Figure 5. Synchrotron SAXS data of taxol-stabilized microtubule bundles (B_MT) and bundles of inverted tubulin tubules (B_ITT).

a, Bottom profile shows synchrotron small-angle-x-ray-scattering (SAXS) data of taxol-stabilized microtubules (MT). The second through fourth profiles from bottom are SAXS data from room temperature samples taken 10 days after mixing increasing amounts of spermine with MTs: 5 mM spermine shows 2D hexagonal bundles of MTs (B_MT); 15 mM spermine shows coexistence of the B_MT phase with the new phase of bundles of inverted tubulin tubules (B_ITT, arrow points to first order diffraction peak); and 30 mM spermine shows the B_ITT phase. Top profile shows SAXS of the B_ITT phase formed at ≈ 2.5 ± 1.5 °C 12 hours after placing microtubules in the B_MT phase, with 2.5 mM spermine. b, Three scattering profiles from (a) (bottom two and top curves) after background subtraction with fitted model scattering curves (solid lines) as described in the text. Twelve peaks of the B_ITT phase can be indexed to a 2D hexagonal lattice. c, Expanded high q region showing comparison of scattering data to a model where the inverted tubulin columns consist of either helical protofilaments with a tight pitch (solid line, which fits the data well) or stacks of rings of curved protofilaments (dotted line, which does not fit the data). All samples were at taxol/tubulin molar ratio = 0.55.