The constitutive androstane receptor (CAR) is a member of the nuclear receptor superfamily. The CAR is normally located in the cytoplasmic compartment of untreated liver cells and translocates to the nucleus after exposure to phenobarbital (PB) or PB-like chemicals. Previously, we identified two nuclear localization signals (NLS) in the rat constitutive androstane/active receptor (CAR), NLS1, which is located in the hinge region, and NLS2, which overlaps with the ligand-binding domain. However, the nuclear import mechanism of CAR is unclear. In this study, we show that nuclear import of CAR is regulated by importin/Ran-GTP systems. The regulation of CAR nuclear import by a Ran-GTP concentration gradient was confirmed using the dominant negative, GTPase-deficient form of Ran (RanQ69L), suggesting the involvement of transport receptors of the importinbeta family. IPO13 was shown to be involved in the PB-mediated nuclear translocation of CAR, which was found to be susceptible to inhibition by a dominant negative mutant of IPO13 in primary hepatocytes.
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