Phosphorylation Alters Long-Range Intramolecular Interactions in Tau

Tau is an intrinsically disordered microtubule-associated protein with a role in modulating microtubule stability and growth in the axons of neurons. In Alzheimer’s disease and other tauopathies, tau forms fibrillar aggregates. Phosphorylation within tau’s proline rich region (PRR) is widely used as a biomarker in disease, but also has an important role in regulating tau’s functional interactions with tubulin and microtubules. Here, we use a “hyperphosphorylated” tau phosphomimic to investigate how intramolecular electrostatic interactions between tau’s N-terminal domain (NTD) and its PRR and microtubule binding region (MTBR) are impacted by phosphorylation. We use single-molecule Förster resonance energy transfer (smFRET) to characterize the interaction between the NTD and the PRR/MTBR in unmodified and ‘hyperphosphorylated’ tau. We find that phosphomimics in the PRR diminish long-range intramolecular electrostatic interactions between the NTD and PRR/MTBR. Donor lifetime analysis of smFRET bursts reveals fast (ns to μs) dynamics between the NTD and PRR/MTBR that are screened with increasing salt. Coarse-grained molecular dynamics simulations demonstrate that ionic screening shifts the NTD-PRR from compact to expanded conformational ensembles. Overall, our findings suggest that PRR phosphorylation may contribute to regulation of tau function by reshaping electrostatically driven interactions with the NTD.