A C-terminal domain in FosB, absent in FosB/SF and Fra-1, which is able to interact with the TATA binding protein, is required for altered cell growth.

Transcriptional regulation in eukaryotes is thought to occur through interactions between specific transcription factors and the general transcription machinery. We show that the regulatory protein FosB, but not FosB/SF or Fra-1, specifically and stably associates with the TATA box binding protein (TBP) and the multiprotein complex TFIID. The binding to TBP is specified by the last 55 C-terminal amino acids of FosB, requiring a small amino acid sequence, termed the 'TBP binding motif' (TBM). Deletion of the TBM affects transcriptional activity slightly, but it is adjacent to a proline-rich sequence which constitutes the major transactivation domain. However, both regions are required for the transformation of Rat-1A cells by FosB. Transfection experiments demonstrate that inhibition of transactivation due to excess levels of Gal4-FosB (squelching) can be partially relieved by the co-expression of TBP, which establishes that TFIID is a functional target of FosB. Since TBP binding is not exhibited by FosB/SF or Fra-1, we suggest that the activity mediated by the TBP interaction is one differentiating characteristic that distinguishes the FosB functions from those of FosB/SF and Fra-1.