Abstract
Modification of serine and threonine residues in proteins by O-linked β-N-acetylglucosamine (O-GlcNAc) glycosylation is a feature of many cellular responses to the nutritional state and to stress. O-GlcNAc modification is reversibly regulated by O-linked β-N-acetylglucosamine transferase (OGT) and β-D-N-acetylglucosaminase (O-GlcNAcase). O-GlcNAc modification of proteins is dependent on the concentration of uridine 5'-diphospho-N-acetylglucosamine (UDP-GlcNAc), which is a substrate of OGT and is synthesized via the hexosamine biosynthetic pathway. Immunoblot analysis using the O-GlcNAc-specific antibody CTD110.6 has indicated that glucose deprivation increases protein O-GlcNAcylation in some cancer cells. The mechanism of this paradoxical phenomenon has remained unclear. Here we show that the increased glycosylation induced by glucose deprivation and detected by CTD110.6 antibodies is actually modification by N-GlcNAc(2), rather than by O-GlcNAc. We found that this induced glycosylation was not regulated by OGT and O-GlcNAcase, unlike typical O-GlcNAcylation, and it was inhibited by treatment with tunicamycin, an N-glycosylation inhibitor. Proteomics analysis showed that proteins modified by this induced glycosylation were N-GlcNAc(2)-modified glycoproteins. Furthermore, CTD110.6 antibodies reacted with N-GlcNAc(2)-modified glycoproteins produced by a yeast strain with a ts-mutant of ALG1 that could not add a mannose residue to dolichol-PP-GlcNAc(2). Our results demonstrated that N-GlcNAc(2)-modified glycoproteins were induced under glucose deprivation and that they cross-reacted with the O-GlcNAc-specific antibody CTD110.6. We therefore propose that the glycosylation status of proteins previously classified as O-GlcNAc-modified proteins according to their reactivity with CTD110.6 antibodies must be re-examined. We also suggest that the repression of mature N-linked glycoproteins due to increased levels of N-GlcNAc(2)-modified proteins is a newly recognized pathway for effective use of sugar under stress and deprivation conditions. Further research is needed to clarify the physiological and pathological roles of N-GlcNAc(2)-modified proteins.