The c-fos protein interacts with c-JunAP-1 to stimulate transcription of AP-1 responsive genes

Cell lines stably transfected with metal inducible, MT- fos chimeric genes were used to study the ability of the c- fos gene product, Fos, to act as a transcriptional trans -activator. In 3T3MT fos cells, induction of Fos expression led to specific trans -activation of an AP-1 responsive reporter gene. Induction of Fos expression in F9MT fos cells, however, did not lead to trans -activation. Since, unlike NIH3T3 cells, F9 cells do not contain detectable levels of AP-1, we examined whether a c - Jun AP -1 expression vector can restore the trans -activating effect of Fos in F9MT fos cells. Transfection with a functional c - Jun AP -1 vector restored the specific trans -activating effect of Fos on AP-1 responsive constructs. When incubated with nondenatured cell extracts, anti-cFos antisera precipitated a protein complex composed of Fos and several Fos associated proteins (FAP). One of these, FAP p39, is structurally identical to c - Jun AP -1 . These results suggest that Fos is a trans -acting factor that is capable of stimulating gene expression not by direct binding to DNA but by interaction with the sequence-specific transcription factor AP-1. Therefore recognition of specific cis -elements by AP-1 is a prerequisite for Fos-mediated stimulation of gene expression.