The nuclear I kappaB protein I kappaB zeta specifically binds NF-kappaB p50 homodimers and forms a ternary complex on kappaB DNA.
Although they share sequence homology to classical cytoplasmic I kappaB inhibitors of transcription factor NF-kappaB, the proteins I kappaB zeta, Bcl-3, and I kappa BNS function in the nucleus as factors that influence NF-kappaB-dependent gene expression profiles. Through the use of purified recombinant proteins and by comparison with the classical I kappaB protein I kappaB alpha, we have discovered mechanistic details of the interaction between I kappaB zeta and functional NF-kappaB dimers. Whereas I kappaB alpha and other classical I kappaB proteins bind tightly to NF-kappaB dimers that possess the p65 subunit, I kappaB zeta binds preferentially to NF-kappaB p50 homodimers. This altered specificity is particularly interesting in light of the fact that both NF-kappaB subunits exhibit high sequence and structural homology, while the I kappaB alpha and I kappaB zeta proteins are also conserved in primary amino acid sequence. We further show that I kappaB zeta is capable of forming a stable ternary complex with the NF-kappaB p50 homodimer and kappaB DNA. Again, this is a stark contrast from I kappaB alpha, which inhibits NF-kappaB p65 homodimer binding to NF-kappaB target DNA sequences. Removal of the DNA sequences flanking the NF-kappaB binding site does not directly affect the interaction of p50 and I kappaB zeta. Rather, we have discovered that the carboxy-terminal glycine-rich region of the NF-kappaB p50 homodimer is involved in mediating high-affinity binding of I kappaB zeta and NF-kappaB p50. We conclude that the NF-kappaB p50 homodimer functions as a legitimate activator of gene expression through formation of a ternary complex between itself, I kappaB zeta, and DNA. The requirement for formation of this complex could explain why the nuclear I kappaB protein I kappaB zeta is absolutely required for expression of the pluripotent pro-inflammatory cytokine interleukin-6 in peritoneal macrophages.
Trinh
Dan V
DV
Structural Biochemistry Laboratory, Department of Chemistry and Biochemistry, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182-1030, USA.
Zhu
Norman
N
Farhang
Ghazal
G
Kim
Brian J
BJ
Huxford
Tom
T
D048868:Adaptor Proteins, Signal Transducing; D000818:Animals; D001483:Base Sequence; D004247:DNA / Q000737:chemistry* / Q000378:metabolism; D019281:Dimerization; D024202:Electrophoretic Mobility Shift Assay; D005998:Glycine / Q000737:chemistry; D006801:Humans; D020672:I-kappa B Proteins; D015850:Interleukin-6 / Q000235:genetics; D051379:Mice; D008969:Molecular Sequence Data; D052002:NF-kappa B p50 Subunit / Q000737:chemistry* / Q000378:metabolism; D009687:Nuclear Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D017434:Protein Structure, Tertiary / Q000235:genetics; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism
D048868:Adaptor Proteins, Signal Transducing; D020672:I-kappa B Proteins; D015850:Interleukin-6; D052002:NF-kappa B p50 Subunit; C417234:NFKBIZ protein, human; D009687:Nuclear Proteins; D011994:Recombinant Proteins; D004247:DNA; D005998:Glycine