ON THE NATURE OF THE TAU R406W MUTATION: INCREASED CIS PSER404-PRO405 AMIDE BOND IN TAU R406W DUE TO A PHOSPHORYLATION-SENSITIVE CIS-PROLINE-AROMATIC INTERACTION

Abstract

Tau aggregation as the neurofibrillary tangles observed in Alzheimer's disease is associated with phosphorylation-induced changes in tau structure that promote protein aggregation. The N-terminus and C-terminus of tau interact with the hydrophobic tubulin-binding domains (TBDs), stabilizing their structure. The proline-rich domain is between the N-terminus and TBDs and serves as a linker that allows their dynamic interaction. Phosphorylation and O-GlcNAcylation have opposing functional effects on tau, with phosphorylation promoting tau aggregation and O-GlcNAcylation opposing tau aggregation. At the C-terminus, the R406W mutation accelerates tau aggregation, but the mechanistic basis of this observation is not well understood. Residue 406 of tau follows Ser404-Pro405, with Ser404 phosphorylation observed pathologically and in animal models of Alzheimer's disease. The prolyl isomerase Pin1, which binds and isomerizes pSer-Pro sequences, implicates prolyl isomerization in misfolding of hyperphosphorylated tau. The structural effects of phosphorylation versus O-GlcNAcylation were examined in the tau proline-rich domain and in the C-terminal domain by circular dichroism and heteronuclear NMR spectroscopy via examination of non-phosphorylated, phosphorylated, and O-GlcNAcylated peptides comprising tau174-251 and tau395-411. Peptides were also examined with the R406W mutation in both non-phosphorylated and phosphorylated forms. Phosphorylation was found to induce a strong disorder-to-order transition in the proline-rich domain, with special ordering observed on the residues pT175, pT181, pT212, pT217, and pT231. Phosphothreonine adopts a stable, ordered structure with a strong hydrogen bond to its own amide. O-GlcNAcylation opposed this structural change. We hypothesized that mutation of R406 to tryptophan (R406W) would lead to increased cis amide bond. R406W tau peptides exhibited increased population of cis amide bond, due to a favorable H a -cis-Pro-aromatic C-H/p interaction. Phosphorylation of Ser404 further increased the population of cis-proline at pSer404-Pro405, with greatest effect in R406W. These data suggest a disorder-to-order transition upon phosphorylation in the proline-rich domain as a basis for changes in the dynamic structure of tau. R406W mutation results in higher populations of cis amide bond. Phosphorylation of Ser404 leads to further increases in population of cis amide bond, particularly in R406W. Cis amide bond prevents phosphatase activity, and therefore tau-R406W induces both greater structural changes and longer phosphorylation residence times.