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J Proteome Res. 2008 Sep;7(9):4050-7. doi: 10.1021/pr800368m. Epub 2008 Aug 16.

Phosphorylation of SUMO-1 occurs in vivo and is conserved through evolution.

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Department of Proteomics and Signal Transduction, Max Planck Institute for Biochemistry, Am Klopferspitz 18, D-82152 Martinsried, Germany.


Protein dynamics is regulated by an elaborate interplay between different post-translational modifications. Ubiquitin and ubiquitin-like proteins (Ubls) are small proteins that are covalently conjugated to target proteins with important functional consequences. One such modifier is SUMO, which mainly modifies nuclear proteins. SUMO contains a unique N-terminal arm not present in ubiquitin and other Ubls, which functions in the formation of SUMO polymers. Here, we unambiguously show that serine 2 of the endogenous SUMO-1 N-terminal protrusion is phosphorylated in vivo using very high mass accuracy mass spectrometry at both the MS and the MS/MS level and complementary fragmentation techniques. Strikingly, we detected the same phosphorylation in yeast, Drosophila and human cells, suggesting an evolutionary conserved function for this modification. The nearly identical human SUMO-2 and SUMO-3 isoforms differ in serine 2; thus, only SUMO-3 could be phosphorylated at this position. Our finding that SUMO can be modified may point to an additional level of complexity through modifying a protein-modifier.

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