Functional cloning of the proto-oncogene brain factor-1 (BF-1) as a smad-binding antagonist of transforming growth factor-beta signaling

Using the plasminogen activator inhibitor (PAI) promoter to drive the expre ssion of a reporter gene (mouse CD2), we devised a system to clone negative regulators of the transforming growth factor-beta (TGF-beta) signaling pat hway. We infected a TGF-beta -responsive cell line (MvLu1) with a retrovira l cDNA library, selecting by fluorescence-activated cell sorter single cell s displaying low PAI promoter activity in response to TGF-beta. Using this strategy we cloned the proto-oncogene brain factor-1 (BF-1). BF-1 represses the PAI promoter in part by associating with both unphosphorylated Smad3 ( in the cytoplasm) and phosphorylated Smad3 (in the nucleus), thus preventin g its binding to DNA BF-1 also associates with Smad1, -2, and -4, the Smad MH2 domain binds to BF-1, and the C-terminal segment of BF-1 is uniquely an d solely required for binding to Smads. Further, BF-1 represses another TGF -beta -induced promoter (p15), it upregulates a TGF-beta -repressed promote r (Cyclin A), and it reverses the growth arrest caused by TGF-beta. Our res ults suggest that BF-1 is a general inhibitor of TGF-beta signaling and as such may play a key role during brain development.