Effect of Phosphorylation and O-GlcNAcylation on Proline-Rich Domains of Tau

The microtubule-associated protein Tau (MAPT) is a phosphoprotein in neurons of the brain. Aggregation of Tau is the leading cause of tauopathies such as Alzheimer's disease. Tau undergoes several post-translational modifications of which phosphorylation and O-GlcNAcylation are key chemical modifications. Tau aggregates into paired helical filaments and neurofibrillary tangles upon hyperphosphorylation, whereas O-GlcNAcylation stabilizes the soluble form of Tau. How specific phosphorylation and/or O-GlcNAcylation events influence Tau conformations remains largely unknown due to the disordered nature of Tau. In this study, we have investigated the phosphorylation- and O-GlcNAcylation-induced conformational effects on a Tau segment (Tau_225-246) from the proline-rich domain (P2), by performing metadynamics simulations. We study two different phosphorylation patterns: Tau_225-246, phosphorylated at T231 and S235, and Tau_225-246, phosphorylated at T231, S235, S237, and S238. We also study O-GlcNAcylation at T231 and S235. We find that phosphorylation leads to the formation of strong salt-bridge contacts with adjacent lysine and arginine residues, which disrupts the native β-sheet structure observed in Tau_225-246. We also observe the formation of a transient α-helix (²³⁸SAKSRLQ²⁴⁴) when Tau_225-246 is phosphorylated at four sites. In contrast, O-GlcNAcylation shows only modest structural effects, and the resultant structure resembles the native form of the peptide. Our studies suggest the opposing structural effects of both protein post-translational modifications (PTMs) and the importance of salt bridges in governing the conformational preferences upon phosphorylation, highlighting the role of proximal arginine and lysine upon hyperphosphorylation.