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 a s separated, negatively-stained structures. Unlike relaxed filaments o f scallops, chelicerate arthropods, or vertebrate striated muscle, all of which display a predominantly helical arrangement of surface myosi n heads, insect asynchronous flight muscle filaments appear striped, w ith cross-striations or shelves at spacings of 14.5 nm. Using a bifunc tional agent to cross-link the active sites of nearest-neighbour myosi n heads we previously demonstrated that the helical arrays on the surf aces of scallop, arthropod, fish and frog filaments are produced by th e association of two oppositely-oriented myosin heads, each of which o riginates from an axially sequential molecule within the same helical strand. The effect of similarly cross-linking nearest-neighbour heads with the bifunctional agent obis[3'(2')-O-(6-propionylamino)hexanoyl]a denosine 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 exam ined. 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 ar rangement on rabbit filaments is similar to those previously studied. Treated indirect flight muscles filaments, however, separated into dis tinct segments of variable lengths, usually multiples of 150 nm, while untreated filaments and those with severed cross-links dissolved comp letely. This implies that intermolecular associations on indirect flig ht muscles filaments most likely occur between circumferentially-adjac ent heads within each crown, but originating from different helical st rands. We interpret this difference in the relaxed orientations of spl ayed myosin heads on the two types of filament as reflecting a differe nce in functional requirements at the onset of, or during, contractile activity.