No abstract

We present evidence that retinoic acid can down-regulate transcriptional activation by the nuclear protooncogene c-jun. All three members of the retinoic acid receptor (RAR) subfamily (RARa, RARl, and RARy) can repress transcriptional induction of the human collagenase gene or a heterologous promoter that contains the collagenase promoter AP-1-bindlng site. In contrast, the retinoid X receptor fails to repress Jun/AP-1 activity, demonstrating a significant difference between the two regulatory systems through which retinoids exert their transcriptional control. Analysis of RARa mutants in transfection studies reveals that the DNAbinding domain is important for the inhibition of Jun/AP-l activity, even though the RAR does not bind the collagenase AP-1 site. Rather, gel-retardation assays reveal that bacterially expressed full-leugth RARa inhibits binding of Jun protein to target DNA. These data suggest that the RARa may form a nonproductive complex with c-Jun and provides a simple mechanism by which retinoic acid may limit cell growth and possibly malignant progression.

No abstract

Gene expression is modulated by the specific interactions of nuclear proteins with unique regulatory sequences in the genome. Proteins involved in transcriptional regulation seem to be either transcription factors or transcription modulators and their interactions are crucial in determining whether the expression of a specific gene is activated or repressed. Recently, the product of the proto-oncogene jun has been identified as the transcription factor AP-1, whereas nuclear oncoproteins fos and myc have been implicated in transcriptional transregulation of several promoters. Furthermore, the products of the fos and jun proto-oncogenes are associated in some transcription complexes. Although the nature of the association is unclear, the two proteins co-immunoprecipitate with fos antibodies in nuclear extracts. Here, we report studies that demonstrate that the fos protein directly modulates jun function by means of a heterodimer of fos and jun proteins. The fos 'leucine zipper' domain is necessary for the DNA binding of the heterodimer; a distinct domain, localized in the C-terminal region of the fos protein, is responsible for transcriptional regulation.

No abstract

We show that the product of the protooncogene c-rel is a constituent of an NF-#cB-like complex that binds to the id site originally identified in the enhancer of immunoglobulin K light chain gene. c-rel protein synthesized in bacteria binds to the ic site in a sequence-specific manner. The rel-,cB complex can be disrupted by incubation with anti-rel antibodies. The rel protein can form oligomers. The c-rel protein can activate transcription from promoters containing icB sites; v-rel, on the other hand, suppresses the transcription of genes linked to #cB sites. Thus, v-rel may interfere with the normal transcriptional machinery of the cell by acting as a dominant negative mutant.

Jun and Fos oncoproteins form a complex that regulates transcription from promoters containing AP-1 binding sites. The 'leucine zipper' domain of both Fos and Jun is necessary for the formation of the heterodimer, but the role of specific leucine residues is unclear. We have used site-specific mutagenesis to examine the contribution of individual leucine residues to the formation of a stable Fos-Jun protein complex and the binding of this complex to the AP-1 site. Mutation of a single leucine in either Fos or Jun had no effect on protein complex formation. Furthermore, mutations of two consecutive leucines in Jun did not interfere with heterodimer formation; however, in the case of Fos, two consecutive mutations resulted in an inability to form a heterodimer. Although mutagenesis of the first leucine of the heptad repeat had no effect on protein complex formation, this mutation in either Fos or Jun drastically reduced the affinity of the complex for DNA. Thus, both Fos and Jun contribute directly to the DNA-binding potential of the heterodimer.

The c-Rel protein is able to associate in vitro and in vivo with the TATA-binding protein (TBP) of the TFIID complex. Coexpression of TBP with c-Rel augments transactivation from the kappa B site in Drosophila Schneider cells. DNA-binding mutants of TBP not only fail to cooperate, but they repress transactivation by c-Rel. There may be a direct communication between kappa B enhancer binding proteins and basal transcription factors which leads to enhanced transcription.