The role of O-linked N-acetylglucosamine (O-GlcNAc) modification in the cell cycle has been enigmatic. Previously, both O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) disruption have been shown to derail the mitotic centrosome numbers, suggesting that mitotic O-GlcNAc oscillation needs to be in concert with mitotic progression to account for centrosome integrity. Here we attempted to address the underlying molecular mechanism by both chemical approaches and biological assays, and observed that Thiamet-G (OGA inhibitor) incubation strikingly elevated centrosomal distances, suggestive of premature centrosome disjuction. These aberrancies could be overcome by inhibiting Polo-like kinase 1 (Plk1), a mitotic master kinase. Plk1 inactivation is modulated by the Myosin Phosphatase Targeting Subunit 1 (MYPT1)-Protein Phosphatase 1 cβ (PP1cβ) complex. Interestingly, MYPT1 is abundantly O-GlcNAcylated and the modified residues have been detected in a recent O-GlcNAc profiling screen utilizing chemoenzymatic labeling and bioorthogonal conjugation. We demonstrate that MYPT1 is O-GlcNAcylated at T577, S585, S589 and S601, which antagonizes CDK1-dependent phosphorylation at S473, subsequently attenuating the association between MYPT1 and Plk1, and promoting PLK1 activity. Thus under high O-GlcNAc, Plk1 is untimely activated, conducive to inopportune centrosome separation and disrupting the cell cycle. We propose that too much O-GlcNAc is equally deleterious as too little O-GlcNAc, and a fine balance between the OGT/OGA duo is indispensible for successful mitotic divisions.
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