Subcellular trafficking signals of constitutive androstane receptor: evidence for a nuclear export signal in the DNA-binding domain.
Translocation of constitutive androstane receptor (CAR) from the cytoplasm to the nucleus is induced by phenobarbital-like drugs. Nuclear localization signals (NLSs) and a sequence [xenochemical response signal (XRS)] required for xenobiotic-induced nuclear translocation have been defined in rat and human CAR, but a nuclear export signal (NES) has not been identified. To identify cellular localization signals of CAR, the localization of fragments and mutants of mouse CAR expressed in mouse hepatocytes in vivo was examined. Consistent with other studies, an NLS in the hinge region, a diffuse NLS in the ligand-binding domain, and a cytoplasmic retention sequence were identified, and mutation of the XRS blocked nuclear accumulation both in phenobarbital-treated mice in vivo and in untreated HepG2 cells. Fusing the simian virus 40 NLS to the mutant proteins reversed the localization defect resulting from mutation of the hinge NLS but not that from mutation of the XRS, indicating that the XRS is not simply a novel phenobarbital-responsive NLS. In the DNA-binding domain, a sequence in CAR is conserved with an NES identified in other nuclear receptors. Mutation of two conserved phenylalanines in this sequence resulted in increased nuclear localization of both full-length CAR and a CAR fragment containing the DNA-binding domain. The DNA-binding domain sequence, therefore, may contain an NES, which is consistent with nucleocytoplasmic shuttling of CAR. The results demonstrate that regulation of the cellular localization of CAR is complex, with multiple sequences mediating nuclear import and export and retention in the cytoplasm.
Xia
Jun
J
Department of Cell and Development Biology, University of Illinois at Urbana-Champaign, 524 Burrill Hall, 407 S. Goodwin Ave., Urbana, IL 61801, USA.
Kemper
Byron
B
D021581:Active Transport, Cell Nucleus / Q000502:physiology*; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism; D002460:Cell Line; D045744:Cell Line, Tumor; D000090702:Constitutive Androstane Receptor; D004247:DNA / Q000378:metabolism*; D004268:DNA-Binding Proteins; D049452:Green Fluorescent Proteins / Q000378:metabolism; D022781:Hepatocytes / Q000187:drug effects / Q000378:metabolism; D052058:Hinge Exons / Q000235:genetics; D006801:Humans; D008024:Ligands; D051379:Mice; D009154:Mutation / Q000235:genetics; D009687:Nuclear Proteins / Q000378:metabolism; D010957:Plasmids / Q000235:genetics; D016601:RNA-Binding Proteins; D051381:Rats; D018160:Receptors, Cytoplasmic and Nuclear / Q000502:physiology*; D015398:Signal Transduction / Q000502:physiology*; D013347:Subcellular Fractions / Q000378:metabolism*; D014157:Transcription Factors / Q000502:physiology*
D002352:Carrier Proteins; D000090702:Constitutive Androstane Receptor; D004268:DNA-Binding Proteins; D008024:Ligands; C110421:Mybbp1a protein, mouse; D009687:Nuclear Proteins; D016601:RNA-Binding Proteins; D018160:Receptors, Cytoplasmic and Nuclear; D014157:Transcription Factors; D049452:Green Fluorescent Proteins; D004247:DNA