C O M P L E X S Y S T E M S A N D B I O M E D I C A L S C I E N C E S

S C I E N T I F I C H I G H L I G H T S

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Hierarchical assembly pathways of conical-spiral architectures

Tubulin is an essential cytoskeletal protein involved in cell division and organelle transport. Time- resolved SAXS and electron microscopy revealed the assembly pathways after mixing tubulin with spermine into hierarchical architectures, based on tubulin conical-spirals. Assemblies formed at higher spermine concentrations assembled from intermediates, similar to those formed at low spermine concentrations.

Tubulin is an essential cytoskeletal protein that assembles into microtubule, a helical filamentous superstructure containing thousands of identical copies of αb-tubulin- dimer subunits. Under a wide range of conditions, tubulin can assemble into various structural phases, including tubular (microtubule and inverted tubules), single- or double-rings, single- and double-layered, double-helix tubulin tubules, or sheets [1-3]. The large number of structures, assembled from the same basic tubulin dimer, points to a flexible-binding subunit, bearing several binding sites at multiple locations and enabling several self-association modes affected by various factors.

Fig. 50: Tubulin conical-spirals and their hierarchical assemblies. a-c) Time-resolved synchrotron SAXS data (black curves) and models (red curves) of the various structures, shown in the cartoons. Schematic illustrations of the models are also shown. Black circles represent the lattice of the tubule models; the change in colour of the circles represents the anti-parallel alignment.

d-f) Cryo-EM images of (d) short-spiral, (e) tubules of conical spirals and (f) inverted tubules. g-i) Diameter distributions of (g) short spirals, (h) conical spirals and (i) inverted tubules, derived from cryo-EM images. Units in nm.