Transformation and transactivation suppressor activity of the c-Jun leucine zipper fused to a bacterial repressor

Transcription factor c-Jun appears to be a nuclear target of the Ras-induced signal transduction pathway. In fact, some experiments show that transforming forms of the Ras protein cooperate with Jun in transcriptional activation mediated by an AP-1 site and others indicate that the two oncoproteins cooperate in cellular transformation. Although it is likely that intracellular signaling systems activated by Ras might act directly on c-Jun by inducing specific phosphorylation, it is unclear how c-Jun participates in the transformation process. Here, we present results obtained with a LexA-Jun zipper fusion that lacks both the transcriptional activation domains and the basic region of the DNA-binding domain of c-Jun and contains only the intact leucine-zipper domain. This fusion product has a dominant negative effect on the transcriptional activation elicited by phorbol esters, c-Jun, c-Fos, Ras and E1A on an AP-1-responsive site. An analogous LexA-Fos zipper fusion has similar effects on transcriptional induction. The LexA-Jun zipper fusion acts further as a transformation suppressor, since it causes the generation of nontransformed revertants of ras-transformed cells. This effect is likely to be elicited by the dimerization potential of the Jun leucine zipper trapping cellular Jun and/or Fos in a protein complex unable to bind to DNA. These data implicate further that Ras-mediated transformation involves functional transcription factor AP-1 and that it is possible to interfere with cell transformation by interfering simply with the dimerization of transcription factors involved in the transformation process.