Sumoylation and ubiquitinylation reversibly regulate the activity of 
transcription factors through covalent attachment to lysine residues of target 
proteins. We examined whether the Ets-1 transcription factor is modified by 
sumoylation and/or ubiquitinylation. Among four potential SUMO motifs in Ets-1, 
we identified lysines 15 and 227 within the LK(15)YE and IK(227)QE motifs, as 
being the sumoylation acceptor sites. Using transfection of Ets-1 wildtype (WT) 
or its sumoylation deficient version (Ets-1 K15R/K227R), as well as WT or mutant 
proteins of the SUMO pathway, we further demonstrated that the E2 
SUMO-conjugating enzyme Ubc9 and a E3 SUMO ligase, PIASy, can enhance Ets-1 
sumoylation, while a SUMO protease, SENP1, can desumoylate Ets-1. We also found 
that Ets-1 is modified by K48-linked polyubiquitinylation independently of the 
sumoylation acceptor sites and is degraded through the 26S proteasome pathway, 
while sumoylation of Ets-1 does not affect its stability. Finally, sumoylation 
of Ets-1 leads to reduced transactivation and we demonstrated that previously 
identified critical lysine residues in Synergistic Control motifs are the 
sumoylation acceptor sites of Ets-1. These data show that Ets-1 can be modified 
by sumoylation and/or ubiquitinylation, with sumoylation repressing 
transcriptional activity of Ets-1 and having no clear antagonistic action on the 
ubiquitin-proteasome degradation pathway.