Mishra et al. Cell Communication and Signaling 2011, 9:1 http://www.biosignaling.com/content/9/1/1 HYPOTHESIS Open Access O-GlcNAc modification: why so intimately associated with phosphorylation? Suresh Mishra1,2*, Sudharsana R Ande1, Neil W Salter2 Abstract Post-translational modification of proteins at serine and threonine side chains by b-N-acetylglucosamine (O-GlcNAc) mediated by the enzyme b-N-acetylglucosamine transferase has been emerging as a fundamental regulatory mechanism encompassing a wide range of proteins involved in cell division, metabolism, transcription and cell signaling. Furthermore, an extensive interplay between O-GlcNAc modification and serine/threonine phosphorylation in a variety of proteins has been reported to exist. However, our understanding of the regulatory mechanisms involved in O-GlcNAc modification and its interplay with serine/threonine phosphorylation in proteins is still elusive. Recent success in the mapping of O-GlcNAc modification sites in proteins as a result of technological advancement in mass spectrometry have revealed two important clues which may be inherently connected to the regulation of O-GlcNAc modification and its interplay with phosphorylation in proteins. First, almost all O-GlcNAc modified proteins are known phospho proteins. Second, the prevalence of tyrosine phosphorylation among O-GlcNAc modified proteins is exceptionally higher (~68%) than its normal occurrence (~2%) alone. We hypothesize that phosphorylation may be a requisite for O-GlcNAc modification and tyrosine phosphorylation plays a role in the interplay between O-GlcNAc modification and serine/threonine phosphorylation in proteins. In other words, the interplay between O-GlcNAc modification and phosphorylation is not limited to serine/threonine phosphorylation but also includes tyrosine phosphorylation. Our hypothesis provides an opportunity to understand the underlying mechanism involved in O-GlcNAc modification and its interplay with serine/threonine phosphorylation in proteins. Furthermore, implication of our hypothesis extends to tyrosine kinase signaling. Background at serine/threonine residues, the potential for interplay O-GlcNAc cycling in proteins, mediated by the enzymes between serine/threonine phosphorylation and O-GlcNAc transferase (OGT) and O-GlcNAc amidase O-GlcNAc modification has been realized very early on (OGA), is dynamically regulated in response to various [4]. Then it was indeed found to be the most common stimuli and is remarkably similar to phosphorylation [1]. feature associated with O-GlcNAc modification and It has been more than 25 years since it was first discov- attributed to the further development in this area [5-7]. ered by Torres and Hart [2]. However, relative develop- However, unlike phosphorylation which is regulated by ment in this field has remained sluggish for almost two hundreds of kinases and phosphatases, O-GlcNAc decades, mainly due to the lack of tools and techniques cycling has only two mediators: OGT and OGA [1]. for the identification and quantification of O-GlcNAc This would imply that there must be fundamental differ- modification in proteins. As a result, our knowledge of ences in the way O-GlcNAc modification in proteins is the site-specific functions of O-GlcNAc modified pro- regulated in relation to phosphorylation. teins is very limited. Development of an O-GlcNAc spe- cificantibodyin2001byHartandcolleagues[3]has Why the Occurrence of Tyrosine Phosphorylation added some momentum in this field and contributed among O-GlcNAc Modified Proteins is significantly in the identification of O-GlcNAc modified Exceptionally High proteins. As O-GlcNAc modificationinproteinoccurs Earlier evidence regarding the interplay between O-GlcNAc modification and serine/threonine phosphory- * Correspondence: [email protected] lation has pointed towards an inverse relationship 1Department of Internal Medicine University of Manitoba, Winnipeg, Canada Full list of author information is available at the end of the article between these two modifications [4-6]. However, © 2011 Mishra et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Mishra et al. Cell Communication and Signaling 2011, 9:1 Page 2 of 4 http://www.biosignaling.com/content/9/1/1 emerging evidence suggests that the relationship between other. It is of note that the common features surrounding O-GlcNAc modification and serine/threonine phosphory- O-GlcNAc modification sites described here and before lation is more extensive than initially thought [7,8]. [9], which are apparent in the primary structure of a pro- Recently we have reported that tyrosine phosphorylation tein, can