A study on the structure of paracrystals of F-actin.

The structure of three types of paracrystals formed by a muscle protein, actin, was studied by electron microscopy using the technique of optical diffraction and filtering methods.The type I paracrystal of F-actin⁴ had a flat net structure and each thread of the net appeared to be made of a single double-stranded filament of F-actin. Its unit cell was rhombic with sides of about 340 Å in length. The narrower angle of the rhomb was about 30 °. A side of the rhomb corresponded to one repeating unit of F-actin. The cross-connecting point of the net appeared to occur at a cross-over point of the double helical F-actin filament when the paracrystal plane was observed perpendicularly. A set of parallel filaments running in one direction seem to simply overlie another set of parallel filaments running in another direction.The type II paracrystal also had a flat net structure with a unit cell of the same size and shape as type I, but had twice the amount of material in the unit cell in comparison with that of type I; a thread of type II was made of a pair of F-actin filaments. The type II paracrystal seemed to be made by attaching the F-actin filaments side-by-side to filaments of the type I paracrystal. These newly associated filaments cross-connected with each other in the same manner as those of the type I paracrystal.The type III paracrystal was a side-by-side aggregate of F-actin filaments. There was no lateral order between the neighbouring filaments.