Figure 2. TEM of taxol-stabilized microtubule bundles (B_MT) and the new spermine-induced phase of bundles of inverted tubulin tubules (B_ITT).

a, A typical transmission electron microscopy (TEM) image of a taxol-stabilized microtubule bundle (10 mM spermine, room T) showing striations parallel to the cylinder axis due to the protofilaments. The bundle phase is dominant for less than 10 days. Inset, Taxol-stabilized microtubules with straight protofilaments. b and c, An example of a large bundle of inverted tubulin tubules (b, B_ITT) and a higher magnification of a smaller B_ITT (c) where protofilaments appear as striations perpendicular to the cylinder axis. TEMs are for 25 mM spermine mixed with taxol-stabilized MTs and imaged after 10 days at room T. d, Example of the B_ITT phase formed 24 hours after addition of 12.5 mM spermine at 4 °C. Arrow points to overlapping protofilament rings. In this sample preparation a sucrose cushion to remove unpolymerized tubulin (which was used for TEM samples (a–c)) was not employed and inverted tubulin tubules depicted here co-exist with double-walled structures shown in Supplementary Fig. S4 (see Sample Preparation in the Methods section). All TEMs were at taxol/tubulin molar ratio = 0.55.