Cory Abate

The proto-oncogenes c-fos and c-jun function cooperatively as inducible transcription factors in signal transduction processes. Their protein products, Fos and Jun, form a heterodimeric complex that interacts with the DNA regulatory element known as the activator protein-1 (AP-1) binding site. Dimerization occurs via interaction between leucine zipper domains and serves to bring into proper juxtaposition a region in each protein that is rich in basic amino acids and that forms a DNA-binding domain. DNA binding of the Fos-Jun heterodimer was modulated by reduction-oxidation (redox) of a single conserved cysteine residue in the DNA-binding domains of the two proteins. Furthermore, a nuclear protein was identified that reduced Fos and Jun and stimulated DNA-binding activity in vitro. These results suggest that transcriptional activity mediated by AP-1 binding factors may be regulated by a redox mechanism.

The protein products of the fos and jun proto-oncogenes form a heterodimeric complex that participates in a stable high affinity interaction with DNA elements containing AP-1 binding sites. The effects of deletions and point mutations in Fos and Jun on protein complex formation and DNA binding have been examined. The data suggest that Fos and Jun dimerize via a parallel interaction of helical domains containing a heptad repeat of leucine residues (the leucine zipper). Dimerization is required for DNA binding and results in the appropriate juxtaposition of basic amino acid regions from Fos and Jun, both of which are required for association with DNA.

This study investigates the transcriptional properties of Msx-1, a murine homeodomain protein which has been proposed to play a key role in regulating the differentiation and/or proliferation state of specific cell populations during embryogenesis. We show, using basal and activated transcription templates, that Msx-1 is a potent repressor of transcription and can function through both TATA-containing and TATA-less promoters. Moreover, repression in vivo and in vitro occurs in the absence of DNA-binding sites for the Msx-1 homeodomain. Utilizing a series of truncated Msx-1 polypeptides, we show that multiple regions of Msx-1 contribute to repression, and these are rich in alanine, glycine, and proline residues. When fused to a heterologous DNA-binding domain, both N- and C-terminal regions of Msx-1 retain repressor function, which is dependent upon the presence of the heterologous DNA-binding site. Moreover, a polypeptide consisting of the full-length Msx-1 fused to a heterologous DNA-binding domain is a more potent repressor than either the N- or C-terminal regions alone, and this fusion retains the ability to repress transcription in the absence of the heterologous DNA site. We further show that Msx-1 represses transcription in vitro in a purified reconstituted assay system and interacts with protein complexes composed of TBP and TFIIA (DA) and TBP, TFIIA, and TFIIB (DAB) in gel retardation assays, suggesting that the mechanism of repression is mediated through interaction(s) with a component(s) of the core transcription complex. We speculate that the repressor function of Msx-1 is critical for its proposed role in embryogenesis as a regulator of cellular differentiation.