ALY Is a Common Coactivator of RUNX1 and c-Myb on the Type B Leukemogenic Virus Enhancer (cid:1)

Type B leukemogenic virus (TBLV), a mouse mammary tumor virus (MMTV) variant, often induces T-cell leukemias and lymphomas by c- myc activation following viral DNA integration. Transfection assays using a c- myc reporter plasmid indicated that the TBLV long terminal repeat (LTR) enhancer is necessary for T-cell-specific increases in basal reporter activity. The sequence requirements for this effect were studied using mutations of the 62-bp enhancer region in an MMTV LTR reporter vector. Deletion of a nuclear factor A-binding site dramatically reduced reporter activity in Jurkat T cells. However, a 41-bp enhancer missing the RUNX1 site still retained minimal enhancer function. DNA affinity purification using a TBLV enhancer oligomer containing the RUNX1 binding site followed by mass spectrometry resulted in the identification of ALY. Subsequent experiments focused on the reconstitution of enhancer activity in epithelial cells. ALY overexpression synergized with RUNX1B on TBLV enhancer activity, and synergism required the RUNX1B- binding site. A predicted c-Myb binding site in the enhancer was confirmed after c- myb overexpression elevated TBLV LTR reporter activity, and overexpression of c-Myb and RUNX1B together showed additive effects on reporter gene levels. ALY also synergized with c-Myb, and coimmunoprecipitation experiments demonstrated an interaction between ALY and c-Myb. These experiments suggest a central role for ALY in T-cell enhancer function and oncogene activation. Madison, WI). A cloned con-catemer containing 10 copies of the 556WT oligomer was excised and end labeled with biotinylated dCTP prior to binding to streptavidin beads. Bound proteins were washed extensively with EMSA-binding buffer containing 1 (cid:4) g/ml or 2.5 (cid:4) g/ml of poly(dA-dT) (GE Healthcare Biosciences, Piscataway, NJ) as a nonspecific competitor in the first two or second two washes, respectively. Pro- teins were eluted with EMSA-binding buffer containing 2 M NaCl and dialyzed against EMSA-binding buffer before analysis on 10% polyacrylamide gels con- taining SDS. Gels were stained in 50% (vol/vol) methanol, 0.05% (vol/vol) Coomassie brilliant blue G-250 (Sigma), 10% (vol/vol) acetic acid, and 40% (vol/vol) water, followed by multiple changes, until a minimal background was achieved. The protein bands were excised from the gel and subjected to in-gel trypsin digestion. The extracted peptide fragments were then analyzed by tandem mass spectrometry as previously described (17).