The structure of spindle-shaped paracrystals of light meromyosin

Light meromyosin paracrystals have been studied by electron microscopy combined with optical diffraction in order to understand how the tails of the myosin molecules might pack in the backbone of muscle thick filaments. The forms of paracrystal investigated were all spindle-shaped structures with an axial periodicity of either 43 nm or 14.3 nm or hybrids involving aspects of both repeats. Transverse sections show that they are not smooth but polygonal in outline. Analysis of the band patterns in negatively stained specimens indicates that the molecular arrangement in the paracrystals involves both parallel and antiparallel interactions. A parallel axial displacement of the molecules by 43 nm is intrinsic to all forms of paracrystal investigated. The principal antiparallel overlap between molecules appears to be one of 84 nm, and it is suggested that an antiparallel dimer is the structural unit in the paracrystals. The role of the interactions leading to these displacements in the formation of the thick filament backbone is discussed.