Evidence for overlapping myosin heads on relaxed thick filaments of fish, frog, and scallop striated muscles.

The organization of surface subunits on muscle thick filaments has been demonstrated by three-dimensional reconstructions of electron microscopic images. Rarely, however, do the resolutions attained allow unequivocal assignment of orientation to individual myosin heads within the subunits. Using a bifunctional agent to cross-link the active sites of nearest-neighbor myosin heads, we recently demonstrated that the two heads within each surface subunit of relaxed Limulus thick filaments are oppositely oriented and arise from axially sequential myosin molecules. Here, the effect of similarly cross-linking nearest-neighbor myosin heads with the bifunctional agent 3,3'-dithio-bis[3'(2')-O-(6-propionylamino)hexanoyl]adenosine 5'-triphosphate in the presence of vanadate was studied on thick filaments from scallop, goldfish, and frog striated muscles. After incubation on high salt, treated filaments remained intact but untreated filaments and those with severed cross-links dissolved. This indicates that intermolecular bonds, formed between active sites, prevented myosin disaggregation. Optical transforms of images of treated, intact filaments showed retention of many features of patterns obtained from relaxed filaments, including layer-line reflections. Thus, it seems most likely that the two heads originating from a single myosin molecule are widely splayed and each overlaps with one originating from an axially sequential molecule on these, as on Limulus, filaments. These findings are discussed with respect to other structural and functional parameters of different muscles.