The transcription factor Ets-1 is involved in many physiological processes, including angiogenesis, hematopoietic development, and tumor progression, and its activity can be regulated by interactions with other proteins and post-translational modifications, such as phosphorylation. Here, we show that Ets-1 is a target for SUMO modification both in vivo and in vitro. Mutational analysis reveals that sumoylation of Ets-1 occurs at two lysine residues at amino acid positions 15 and 227, which lie within previously identified synergy control motifs. Replacement of sumoylation site lysines with arginine or overexpression of SENP1, a desumoylation enzyme, enhances the transactivation ability of Ets-1. Furthermore, we identify PIASy as a novel interaction partner and a specific SUMO-E3 ligase of Ets-1. PIASy represses the Ets-1-dependent transcription, and its repression is independent of the sumoylation status of Ets-1, but it is dependent on the sumoylation of other factors. (c) 2006 Elsevier Inc. All rights reserved.