Global Identification and Characterization of Both O-GlcNAcylation and Phosphorylation at

reversible monosaccharide modifier of serine and threo-nine residues on intracellular protein domains. Crosstalk between O-GlcNAcylation and phosphorylation has been hypothesized. Here, we identified over 1750 and 16,500 sites of O-GlcNAcylation and phosphorylation from mu-rine synaptosomes, respectively. In total, 135 (7%) of all O-GlcNAcylation sites were also found to be sites of phosphorylation. Although many proteins were exten-sively phosphorylated and minimally O-GlcNAcylated, proteins found to be extensively O-GlcNAcylated were almost always phosphorylated to a similar or greater extent, indicating the O-GlcNAcylation system is specif-ically targeting a subset of the proteome that is also phosphorylated. Both PTMs usually occur on disordered regions of protein structure, within which, the location of O-GlcNAcylation and phosphorylation is virtually ran-dom with respect to each other, suggesting that nega-tive crosstalk at the structural level is not a common phenomenon. As a class, protein kinases are found to be more extensively O-GlcNAcylated than proteins in gen-eral, indicating the potential for crosstalk of phosphor-ylation with O-GlcNAcylation via regulation of enzymatic activity. Molecular & Cellular Proteomics 11: 10.1074