SUMOylation is a reversible post-translational modification essential for genome 
stability. Using high-resolution MS, we have studied global SUMOylation in human 
cells in a site-specific manner, identifying a total of >4,300 SUMOylation sites 
in >1,600 proteins. To our knowledge, this is the first time that >1,000 
SUMOylation sites have been identified under standard growth conditions. We 
quantitatively studied SUMOylation dynamics in response to SUMO protease 
inhibition, proteasome inhibition and heat shock. Many SUMOylated lysines have 
previously been reported to be ubiquitinated, acetylated or methylated, thus 
indicating cross-talk between SUMO and other post-translational modifications. 
We identified 70 phosphorylation and four acetylation events in proximity to 
SUMOylation sites, and we provide evidence for acetylation-dependent SUMOylation 
of endogenous histone H3. SUMOylation regulates target proteins involved in all 
nuclear processes including transcription, DNA repair, chromatin remodeling, 
precursor-mRNA splicing and ribosome assembly.