Two-dimensional crystalline arrays of periodic mesoporous nanostructures, consisting of α/β-tubulin ring-shaped oligomers in the double-layered architecture in an apolar arrangement are successfully prepared with PEM-G buffer containing Arginine nonamers and observed by electron microscopy. The ring-shaped α/β-tubulin assemblies are conformations related to a protofilament, although protofilaments are usually highly constrained in a straight conformation in microtubules. The two-dimensional crystalline arrays appear to have the structure of a square lattice with the layer group P4, the unit cell dimensions of a = b = 36.9 nm and γ = 90°. The isolated torus-like shaped projections of a pseudo-repeat structural unit are analysed with the method for single particle reconstruction. The average values of outer- and inner-diameters for the torus-like shapes are 49.0 ± 1.0, and 25.4 ± 1.1 nm, respectively. The thickness of the peripheral masses consisting of two concentric sub-rings is 11.8 ± 1.0 nm, where α/β- tubulin heterodimers appear to reside alternately. Taking the average sizes of the rings and the crystallographic unit cell dimensions into consideration, the discussion is focused on the most probable arrangement of the α/β-tubulin rings in the two-dimensional crystalline arrays.
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