A fragment of MLL functions as a dominant negative to disrupt the MLL-PAFc interaction. (A) Schematic diagram of the MLL protein and functional domains with magnified view of the CxxC domain and the RD2 region with illustration of the engineered MLLDN fragment. Amino acid numbers for regions of MLL are indicated. (B) Expression of MLLDN was confirmed to coimmunoprecipitate CDC73 by transient transfection of 293 cells. (C) Purified GST-tagged MLLDN bound directly and specifically to MBP-tagged full-length PAF1 in GST pull-down experiments. (D) Coimmunoprecipitation assays performed in 293 cells demonstrate the MLL fragment Myc-CxxC-RD2 associates with endogenous PAF1 (lane 2). Increasing doses of MLLDN (1.5 and 3.0 μM, lanes 3 and 4) disrupt the interaction between the CxxC-RD2 and PAF1, whereas increasing doses of the GST tag (1.5 and 3.0 μM, lanes 5 and 6) do not. Arrow indicates PAF1 protein. One of 2 representative experiments is shown.