The overlap between the thin- and thick-filament reflections in the small-angle X-ray diffraction pattern from a molluscan smooth muscle

Small-angle X-ray diffraction studies of the living anterior byssus retractor muscle (ABRM) of Mytilus edulis have been carried out using a long (2.2 m) point-focusing camera to investigate the overlap between the thin- and thick-filament reflections. While the reflections due to the cross-bridge lattice on the thick filaments in the resting ABRM are distant in the axial direction from the thin-filament layer lines, the paramyosin cores of the thick filaments produce reflections very close to the thin-filament reflections at axial spacings of 387, 59 and 51 Å. Using such a long camera it becomes possible to separate these reflections due to an improvement in the angular resolution and increase in the difference between the widths of the thin- and thick-filament reflections, and it is shown that the intensities of the 387 and 59 Å thin-filament layer lines gradually drop to zero close to the meridian. It becomes relatively easy to observe the thin-filament-based X-ray pattern by use of the long camera, making it possible to show a partial resemblance of the small-angle X-ray pattern from the contracting ABRM with the pattern from the rigorized ABRM.