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Subunits of the Heterotrimeric Transcription Factor NF-Y Are Imported into the Nucleus by Distinct Pathways Involving Importin β and Importin 13

  • Joerg Kahle
  • Matthias Baake
  • Detlef Doenecke
  • Werner Albig
American Society for Microbiology
Publication Date
Jul 01, 2005


The transcriptional activator NF-Y is a heterotrimeric complex composed of NF-YA, NF-YB, and NF-YC, which specifically binds the CCAAT consensus present in about 30% of eukaryotic promoters. All three subunits contain evolutionarily conserved core regions, which comprise a histone fold motif (HFM) in the case of NF-YB and NF-YC. Our results of in vitro binding studies and nuclear import assays reveal two different transport mechanisms for NF-Y subunits. While NF-YA is imported by an importin β-mediated pathway, the NF-YB/NF-YC heterodimer is translocated into the nucleus in an importin 13-dependent manner. We define a nonclassical nuclear localization signal (ncNLS) in NF-YA, and mutational analysis indicates that positively charged amino acid residues in the ncNLS are required for nuclear targeting of NF-YA. Importin β binding is restricted to the monomeric, uncomplexed NF-YA subunit. In contrast, the nuclear import of NF-YB and NF-YC requires dimer formation. Only the NF-YB/NF-YC dimer, but not the monomeric components, are recognized by importin 13 and are imported into the nucleus. Importin 13 competes with NF-YA for binding to the NF-YB/NF-YC dimer. Our data suggest that a distinct binding platform derived from the HFM of both subunits, NF-YB/NF-YC, mediates those interactions.

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