Fig. 5

Identification of residues that form the K18/phospholipid oligomer core. a Valine and isoleucine spin system assignments showing the presence of two Val residues and one Ile residue in a β-strand conformation (signals shifted down and right reflect decreased Cα and increased Cβ shifts characteristic of β-strand). b Proton-nitrogen HSQC spectrum of a K18/phospholipid complex preparation in solution (red), compared with a matching spectrum of the monomeric protein (black). Resonance assignments are indicated, with red labels for the PHF6 and PHF6* regions. c Secondary carbon chemical shifts (Cα-Cα_rc + Cβ-Cβ_rc) by amino acid type for signals observed in DARR spectra of K18/phospholipid complexes. Lys-2 is marked with an asterisk to indicate that this tentatively assigned lysine residue exhibits chemical shifts consistent with a pre-proline residue, indicating that it corresponds to residue Lys-311. d–f Regions from the proton-nitrogen correlation HSQC spectra of complexes suspended in solution (red), compared with matching spectra of the monomeric protein (black). The signals from the PHF6 region (Tyr-310 and Leu-315), from the PHF6* region (Gln-276), and from the ~9 residues subsequent to each PHF motif (Val-287 and Val-318) are highly attenuated, indicating their immobilization in the oligomer core. Signals from other regions of K18 are much less affected, indicating that they remain highly flexible outside the oligomer core. Signals from Ile-297 and Ile-328 also show some attenuation, possibly resulting from their close proximity to regions within the core, which likely restricts their mobility. g Schematic of the K18 sequence and its location within full-length Tau, highlighting in red the residues that appear to form the oligomer core. The hexapeptide motifs PHF6 and PHF6* are underlined and in bold type. h Ratios of intensities for well resolved signals in HSQC spectra of protein/phospholipid complexes versus monomers, showing that the PHF6 region and subsequent ~9 residues, are most highly attenuated, but that the corresponding region in repeat 2, including the PHF6* motif and subsequent ~9 residues are also significantly attenuated compared to the remainder of the protein