also be achieved or constituted in the secondary interacts with O-GlcNAc modification, a phenomenon structure of proteins by residues present distantly from which was previously not known [9]. Subsequently, two each other. Intriguingly such a high prevalence of tyro- more articles were published showing that O-GlcNAc sine phosphorylation among O-GlcNAc modified pro- modification of insulin receptor substrate 1 (IRS1) occurs teins, which is many folds higher than its normal in close proximity of tyrosine phosphorylation sites and occurrence [~2%, [13]], further supports our hypothesis affects the tyrosine phosphorylation dependent function of a role of tyrosine phosphorylation in this dynamic of IRS1 [10,11]. Taken together, these evidences would process. Furthermore, a recent report on the recruitment suggest that the interaction between O-GlcNAc modifi- of OGT in response to insulin stimulation and the sub- cation and phosphorylation is not limited to serine/threo- sequent O-GlcNAc modification of insulin signaling nine phosphorylation (as initially thought) though rather intermediates as a part of an intrinsic mechanism also includes tyrosine phosphorylation. To further sub- involved in the attenuation of insulin’styrosinephos- stantiate our hypothesis of the interaction between tyro- phorylation dependent signaling also support our sine phosphorylation and O-GlcNAc modification, we hypothesis [14]. analyzed the tyrosine phosphorylation status of all O-GlcNAc modified proteins curated at PhosphoSite- Is Phosphorylation a Requisite for O-GlcNAc ® Plus http://www.phosphosite.org along with phospho- Modification proteomes [12]. Analysis of O-GlcNAc modified proteins The wide range of simultaneous changes in O-GlcNAc revealed that 68.02% of them are known to be tyrosine modification in a number of proteins under different phosphorylated (Figure 1A). As our knowledge of phos- experimental conditions as previously observed by phoproteomes is currently increasing rapidly it is Wang et al [15] and in a recent report by Gu et al [16], expected that this percentage will increase further. Most could not be explained by changes in the expression importantly, 65.11% of the O-GlcNAc modified proteins and/or the activity of O-GlcNAc cycling enzymes. Espe- were found to be serine/threonine and tyrosine phos- cially when there is only one OGT and one OGA [1]. It phorylated (Figure 1A). This would mean that all is possible that the phosphorylation status of proteins is O-GlcNAc modified proteins which are tyrosine phos- the major determinant of subsequent O-GlcNAc modifi- phorylated are also serine/threonine phosphorylated. cation (i.e. regulation directed by the substrate itself). FurtheranalysisofmotifsaroundO-GlcNAc modifica- This may be mediated by the recruitment of various tion sites revealed that in the majority of the cases these interacting partners in the form of a protein complex, dynamic modifications occur in close proximity of each which may modulate the binding and/or the catalytic activity of O-GlcNAc cycling enzymes. A prerequisite forthishypothesisisthatallO-GlcNAc modified pro- pY teins must be phosphoproteins. To confirm if this is the 117 AB(68.02% case, we checked the phosphorylation status of all O-GlcNAc modified proteins curated at PhopshoSite- O- GlcNAcylated ® 29 (100%) Plus database along with phosphoproteomes [12]. PNR pS/pT/pY gS/gT O 7 112 Interestingly, ~93% of the -GlcNAc modified proteins 172 Tyrosine (4.06%) (65.11%) Phosphory- (100%) phosphory- lated are known phosphoproteins (Figure 1A). The phosphor- lated 28 (96.5%) 27 (93.1%) ylation status of ~4% of the O-GlcNAc modified pro- pS/pT teins is not known (Figure 1A). As the number of 159 proteins in phosphoproteomes are currently increasing (92.44%) rapidly it is expected that this minor gap will be further Figure 1 Venn diagrams showing exceptionally high prevalence dwindled down. Therefore, a possibility of phosphoryla- of tyrosine phosphorylation in O-GlcNAc modified proteins. (A) tion as a requisite for O-GlcNAc modification may not Serine, threonine and tyrosine phosphorylation status of all known ® be ruled out. O-GlcNAc modified proteins curated at PhosphoSitePlus http:// O www.phosphosite.org (B) Phosphorylation (i.e. pS/pT/pY) and tyrosine It appears that -GlcNAc modification of proteins phosphorylation status of dynamically altered O-GlcNAc modified (a occurs in a small subset of phosphoproteins. This raises total of 29 proteins) in response to inhibition of glycogen synthase another question: what makes a small subset of phos- kinase-3 (GSK-3). gS, GlcNAcylated serine; gT, GlcNAcylated threonine; phoproteins