Three classes of mutations in the A subunit of the CCAAT-binding factor CBF delineate functional domains involved in the three-step assembly of the CBF-DNA complex.
The mammalian CCAAT-binding factor CBF (also called NF-Y or CP1) consists of three subunits, CBF-A, CBF-B, and CBF-C, all of which are required for DNA binding and present in the CBF-DNA complex. In this study we first established the stoichiometries of the CBF subunits, both in the CBF molecule and in the CBF-DNA complex, and showed that one molecule of each subunit is present in the complex. To begin to understand the interactions between the CBF subunits and DNA, we performed a mutational analysis of the CBF-A subunit. This analysis identified three classes of mutations in the segment of CBF-A that is conserved in Saccharomyces cerevisiae and mammals. Analysis of the first class of mutants revealed that a major part of the conserved segment was essential for interactions with CBF-C to form a heterodimeric CBF-A/CBF-C complex. The second class of mutants identified a segment of CBF-A that is necessary for interactions between the CBF-A/CBF-C heterodimer and CBF-B to form a CBF heterotrimer. The third class defined a domain of CBF-A involved in binding the CBF heterotrimer to DNA. The second and third classes of mutants acted as dominant negative mutants inhibiting the formation of a complex between the wild-type CBF subunits and DNA. The segment of CBF-A necessary for DNA binding showed sequence homology to a segment of CBF-C. Interestingly, these sequences in CBF-A and CBF-C were also homologous to the sequences in the histone-fold motifs of histones H2B and H2A, respectively, and to the archaebacterial histone-like protein HMf-2. We discuss the functional domains of CBF-A and the properties of CBF in light of these sequence homologies and propose that an ancient histone-like motif in two CBF subunits controls the formation of a heterodimer between these subunits and the assembly of a sequence-specific DNA-protein complex.
Department of Molecular Genetics, University of Texas M. D. Anderson Cancer Center, Houston 77030, USA.
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Sinha
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Kim
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Sohn
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de Crombrugghe
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Maity
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites / Q000235:genetics; D022762:CCAAT-Enhancer-Binding Proteins; D017124:Conserved Sequence; D004247:DNA / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D008969:Molecular Sequence Data; D009154:Mutation; D011487:Protein Conformation; D051381:Rats; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid
D022762:CCAAT-Enhancer-Binding Proteins; D004268:DNA-Binding Proteins; D004247:DNA