Differences in myosin head arrangement on relaxed thick filaments from Lethocerus and rabbit muscles

Relaxed thick filaments from insect asynchronous flight muscle appear different from those of other striated muscles, both in sections and as separated, negatively-stained structures. Unlike relaxed filaments of scallops, chelicerate arthropods, or vertebrate striated muscle, all of which display a predominantly helical arrangement of surface myosin heads, insect asynchronous flight muscle filaments appear striped, with cross-striations or shelves at spacings of 14.5 nm. Using a bifunctional agent to cross-link the active sites of nearest-neighbour myosin heads we previously demonstrated that the helical arrays on the surfaces of scallop, arthropod, fish and frog filaments are produced by the association of two oppositely-oriented myosin heads, each of which originates from an axially sequential molecule within the same helical strand. The effect of similarly cross-linking nearest-neighbour heads with the bifunctional agent 3,3'-dithiobis[3'(2')-O-(6-propionylamino)hexanoyl]adenosine 5'-triphosphate in the presence of vanadate on the solubility of thick filaments separated from Lethocerus indirect flight muscle (an insect asynchronous flight muscle) and rabbit psoas muscle was examined. After incubation on high salt, treated rabbit filaments retained their length and surface myosin, while untreated filaments and those with severed cross-links dissolved, indicating that the myosin head arrangement on rabbit filaments is similar to those previously studied. Treated indirect flight muscles filaments, however, separated into distinct segments of variable lengths, usually multiples of 150 nm, while untreated filaments and those with severed cross-links dissolved completely. This implies that intermolecular associations on indirect flight muscles filaments most likely occur between circumferentially-adjacent heads within each crown, but originating from different helical strands. We interpret this difference in the relaxed orientations of splayed myosin heads on the two types of filament as reflecting a difference in functional requirements at the onset of, or during, contractile activity.