Determination of functional domains in the C subunit of the CCAAT-binding factor (CBF) necessary for formation of a CBF-DNA complex: CBF-B interacts simultaneously with both the CBF-A and CBF-C subunits to form a heterotrimeric CBF molecule.
The mammalian CCAAT-binding factor (CBF; also called NF-Y and CP1) is a heterotrimeric protein consisting of three subunits, CBF-A, CBF-B, and CBF-C, all of which are required for DNA binding and all of which are present in the CBF-DNA complex. In this study using cross-linking and immunoprecipitation methods, we first established that CBF-B interacts simultaneously with both subunits of the CBF-A-CBF-C heterodimer to form a heterotrimeric CBF molecule. We then performed a mutational analysis of CBF-C to define functional interactions with the other two CBF subunits and with DNA using several in vitro assays and an in vivo yeast two-hybrid system. Our experiments established that the evolutionarily conserved segment of CBF-C, which shows similarities with the histone-fold motif of histone H2A, was necessary for formation of the CBF-DNA complex. The domain of CBF-C which interacts with CBF-A included a large portion of this segment, one that corresponds to the segment of the histone-fold motif in H2A used for interaction with H2B. Two classes of interactions involved in formation of the CBF-A-CBF-C heterodimer were detected; one class, provided by residues in the middle of the interaction domain, was needed for formation of the CBF-A-CBF-C heterodimer. The other, provided by sequences flanking those of the first class was needed for stabilization of the heterodimer. Two separate domains were identified in the conserved segment of CBF-C for interaction with CBF-B; these were located on each side of the CBF-A interaction domain. Since our previous experiments identified a single CBF-B interaction domain in the histone-fold motif of CBF-A, we propose that a tridentate interaction domain in the CBF-A-CBF-C heterodimer interacts with the 21-amino-acid-long subunit interaction domain of CBF-B. Together with our previous mutational analysis of CBF-A (S. Sinha, I.-S. Kim, K.-Y. Sohn, B. de Crombrugghe, and S. N. Maity, Mol. Cell. Biol. 16:328-337, 1996), this study demonstrates that the histone fold-motifs of CBF-A and CBF-C interact with each other to form the CBF-A-CBF-C heterodimer and generate a hybrid surface which then interacts with CBF-B to form the heterotrimeric CBF molecule.
Kim
I S
IS
Department of Molecular Genetics, M.D. Anderson Cancer Center, Univerity of Texas, Houston 77030, USA.
Sinha
S
S
de Crombrugghe
B
B
Maity
S N
SN
D000595:Amino Acid Sequence; D000818:Animals; D022762:CCAAT-Enhancer-Binding Proteins; D003432:Cross-Linking Reagents; D004268:DNA-Binding Proteins / Q000737:chemistry*; D018257:Helix-Loop-Helix Motifs; D046911:Macromolecular Substances; D008969:Molecular Sequence Data; D011233:Precipitin Tests; D011485:Protein Binding; D011994:Recombinant Proteins; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D013329:Structure-Activity Relationship; D014157:Transcription Factors / Q000737:chemistry*
D022762:CCAAT-Enhancer-Binding Proteins; D003432:Cross-Linking Reagents; D004268:DNA-Binding Proteins; D046911:Macromolecular Substances; D011994:Recombinant Proteins; D014157:Transcription Factors