Fig. 3. R′ binds both dynamically unstable MTs.

(A) 2D NCα spectrum of tubulin-tau (1:2) coassembly (blue), overlaid with the spectrum of taxol-stabilized MT-P2R tau (1:1) complex (red). (B) 2D CC correlation spectrum of tubulin-tau coassembly. Joint analysis of the two 2D spectra allows assignment of the immobilized residues. Thr, Asn, Asp, Ala, and Ile signals are observed. (C) ¹⁵N CP spectra of P2R tau bound to unstable (top) and stabilized (bottom) microtubules. Arginine side-chain signals are detected in both spectra, indicating that some Arg side chains are immobilized by the MTs. The Gly ¹⁵N signals are broadened in the coassembled sample relative to the taxol-stabilized sample. (D) Amino acid sequence of P2R tau and the location of key residue types that identify the immobilized segment in the tubulin:tau coassembly. R′ is the only segment that contains all residue types (Ala, Arg, Thr, Ile, Tyr/Phe, and Asp/Asn) in the dipolar spectra. The cryo-EM reported MT-bound R1 and R2 segments in chimeric tau constructs are underlined.