Transactivation mediated by B-Myb is dependent on TAF(II)250

B-Myb is a highly conserved member of the Myb family of transcription factors, which has been implicated in cell cycle regulation. B-Myb is expressed in most proliferating cells and its activity is highly regulated around the G1/S-phase border of the cell cycle. It is generally assumed that B-Myb regulates the expression of genes that are crucial for cell proliferation; however, the identity of these genes, the molecular mechanisms by which B-Myb stimulates their expression and the involvement of other proteins have not been sufficiently clarified. We have employed the hamster cell line ts13 as a tool to demonstrate a functional link between B-Myb and the coactivator TAF(II)250, a key component of the transcriptional machinery which itself is essential for cell proliferation. ts13 cells express a point-mutated version of TAF(II)250 whose intrinsic histone acetyl transferase activity is temperature sensitive. Transactivation of Myb-responsive reporter genes by B-Myb is temperature-dependent in ts13 cells but not in ts13 cells, which have been rescued by transfection with an expression vector for wild-type TAF(II)250. Furthermore, B-Myb and TAF(II)250 can be coprecipitated, suggesting that both proteins are present in a complex. The formation of this complex is dependent on the DNA-binding domain of B-Myb and not on its transactivation domain. Taken together, these observations provide the first evidence that the coactivator TAF(II)250 is involved in the activation of Myb responsive promoters by B-Myb. The finding that B-Myb transactivation is dependent on a key coactivator involved in cell cycle control is consistent with and strengthens the idea that B-Myb plays a crucial role as a transcription factor in proliferating cells.