Figure 4. Direct force measurements of Tau-mediated active microtubule bundles reveal distinct energy minima.

(a) Representative azimuthally averaged SAXS data for Φ_3RS=1/20 at selected increasing pressures resulting in increasing peak position of q₁₀, reflecting the decrease in the hexagonal lattice parameter a_H. (b) The measured wall-to-wall distances (D_w–w) for 3RS- and 3RL-mediated microtubule bundles show a sudden transition (≈5 nm decrease) at applied osmotic pressure P≈300–400 Pa, indicative of a secondary energy minimum for microtubule bundles. (c) A cartoon of the osmotic-pressure-induced secondary minimum at intermediate D_w–w with antiparallel dimerization occurring between the anionic section of the NTT (green) and the cationic part of the PD (purple) plus cationic proline-rich region (grey). (d) A cartoon of the widely spaced energy minimum, with Tau-mediating microtubule bundles by transient charge–charge attractions between the cationic residues (purple/grey) and the anionic residues (green) in the amino-terminal tail.