Crystalline actin sheets: their structure and polymorphism.

Crystalline sheets of Acanthamoeba actin induced by the trivalent lanthanide gadolinium exist in three different polymorphic forms, which show different striation patterns and surface topographies. We have called these different forms "rectangular" and "square" sheets, and "cylinders" and have shown that each of the three forms is constructed from common "basic" lattices associated in different ways. We have used image processing of electron micrographs 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 x 3.3 x 4.5 nm, and the molecule itself appears distinctly bilobed with the two lobes separated by a cleft. actin monomers in the sheets are arranged with P2 symmetry and are therefore packed in a manner different from that of the molecules in actin filaments. Because approximately 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.