Structure of the myosin crossbridge lattice in insect flight muscle

Freeze fracture and deep-etching of quick-frozen insect flight muscles provides unusually clear views of thick filament projections in rigor and relaxed states. In rigor, these projections form crossbridges that are deployed helically. The tracks of these helices are left-handed, repeat every approximately 38 nm, tilt at approximately 42 degrees to the muscle axis, and, when viewed on edge, produce the unique "double chevron" pattern of crossbridges that characterizes this muscle type in thin sections (Reedy, 1968). These helical parameters substantiate Reedy's earlier deduction that rigor crossbridges form two-start helices in this muscle. On the other hand, deep-etchings of insect flight muscles relaxed with Mg-ATP before freezing do not fit with earlier results. Contrary to earlier thin section views and the expectations of X-ray diffraction, thick filaments in such relaxed muscles display no hint of a 14.5 nm axial periodicity; instead, their projections appear to be very disordered. This suggests that when crossbridges are detached, they are free to "wobble" by at least +/- 7 nm in the axial direction and thus obscure their points of origin from the thick filaments. With the images of detached crossbridges in mind, closer inspection of rigor thick filaments yields no indication of any "extra" projections between the helically deployed ones, i.e. there is no indication of any detached crossbridges in rigor muscles. Thus in this type of muscle, at least, the establishment of a rigor pattern may not involve a "selection" of suitably located myosin heads from a larger population, as is generally thought, but may instead involve a systematic redistribution of the whole population of heads until all of them became crossbridges.