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¹ Cellular Dynamics Laboratory, Discovery Research Institute, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
² Department of Frontier Biosciences, Graduate School of Frontier Biosciences, Osaka University, 2-2 Yamada-oka, Suita, Osaka 565-0871, Japan
³ CREST of JST and Communications Research Laboratory, Kansai Advanced Research Centre, Kobe, Hyogo 651-2492, Japan

Heat-shock induces a strong stress response and modifies all aspects of cellular physiology, which involves dynamic changes in the nucleocytoplasmic distributions of a variety of proteins. Many distinct nucleocytoplasmic transport pathways exist in eukaryotic cells, but how a particular transport pathway is regulated under different cellular conditions remains elusive. The finding of this study indicate that conventional nuclear import, which is mediated by importin /ß, is down-regulated, while the nuclear import of 70 kD heat-shock cognate protein is up-regulated in heat-shock cells. Among the factors involved in the mediation of the conventional nuclear import, significant levels of importin accumulate in the nucleus in response to heat-shock. An analysis of the behaviour of importin with fluorescence recovery after photobleaching and fluorescence loss in photobleaching studies show that nuclear importin becomes less mobile and its nucleocytoplasmic recycling is impaired in heat-shock cells. These data coincided well with biochemical and cytological studies. Our present data show that heat-shock induces the nuclear accumulation, nuclear retention, and recycling inhibition of importin , resulting in the suppression of conventional nuclear import. This suggests a new regulatory mechanism for the adaptation of cells to environmental changes, such as heat-shock.

^* Correspondence: E-mail: nimamoto{at}riken.jp