Symmetry and molecular arrangement in paracrystals of reconstituted muscle thin filaments.

Abstract

Paracrystals of thin filaments prepared from actin, tropomyosin and troponin (actin + TM:TN have different filament structure and packing at high Ca2+ concentrations (>10−5 m) than at low (<10−6 m). In the former condition, the paracrystals have periodic transverse stripes, adjacent filaments have alternate polarity, and the presence of tropomyosin in the long-pitch helical grooves of actin can be discerned. In the latter, there are no transverse stripes, all filaments have the same polarity and the paracrystals appear identical to those of pure actin. These features are revealed by electron microscopy, optical diffraction and filtering and, for the demonstration of polarity, by the “decoration” of the filaments with heavy meromyosin.