The product of a novel growth factor activated gene, fos B, interacts with JUN proteins enhancing their DNA binding activity.

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Abstract
We have identified a gene, fos B, encoding a nuclear protein of 338 amino acids presenting a 70% homology with c‐fos, whose expression is activated during G0/G1 transition. Growth factor stimulation of quiescent cells leads to a rapid and transient accumulation of fos B mRNA, with kinetics similar to those of c‐fos. The induction of fos B mRNA levels is in part due to a dramatic increase in the transcription of the gene. The half‐life of fos B mRNA is in the order of 10‐15 min. Both transcriptional activation and mRNA stability are substantially increased in the presence of protein synthesis inhibitors. Immunoprecipitation studies showed that fos B as c‐fos protein, forms a complex in vitro with c‐jun and jun B proteins in the absence of a target binding sequence. Gel retardation assays demonstrated that fos B protein positively influences the binding of c‐jun and jun B proteins to an AP‐1 binding consensus sequence, suggesting that fos B protein plays a role in control of gene expression.