DNA-binding activity of jun is increased through its interaction with Fos

Transcription factor AP‐1 mediates induction of a set of genes in response to the phorbol ester tumor promoter TPA. Recently, AP‐I preparations from HeLa cells were shown to contain a product of the c‐JUN protooncogene (Jun/AP‐l) which forms a tight complex with the Fos protein. In this paper, we examine the role of the Fos protein in the DNA‐binding activity of the AP‐I complex. We show that the DNA‐binding activity of bacterially expressed trpE‐Jun fusion proteins is increased many‐fold upon their interaction with Fos (or a Fos‐relaied antigen) expressed from a baculovirus vector. The site of Fos interaction is within the DNA‐binding domain of Jun/AP‐l, and anti‐Fos antibodies interfere with the binding of affinity purified AP‐1 to DNA. These results suggest that, by associating with Jun/AP‐l, Fos is responsible for the formation of a multimeric protein complex that has greater affinity for the target sequence than does Jun/AP‐l alone.