Characterization of O-GlcNAc cycling and proteomic identification of differentially O-GlcNAcylated proteins during G1/S transition

Background: DNA replication represents a critical step of the cell cycle which requires highly controlled and ordered regulatory mechanisms to ensure the integrity of genome duplication. Among a plethora of elements, post-translational modifications (PTMs) ensure the spatiotemporal regulation of pivotal proteins orchestrating cell division. Despite increasing evidences showing that O-GlcNAcylation regulates mitotic events, the impact of this PTM in the early steps of the cell cycle remains poorly understood. Methods and results: Quiescent MCF7 cells were stimulated by serum mitogens and cell cycle progression was determined by now cytometry. The levels of O-GlcNAc modified proteins, O-GlcNAc Transferase (OCT) and O-GlcNAcase (OGA) were examined by Western blotting and OGA activity was measured during the progression of cells towards S phase. A global decrease in O-GlcNAcylation was observed at S phase entry, concomitantly to an increase in the activity of OGA. A combination of two-dimensional electrophoresis, Western blotting and mass spectrometry was then used to detect and identify cell cycle-dependent putative O-GlcNAcylated proteins. 58 cytoplasmic and nuclear proteins differentially O-GlcNAcylated through Gl/S transition were identified and the O-GlcNAc variations of Cytokeratin 8, hnRNP K. Caprin-1, Minichromosome Maintenance proteins MCM3, MCM6 and MCM7 were validated by immunoprecipitation. Conclusions: The dynamics of O-GlcNAc is regulated during Gl/S transition and observed on key proteins involved in the cytoskeleton networks, mRNA processing, translation, protein folding and DNA replication. General significance: Our results led us to propose that O-GlcNAcylation joins the PTMs that take part in the regulation of DNA replication initiation. (C) 2012 Elsevier B.V. All rights reserved.