TI  - Physical and functional interactions between the transcriptional
      inhibitors  <prot>Id3</prot>  and  <prot>ITF-2b</prot> . Evidence toward a novel mechanism regulating
      muscle-specific gene expression.
PG  - 2459-63
AB  - We have used an interaction cloning strategy to identify an inhibitory
      isoform of the <prot>ITF-2</prot> transcription factor,  <prot>ITF-2b</prot> , that interacts with the
      transcriptional inhibitor  <prot>Id3</prot>/<prot>HLH462</prot> . The interaction was confirmed in
      vitro, and inside intact myogenic C2C12 cells. As expected, overexpression
      of either <prot>Id3</prot>/<prot>HLH462</prot> or <prot>ITF-2b</prot> effectively inhibited the activation of the
      muscle-specific <prot>creatine kinase</prot> promoter by the myogenic transcription
      factor <prot>MyoD</prot>. However, when overexpressed simultaneously, <prot>ITF-2b</prot> and
      Id3/HLH462 counteracted each other's inhibitory effect to produce a
      reduced overall inhibition. Moreover, while <prot>ITF-2b</prot> inhibited the <prot>creatine
      kinase</prot> promoter, it acted as a weak transactivator on an artificial
      promoter consisting of three tandem copies of the consensus myogenic
      factor DNA binding site. Further investigation indicated that the
       <prot>ITF-2b</prot> / <prot>MyoD</prot>  heterodimer bound to its specific DNA binding site in vitro,
      and the DNA binding was effectively blocked by <prot>Id3</prot>/<prot>HLH462</prot>. Additional
      analysis revealed the presence of transcripts for both the activating
      (<prot>ITF-2a</prot>) and inhibitory (<prot>ITF-2b</prot>) isoforms in differentiating C2C12
      cultures, suggesting that both isoforms might participate in regulating
      the differentiation process. Taken together, this study reveals a more
      complex pattern of regulatory interactions involving the helix-loop-helix
      proteins than was previously anticipated.
AD  - Department of Pharmacology, University of Missouri, School of Medicine,
