The N-terminal nuclear localization sequences of liver X receptors alpha and beta bind to importin alpha and are essential for both nuclear import and transactivating functions.
Liver X receptors (LXRs) alpha and beta are nuclear receptors, which form obligate heterodimers with the retinoid X receptor (RXR). The LXRs regulate both redundantly and non-redundantly the transcription of genes controlling cholesterol metabolism and transport as well as lipogenesis. Previously, we showed that mutations in putative N-terminal nuclear localization sequences (NLSs) within both LXRs inhibit nuclear import. Through in vitro studies, we show here that these NLSs bind importin alpha and are both necessary and sufficient for the nuclear import of LXRs. Imaging, transactivation, and electro-mobility shift experiments show that RXR rescues the nuclear import of the LXRalpha NLS mutant yet does not restore its transcriptional activity despite intact DNA binding. In contrast, RXR partially rescues the import of the LXRbeta NLS mutant, but has no effect on its transcriptional activity due to the loss of DNA binding. Experiments with NLS mutant RXR confirmed that RXR may dominate the nuclear import of the RXR/LXRalpha heterodimer, whereas LXRbeta dominates the nuclear import of the RXR/LXRbeta heterodimer. Intriguingly, our data indicate differences between LXRalpha and LXRbeta in their interaction with RXR and in the role their NLSs play in transactivating functions independent of nuclear import.
Miller
Anna
A
Department of Biological Sciences, A243 Life Science Building, Louisiana State University, Baton Rouge, LA 70803, USA.
Crumbley
Christine
C
Prüfer
Kirsten
K
D021581:Active Transport, Cell Nucleus; D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D019556:COS Cells; D002352:Carrier Proteins; D002467:Cell Nucleus / Q000378:metabolism*; D002522:Chlorocebus aethiops; D004247:DNA / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D000071518:Liver X Receptors; D008969:Molecular Sequence Data; D019913:Nuclear Localization Signals / Q000378:metabolism*; D057093:Orphan Nuclear Receptors; D055503:Protein Multimerization; D018160:Receptors, Cytoplasmic and Nuclear / Q000235:genetics / Q000378:metabolism*; D015533:Transcriptional Activation; D014162:Transfection; D028901:alpha Karyopherins / Q000235:genetics / Q000378:metabolism*
D002352:Carrier Proteins; D004268:DNA-Binding Proteins; D000071518:Liver X Receptors; C000605213:NR1H3 protein, human; D019913:Nuclear Localization Signals; D057093:Orphan Nuclear Receptors; D018160:Receptors, Cytoplasmic and Nuclear; D028901:alpha Karyopherins; D004247:DNA