The 4-stranded helical arrangement of myosin heads on insect ( Lethocerus) flight muscle thick filaments

Abstract

Several electron microscope studies of myosin filaments from relaxed insect flight muscles have been published (Reedy and Garrett, 1977; Reedy et al., 1983; Squire et al., 1982; Bullard, 1983; Clarke et al., 1986). However, little evidence has been shown of satisfactory preservation of the helical arrangement of myosin crossbridges with a 116-nm axial repeat known to be present on the basis of X-ray diffraction data (Miller and Tregear, 1972; