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Published online 3 October 2005. doi:10.1083/jcb.200506074
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 171, Number 1, 19-25
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 The 70-kD heat shock cognate protein (hsc70) facilitates the nuclear export of the import receptors

Shingo Kose1, Maiko Furuta1, Makiko Koike1, Yoshihiro Yoneda2, and Naoko Imamoto1

1 Cellular Dynamics Laboratory, Discovery Research Institute, RIKEN, Wako, Saitama 351-0198, Japan
2 Department of Frontier Biosciences, Graduate School of Frontier Biosciences, Osaka University, Osaka 565-0871, Japan

Correspondence to Naoko Imamoto: nimamoto{at}riken.jp


Abstract
Transport receptors of the importin ß family continuously shuttle between the nucleus and cytoplasm. We previously reported that the nuclear export of importin ß involves energy-requiring step(s) in living cells. Here, we show that the in vitro nuclear export of importin ß also requires energy input. Cytosol, depleted of ATP-binding proteins, did not support the sufficient nuclear export of importin ß. Further purification revealed that the active component in the absorbed fraction was a 70-kD heat shock cognate protein (hsc70). The addition of recombinant hsc70, but not an ATPase-deficient hsc70 mutant, to the depleted cytosol restored the export activity. In living cells, depletion of hsc70 caused the significant nuclear accumulation of importin ß. These effects of hsc70 were observed in the nuclear export of importin ß, but also for other import receptors, transportin and importin {alpha}. These results suggest that hsc70 broadly modulates nucleocytoplasmic transport systems by regulating the nuclear export of receptor proteins.

Abbreviations used in this paper: hsc70, 70-kD heat shock cognate protein; NPC, nuclear pore complex; NTF2, nuclear transport factor 2; RCC1, regulator of chromosomal condensation; siRNA, small interfering RNA; TB, transport buffer.


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