Invertebrate myosin filament: subfilament arrangement in the wall of tubular filaments of insect flight muscles.

Transverse sections (100 to 140 nm thick) of the flight muscles of the fleshfly Phormia terrae-novae and the housefly Musca domestica were studied. The images of 56 tubular myosin filaments of the fleshfly and 62 filaments of the housefly were digitized and computer processed by rotational averaging. The rotational power spectra of more than 80% of the filaments showed peaks for 6-fold rotational frequency. The average of these images for each species showed a characteristic pattern consisting of 12 subunits arranged in six pairs around the wall of the filament. This pattern was enhanced by rotationally filtering the average images using the 6-fold components of the rotational power spectrum. On tilting individual images, the subunits behaved like rods perpendicular to the plane of the transverse section and they were therefore considered to be subfilaments essentially parallel to the long axis of the filament. The center-to-center spacing between the subfilaments of a pair is 2.8 nm, and the center-to-center spacing between the adjacent subfilaments of neighboring pairs is 4.0 nm. The observation of 12 subfilaments is consistent with a four-stranded helical arrangement of myosin cross-bridges on the surface of the filaments.