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Identification of Novel Nuclear Export and Nuclear Localization-related Signals in Human Heat Shock Cognate Protein 70*

Heat shock cognate protein 70 (Hsc70) serves nuclear transport of several proteins as a molecular chaperone. We have recently identified a novel variant of human Hsc70, heat shock cognate protein 54 (Hsc54), that lacks amino acid residues 464–616 in the protein binding and variable domains of Hsc70. In the present study, we examined nucleocytoplasmic localization of Hsc70 and Hsc54 by using green fluorescent protein (GFP) fusions. GFP-Hsc70 is localized in both the cytoplasm and the nucleus at 37 °C and accumulated into the nucleolus/nucleus after heat shock, whereas GFP-Hsc54 always remained exclusively in the cytoplasm under these conditions. Mutation studies indicated that 20 amino acid residues of nuclear localization-related signals, which are missing in Hsc54 but are retained in Hsc70, are required for proper nuclear localization of Hsc70. We further found that Hsc54 contains a functional leucine-rich nuclear export signal (NES, 394LDVTPLSL401) which is differently situated from the previously proposed NES in Saccharomyces cerevisiae Ssb1p. The cytoplasmic localization of Hsc54 was impaired by a mutation in NES as well as by a nuclear export inhibitor, leptomycin B, suggesting that Hsc54 is actively exported from the nucleus to the cytoplasm through a CRM1-dependent mechanism. In contrast, the nucleocytoplasmic localization of Hsc70 was not affected by the same mutation of NES or leptomycin B. These results suggest that the nuclear localization-related signal could functionally mask NES leading to prolonged retention of Hsc70 in the nucleus. An additional mechanism for unmasking the NES may regulate nucleocytoplasmic trafficking of Hsc70.