AbstractPleiomorphic adenoma gene (PLAG) family proteins are oncogenes involved in various malignancies including lipoblastomas, hepatoblastomas, and acute myeloid leukemia. Overexpression of PLAGL2 induces cell transformation and proliferation, but little is known about how its activities are regulated. We previously showed that transcriptional activity of PLAGL2 is negatively regulated by sumoylation. Here we report that Tip60 modulates PLAGL2 functions through acetylation. Tip60 associates with PLAGL2 through its zinc finger domain and acetylates PLAGL2. Wild‐type but not the histone acetyltransferase (HAT)‐minus mutant form of Tip60 enhances PLAGL2‐mediated transactivation. In addition, coexpression of Tip60 and PLAGL2 completely abolishes the sumoylation of PLAGL2. Both Tip60 and DN‐Ubc9 increase transactivation activity of wild‐type but not the sumoylation deficient form of PLAGL2 (K250, 269, 356R), indicating that Tip60 acetylates PLAGL2 and abolishes the sumoylation of PLAGL2 possibly through modification of the same lysine residues (K250, 269, 356) within PLAGL2. Tip60 effects vary between different PLAGL2 target gene promoters, suggesting that Tip60 is a novel promoter‐specific coactivator of PLAGL2. This is the first demonstration that Tip60 can function as a sumoylation inhibitor in part through its intrinsic acetyltransferase activity to regulate specific gene expression. J. Cell. Biochem. 103: 730–739, 2008. © 2007 Wiley‐Liss, Inc.