Figure 1. Schematic of a spermine (4+)-induced inversion process from bundles of taxol-stabilized microtubules (B_MT) to bundles of inverted tubulin tubules (B_ITT).

a and b, Taxol-stabilized microtubules (MTs, a) may be induced to form MT bundles above a critical concentration of spermine (4+) counterions (B_MT, b). The bundles result from the nonspecific electrostatic attraction between spermine coated MTs. c and d, For concentrations several times larger than the critical bundling concentration a specific spermine-triggered straight-to-curved conformation transition in protofilaments, leads to MT disassembly into curved protofilaments (c-PFs) within the bundles (c). Concurrent to MT disassembly spermine counterions induce non-specific assembly of c-PFs into the B_ITT phase (d). Both phases are hierarchically ordered, liquid crystalline nanotubes, but the tubes are inverted: the tubulin surface, which is on the inside of the tubes in the B_MT phase is on the outside in the B_ITT phase.