title: Constitutive nuclear import and stress-regulated nucleocytoplasmic shuttling of mammalian heat-shock factor 1.
abstract: Inducible expression of major cytosolic and nuclear chaperone proteins is mediated by the heat-shock transcription factor HSF1 that is activated by derepressive mechanisms triggered by transient heat stress and sustained proteotoxicity. Despite progress in defining essential aspects of HSF1 regulation, little is known about the cellular dynamics enabling this factor to mediate gene responses to cytosolic stress signals. We report that the inactive, stress-responsive form of HSF1 accumulates in the nucleus due to a relatively potent import signal, which can be recognized by importin-alpha/beta, and simultaneously undergoes continuous nucleocytoplasmic shuttling due to a comparatively weak, nonetheless efficient, export activity not involving the classical exportin-1 pathway. Strikingly, experimental stresses at physiological or elevated temperature reversibly inactivate the export competence of HSF1. Likewise, mutations mimicking stress-induced derepression impair export but not import. These findings are consistent with a dynamic process whereby exported molecules that are derepressed in an inductive cytosolic environment are recollected and pause in the nucleoplasm, replacing progressively the inactive pool. While steady-state nuclear distribution of the bulk of HSF1 ensures a rapid gene response to acute heat stress, our results suggest that the capture in the nucleus of molecules primed for activation in the cytosol may underlie responses to sustained proteotoxicity.
authors.affiliation: DIBIT--San Raffaele Scientific Institute, Via Olgettina 58, 20132 Milano, Italy.
authors.forename: Milos
authors.initials: M
authors.lastname: Vujanac
authors.forename: Angelia
authors.initials: A
authors.lastname: Fenaroli
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Zimarino
mesh_terms: D021581:Active Transport, Cell Nucleus / Q000502:physiology*; D000818:Animals; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism*; D005231:Fatty Acids, Unsaturated / Q000494:pharmacology; D005786:Gene Expression Regulation; D000076249:Heat Shock Transcription Factors; D006360:Heat-Shock Proteins / Q000378:metabolism*; D006358:Hot Temperature; D006801:Humans; D051379:Mice; D018832:Molecular Chaperones / Q000502:physiology*; D019913:Nuclear Localization Signals / Q000502:physiology; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D014157:Transcription Factors; D001616:beta-Galactosidase / Q000378:metabolism
chemical_list: D004268:DNA-Binding Proteins; D005231:Fatty Acids, Unsaturated; C000619706:HSF1 protein, human; D000076249:Heat Shock Transcription Factors; D006360:Heat-Shock Proteins; C000619709:Hsf1 protein, rat; D018832:Molecular Chaperones; D019913:Nuclear Localization Signals; D011993:Recombinant Fusion Proteins; D014157:Transcription Factors; D001616:beta-Galactosidase; C038753:leptomycin B