OGlcNAcylation and phosphorylation have opposing structural effects in tau: Phosphothreonine induces particular conformational order

Phosphorylation and OGlcNAcylation are dynamic intracellular protein post-translational modifications that frequently are alternatively observed on the same serine and threonine residues. Phosphorylation and OGlcNAcylation commonly occur in natively disordered regions of proteins, and often have opposing functional effects. In the microtubule-associated protein tau, hyperphosphorylation is associated with protein misfolding and aggregation as the neurofibrillary tangles of Alzheimer's disease, whereas OGlcNAcylation stabilizes the soluble form of tau. A series of peptides derived from the proline-rich domain (residues 174-251) of tau was synthesized, with free Ser/Thr hydroxyls, phosphorylated Ser/Thr (pSer/pThr), OGlcNAcylated Ser/Thr, and diethylphosphorylated Ser/Thr. Phosphorylation and OGlcNAcylation were found by CD and NMR to have opposing structural effects on polyproline helix (PPII) formation, with phosphorylation favoring PPII, OGlcNAcylation opposing PPII, and the free hydroxyls intermediate in structure, and with phosphorylation structural effects greater than OGlcNAcylation. For tau_196-209, phosphorylation and OGlcNAcylation had similar structural effects, opposing a nascent α-helix. Phosphomimic Glu exhibited PPII-favoring structural effects. Structural changes due to Thr phosphorylation were greater than those of Ser phosphorylation or Glu, with particular conformational restriction as the dianion, with mean ³J_αN = 3.5 Hz (pThr) versus 5.4 Hz (pSer), compared to 7.2, 6.8, and 6.2 Hz for Thr, Ser, and Glu, respectively, values that correlate with the backbone torsion angle. Dianionic phosphothreonine induced strong phosphothreonine amide protection and downfield amide chemical shifts (δ_mean = 9.63 ppm), consistent with formation of a stable phosphate-amide hydrogen bond. These data suggest potentially greater structural importance of threonine phosphorylation than serine phosphorylation due to larger induced structural effects.