Fig. 7: Time-dependent synchrotron SAXS data at 0.6 and 1.8 mM Mg2+ in the wide-spacing (Bws) and intermediate (Bint) microtubule (MT) bundle states reveal prevalence of rings in the Bint state.
A SAXS data (open circles, profiles offset for clarity) and corresponding fits (solid lines) of a sample at 0.6 mM Mg2+ (below clower ≈ 1.6 mM Mg2+) in the Bws state (2D Hexagonal peaks indexed for profile at t0 + 3 hrs). Dashed lines are non-MT scattering contribution obtained from fits. Bottom profile (solid blue curve) depicts the form factor of a MT. B Data from A plotted without offset. Solid arrows point to MT form factor minima, highlighting nominal change in non-MT scattering over time. (C) SAXS data (open circles, profiles offset for clarity) and corresponding fits (solid lines) at 1.8 mM Mg2+ (between clower ≈ 1.6 mM Mg2+ and cupper ≈ 2.4 mM Mg2+) show a transition with increasing time from the Bws (profiles at t0 + 3 hrs and t0 + 12 hrs) to the Bint (profiles at t0 + 15 hrs and t0 + 18 hrs) and to the tubulin ring state (profile at t0 + 33 hrs). 2D Hexagonal peaks indexed for profiles at t0 + 12 hrs (Bws) and t0 + 15 hrs (Bint). D Data from C without offset. An abrupt increase in tubulin ring scattering fills in the minima of the MT Form Factor (open arrows) at the transition between t0 + 12 and t0 + 15 h. The fit lines in A–D are color-coded (green, yellow, and magenta represent the Bws, Bint, and tubulin ring states, respectively). Parallel whole-mount TEM image taken at t0 + 18 hrs of a sample prepared with 1.8 mM Mg 2+ added, showing E coexistence of tubulin rings with bundled MTs and F approximate dimensions of single tubulin rings. Samples contained the stated Mg2+ concentrations added to PIPES buffer at pH 6.8, which includes 1 mM of Mg2+. All data shown is provided in the Source Data file.
