Modulating the Curvature of Protein Self-Assembled Spiral Nanotubules.

Structural transformations from ribbons to twisted ribbons to helical ribbons are often observed across supramolecular assemblies and macroscopic structures and can be described under a consistent theoretical framework. Conical molecular self-assembled structures, however, are rarely observed, may require more than one subunit, their dimensions are hard to control, and are poorly understood. Cytoskeleton microtubule (MT) is a dynamic protein-polymer that self-assembles from αβ-tubulin heterodimer, providing mechanical support to Eukaryotic cells. Colchicine is a drug known to bind the exchangeable nucleotide site on the β-tubulin subunit and suppress MT assembly. The tetravalent polyamine spermine promotes MT assembly and tubulin spiral structures, including conical tubulin spirals, tubules of conical spirals, and inverted helical tubules. Here we show how colchicine as a single agent suppressed MT and tubulin single ring assembly already at substoichiometric concentrations, whereas in the presence of spermine, the tubulin-colchicine stoichiometry controlled the dimensions and curvature of tubulin spiral assemblies. At a fixed spermine concentration, the concentration of colchicine modulated the radii of the nanotubular structures. The radii of the inverted helical nanotubules and conical spiral nanotubules monotonically decreased with colchicine concentration. We attribute our observation to the increased curvature of the tubulin dimer subunit induced by colchicine.