A nuclear export signal prevents Saccharomyces cerevisiae Hsp70 Ssb1p fromstimulating nuclear localization signal-directed nuclear transport

Hsp70 has been implicated in nuclear localization signal (NLS)-directed nuc lear transport. Saccharomyces cerevisiae contains distinct SSA and SSB gene families of cytosolic Hsp70s. The nucleocytoplasmic localization of Ssa1p and Ssb1p was investigated using green fluorescent protein (GFP) fusions. W hereas GFP-Ssa1p localized both to the nucleus and cytoplasm, GFP-Ssb1p app eared only in the cytosol. The C-terminal domain of Ssb1p contains a leucin e-rich nuclear export signal (NES) that is necessary and sufficient to dire ct nuclear export. The accumulation of GFP-Ssb1p in the nuclei of xpo1-1 ce lls suggests that Ssb1p shuttles across the nuclear envelope. Elevated leve ls of SSA1 but not SSB1 suppressed the NLS-GFP nuclear localization defects of nup188-Delta cells. Studies with Ssa1p/Ssb1p chimeras revealed that the Ssb1p NES is sufficient and necessary to inhibit the function of Ssa- or S sb-type Hsp70s in nuclear transport. Thus, NES-less Ssb1p stimulates nuclea r transport in nup188-Delta cells and NES-containing Ssa1p does not. We con clude that the differential function of Ssa1p and Ssb1p in nuclear transpor t is due to the NES-directed export of the Ssb1p and not to functional diff erences in their ATPase or peptide binding domains.