“…The following observations support the notion that OGlcNAc is a regulatory post-translational modification analogous to protein phosphorylation: 1) O-GlcNAc is not further extended into more complex oligo-or polysaccharides with the exception of plant nuclear pore proteins (62,162); 2) the half-life of the O-GlcNAc modification is shorter that that of the protein it modifies (38,86,144); 3) O-GlcNAc levels respond dynamically to both extracellular (e.g., insulin) and intracellular (e.g., stress) stimuli (12,13,22,38,53,74,91,118,153,188,195); 4) sites of O-GlcNAc modification are similar to those used by protein kinases and are identical to those used by kinases on a subset of proteins (57); 5) OGlcNAc is essential for life in mammals, arabidopsis and drosophila (45,59,131,146,149), and the deletion of the O-GlcNAc transferase (OGT) is lethal in mice [embryonic day (E) 4.5] cells and some tissues (131,146); and 6) numerous studies have implicated O-GlcNAc in regulating enzyme activity, DNA binding, protein binding, localization, half-life, and regulating phosphorylation levels either by regulating protein kinases or by blocking amino acids that would otherwise be phosphorylated (58). For a subset of proteins, OGlcNAc can block phosphorylation either by modifying the same Ser/Thr residue that would usually be modified (e.g., C-Myc, Thr 58) (23,24), or sterically by modifying a nearby Ser/Thr residue (e.g., casein kinase 2, Ser347).…”