MULTIPROTEIN-BRIDGING-FACTOR-1 (MBF1) IS AN EVOLUTIONARILY CONSERVED TRANSCRIPTIONAL COACTIVATOR THAT CONNECTS A REGULATORY FACTOR AND TATAELEMENT-BINDING PROTEIN

Multiprotein bridging factor 1 (MBF1) is a transcriptional cofactor th at bridges between the TATA box-binding protein (TBP) and the Drosophi la melanogaster nuclear hormone receptor FTZ-F1 or its silkworm counte rpart BmFTZ-F1. A cDNA clone encoding MBF1 was isolated from the silkw orm Bombyx mori whose sequence predicts a basic protein consisting of 146 amino acids. Bacterially expressed recombinant MBF1 is functional in interactions with TBP and a positive cofactor MBF2. The recombinant MBF1 also makes a direct contact with FTZ-F1 through the C-terminal r egion of the FTZ-F1 DNA-binding domain and stimulates the FTZ-F1 bindi ng to its recognition site. The central region of MBF1 (residues 35-11 3) is essential for the binding of FTZ-F1, MBF2, and TBP. When the rec ombinant MBF1 was added to a HeLa cell nuclear extract in the presence of MBF2 and FTZ622 bearing the FTZ-F1 DNA-binding domain, it supporte d selective transcriptional activation of the fushi tarazu gene as nat ural MBF1 did. Mutations disrupting the binding of FTZ622 to DNA or MB F1, or a MBF2 mutation disrupting the binding to MBF1, all abolished t he selective activation of transcription. These results suggest that t ethering of the positive cofactor MBF2 to a FTZ-F1-binding site throug h FTZ-F1 and MBF1 is essential for the binding site-dependent activati on of transcription. A homology search in the databases revealed that the deduced amino acid sequence of MBF1 is conserved across species fr om yeast to human.