A naturally occurring truncated form of FosB that inhibits Fos/Jun transcriptional activity.

Fos and Jun transcription factors are induced by a variety of extracellular signaling agents. We describe here an unusual member of the Fos family that is also induced, namely, a truncated form of FosB (delta FosB) missing the C-terminal 101 amino acids of FosB. delta FosB retains the dimerization and DNA-binding activities of FosB but has lost the ability in transfection assays to activate a promoter with an AP-1 site and to repress the c-fos promoter. Rather, delta FosB inhibits gene activation by Jun or Jun + Fos and inhibits repression of the c-fos promoter by FosB or c-Fos, presumably by competing with full-length Fos proteins at the steps of dimerization with Jun and binding to DNA. In stimulated cells delta FosB may act to limit the transcriptional effects of Fos and Jun proteins.