Nuclear import of sterol regulatory element-binding protein-2, a basic helix-loop-helix-leucine zipper (bHLH-Zip)-containing transcription factor, occurs through the direct interaction of importin beta with HLH-Zip.
The sterol regulatory element-binding protein-2 (SREBP-2) is produced as a large precursor molecule attached to the endoplasmic reticulum membrane. In response to the sterol depletion, the N-terminal segment of the precursor, which contains a basic helix-loop-helix-leucine zipper domain, is released by two sequential cleavages and is translocated to the nucleus, where it activates the transcription of target genes. The data herein show that released SREBP-2 uses a distinct nuclear transport pathway, which is mediated by importin beta. The mature form of SREBP-2 is actively transported into the nucleus when injected into the cell cytoplasm. SREBP-2 binds directly to importin beta in the absence of importin alpha. Ran-GTP but not Ran-GDP causes the dissociation of the SREBP-2-importin beta complex. G19VRan-GTP inhibits the nuclear import of SREBP-2 in living cells. In the permeabilized cell in vitro transport system, nuclear import of SREBP-2 is reconstituted only by importin beta in conjunction with Ran and its interacting protein p10/NTF2. We further demonstrate that the helix-loop-helix-leucine zipper motif of SREBP-2 contains a novel type of nuclear localization signal, which binds directly to importin beta.
Nagoshi
E
E
Department of Anatomy and Cell Biology, Osaka University Medical School, Osaka 565-0871, Japan.
Imamoto
N
N
Sato
R
R
Yoneda
Y
Y
D001665:Binding Sites; D001692:Biological Transport; D002467:Cell Nucleus / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D006367:HeLa Cells / Q000378:metabolism; D018257:Helix-Loop-Helix Motifs; D006801:Humans; D028884:Karyopherins; D016350:Leucine Zippers; D009687:Nuclear Proteins / Q000378:metabolism*; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D051782:Sterol Regulatory Element Binding Protein 2; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D020931:ran GTP-Binding Protein
D004268:DNA-Binding Proteins; D028884:Karyopherins; D009687:Nuclear Proteins; D011994:Recombinant Proteins; C497390:SREBF2 protein, human; D051782:Sterol Regulatory Element Binding Protein 2; D014157:Transcription Factors; D020931:ran GTP-Binding Protein