Fig. 2.

MT bundling transitions with increasing osmotic pressure are Tau isoform- and concentration-dependent. (A–C) Schematics of the (A) nematic (N_MT; with orientational order and no positional order), (B) buckled rectangular bundle (R^B_MT; characterized by the rectangular lattice constants a_R and b_R), and (C) hexagonal bundle (H_MT; characterized by the hexagonal lattice constant a_H) phases of Tau MTs, with PEO colored in yellow. (D and E) Azimuthally averaged synchrotron SAXS data of Tau MTs plotted as a function of increasing osmotic pressure induced by 20k PEO, with PEO concentration and corresponding pressure shown next to each profile. (D) A reaction mixture of the short projection domain isoform 4RS at Tau/tubulin-dimer molar ratio Φ_4RS = 1/10, exhibits an osmotic pressure-induced phase transition from nematic (N_MT) to buckled rectangular (R^B_MT; a_R = 17.0 nm and b_R = 30.7 nm) and from buckled rectangular to hexagonal (H_MT; a_H = 30.2 nm), with coexistence regimes between phases. (E) However, another mixture (long projection domain isoform 4RL at Φ_4RL = 1/10) displays a transition from nematic (N_MT) to hexagonal (H_MT; a_H = 29.3 nm) phases, bypassing the buckled rectangular phase entirely. SAXS data were analyzed and presented fit parameters extracted with fitted model scattering curves (red lines) corresponding to the model appropriate structure and form factor as described in the text.