Starvation promotes nuclear accumulation of the hsp70 Ssa4p in yeast cells.
Nuclear import of proteins that are too large to passively enter the nucleus requires soluble factors, energy, and a nuclear localization signal (NLS). Nuclear protein transport can be regulated, and different forms of stress affect nucleocytoplasmic trafficking. As such, import of proteins containing a classical NLS is inhibited in starving yeast cells. In contrast, the hsp70 Ssa4p concentrates in nuclei upon starvation. Nuclear concentration of Ssa4p in starving cells is reversible, and transfer of stationary phase cells to fresh medium induces Ssa4p nuclear export. This export reaction represents an active process that is sensitive to oxidative stress. In starving cells, the N-terminal domain of Ssa4p mediates Ssa4p nuclear accumulation, and a short hydrophobic sequence, termed Star (for starvation), is sufficient to localize the reporter proteins green fluorescent protein or beta-galactosidase to nuclei. To determine whether nuclear accumulation of Star-beta-galactosidase depends on a specific nuclear carrier, we have analyzed its distribution in mutant yeast strains that carry a deletion of a single beta-importin gene. With this assay we have identified Nmd5p as a beta-importin required to concentrate Star-beta-galactosidase in nuclei when cells enter stationary phase.
Department of Physiology, McGill University, Montreal, Province of Quebec H3G 1Y6, Canada.
Z S
ZS
Chughtai
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R
Rassadi
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N
Matusiewicz
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Stochaj
D002467:Cell Nucleus / Q000378:metabolism*; D049452:Green Fluorescent Proteins; D018840:HSP70 Heat-Shock Proteins / Q000378:metabolism*; D008164:Luminescent Proteins / Q000378:metabolism; D019913:Nuclear Localization Signals; D021381:Protein Transport; D011993:Recombinant Fusion Proteins / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000378:metabolism*; D013217:Starvation
D018840:HSP70 Heat-Shock Proteins; D008164:Luminescent Proteins; D019913:Nuclear Localization Signals; D011993:Recombinant Fusion Proteins; D049452:Green Fluorescent Proteins