Paracrystals of myosin rod.
To help understand the packing of myosin tails in the backbone of the vertebrate striated muscle thick filament, paracrystals of myosin rod, a proteolytic fragment corresponding to the whole myosin tail, have been examined by electron microscopy and image analysis. Two types of paracrystal were observed. Type I paracrystals were similar to those seen by Moos et al. (1975; J. molec. Biol. 97, 1-9). These showed a 14-nm axial repeat, but yielded no other structural information. Type II paracrystals were long, flexible ribbons with more regularity. When negatively stained they exhibited a weak 43-nm axial striation and appeared to be composed of a layer of narrow filaments. Optical diffraction showed that the paracrystals had a rectangular unit cell of dimensions 43 nm axially and 12.4 nm laterally. Transverse sections indicated a paracrystal depth similar to the lateral dimension of the unit cell. Each unit cell contained two filaments arranged antiparallel and related by a two-fold screw axis perpendicular to the length, and in the plane of the ribbon. The filaments probably consist of parallel rod molecules related by axial displacements of 43 nm and higher multiples of 43 nm. The nature of these paracrystals indicates that the myosin tail alone can form structures like thick filament subfilaments. Their structure, based on distinguishable parallel and antiparallel rod interactions, was sensitive to pH and divalent cations in a similar way to the ionic effects on the structure of thick filaments. This behaviour suggests that some of the interactions present in the paracrystal are the same as those in the thick filament.