Water-Mediated Phosphoryl Wires Stabilize Pathological Tau Fibrils

Hyperphosphorylation of tau is a hallmark of tauopathies, with specific phosphorylation sites elevated in pathological fibrils. However, the molecular role of this post-translational modification (PTM) in driving tau aggregation remains unclear. In-register fibril assembly places phosphoryl groups on adjacent monomers at ∼4.8 Å spacing, requiring an energetically favorable arrangement. Conventional intuition holds that closely packed phosphoryl groups should be electrostatically unfavorable. We test the opposing hypothesis: that phosphoryl groups within the fibril core associate into an extended "wire" that stabilizes the amyloid fibril. We examined two phosphorylation sites linked to neurodegeneration, serine 305 (S305^p) and tyrosine 310 (Y310^p), using seeding-competent fibrils of the tau peptide jR2R3-P301L. Multiple-quantum spin counting (MQ-SC) by ^{3 1}P solid-state NMR with dynamic nuclear polarization (DNP) revealed at least six phosphorus spins linearly arranged within a protofibril, consistent with a MQ coherence order of four. Molecular dynamics simulations identified water-mediated phosphoryl wire geometries, and 2D ¹H-^{3 1}P heteronuclear correlation NMR confirmed water-bridged phosphoryl-phosphoryl contacts. Denaturation experiments showed that S305 phosphorylation increased fibril stability relative to the unmodified peptide. These findings show that phosphorylation within the tau fibril core promotes fibril registry and stability through water-mediated, hydrogen-bonded phosphoryl wires, which may be a structural signature for next-generation pathological tau binders.