Phosphorylation of S396 and S400 Promotes Tau Self-Assembly and Favors the Chronic Traumatic Encephalopathy (CTE) Protofilament Fold

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Abstract
Tau hyperphosphorylation is linked to tauopathy aggregates, but the effects of individual sites on tau assembly remain unclear. Here, we used protein semisynthesis to generate defined Tau(297–407) proteoforms containing specific combinations of phosphorylation within the PHF1 epitope. Spontaneous aggregation revealed that phosphorylation of S396, and S400 to a lesser extent, promoted nucleation, while phosphorylation of T403 and S404 suppressed assembly. A similar reactivity trend of pS396 > pS400 > WT > pT403 > pS404 was observed for seeded assembly, and all proteoforms, even anti-aggregation ones, were incorporated. The fibrils have similar thermodynamic stability, suggesting that phosphorylation selectively tunes reactivity and not thermodynamics. Cryo-EM revealed that pS400 produces a chronic traumatic encephalopathy (CTE) protofilament conformation. Strikingly, one of the structures observed in the pS400 sample appeared to capture a secondary nucleation step. Together, these studies reveal the importance of positional effects of phosphorylation on tau self-assembly.

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