Crystalline actin sheets: their structure and polymorphism.

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Abstract
Crystalline sheets of Acanthamoeba actin induced by the trivalent lanthanide gadolinium exist in 3 different polymorphic forms, which show different striation patterns and surface topographies. These different forms are called rectangular and square sheets, and cylinders. Each of the 3 forms is constructed from common basic lattices associated in different ways. Image processing of electron micrographs was used to obtain a model for the actin molecule in projection to a resolution of 1.5 nm. The overall dimensions observed in these images are 5.6 .times. 3.3 .times. 4.5 nm, and the molecule itself appears distinctly bilobed with the 2 lobes separated by a cleft. Actin monomers in the sheets are arranged with P2 symmetry and are packed differently from the molecules in actin filaments. Because .apprx. 35% of the surface area of the actin molecule is exposed on the surface of these sheets, the sheets should be useful to study the stoichiometric binding of actin-binding proteins to the actin molecule.