Figure 1: Tau-mediated microtubule assemblies and Tau charge distribution.
(a) Cartoon showing Tau binding to a microtubule (red/blue) through its microtubule-binding region (yellow), with the projection domain (green/purple) and CTT (grey/teal) extending off the microtubule surface. (b) The average charge (dark blue for anionic character and dark grey for cationic character) of fully expressed 4RL Tau (top) as a function of primary sequence, with alternative splicing of exons 2 (red rectangle), 3 (orange) and 10 (blue) resulting in the five additional wild-type isoforms. Wild-type Tau consists of the amino-terminal tail (NTT), which includes the projection domain (PD, green/purple background) and proline-rich region (PRR, grey), followed by the microtubule-binding region (MTBR, yellow) and carboxyl-terminal tail (CTT, grey/teal). Truncated Tau constructs were designed to understand the domain dependence of the CTT (3RSΔC, missing CTT), anionic component of the projection domain (3RΔ(N-), missing anionic block of NTT) and the entire NTT (3RΔN, missing NTT). (c) Prior electron microscopy revealed linear microtubule bundles in the axon initial segment (adapted from Peters et al.5 and reprinted by permission of Oxford University Press, USA). (d) Subsequent Tau cDNA transfection of Sf9 cells revealed hexagonal arrays of microtubules in neurite-like processes (adapted from Chen et al.8 and reprinted by permission from Macmillan Publishers Ltd: Nature 360, 674–677, ©1992). (e) While individual microtubules (arrows) can be identified via differential interference contrast (DIC) microscopy, no discernible bundles form in paclitaxel-free microtubules without Tau. (f) With centrifugation (9,500g for 30 min at 37 °C), microtubules form clear and stable bundles when assembled with Tau (Φ3RL=1/20), demonstrating an attractive component of the Tau-mediated microtubule–microtubule interaction. Scale bars, 250 nm (c); 500 nm (d); 20 μm (e–f).
