yasuda, I. Kotera, S. Shibata, M. Shigeta, Y. Hiraoka, T. Haraguchi, Y. Yoneda, Cellular stresses induce the nuclear accumulation of importin α and cause a conventional nuclear import block. J. Cell Biol. 165, 617-623 (2004). [Abstract] [Full Text]