Synchrotron small-angle X-ray scattering and electron microscopy characterization of structures and forces in microtubule/Tau mixtures

Tau, a neuronal protein known to bind to microtubules and thereby regulate microtubule dynamic instability, has been shown recently to not only undergo conformational transitions on the microtubule surface as a function of increasing microtubule coverage density (i.e., with increasing molar ratio of Tau to tubulin dimers) but also to mediate higher-order microtubule architectures, mimicking fascicles of microtubules found in the axon initial segment. These discoveries would not have been possible without fine structure characterization of microtubules, with and without applied osmotic pressure through the use of depletants. Herein, we discuss the two primary techniques used to elucidate the structure, phase behavior, and interactions in microtubule/Tau mixtures: transmission electron microscopy and synchrotron small-angle X-ray scattering. While the former is able to provide striking qualitative images of bundle morphologies and vacancies, the latter provides angstrom-level resolution of bundle structures and allows measurements in the presence of in situ probes, such as osmotic depletants. The presented structural characterization methods have been applied both to equilibrium mixtures, where paclitaxel is used to stabilize microtubules, and also to dissipative nonequilibrium mixtures at 37°C in the presence of GTP and lacking paclitaxel.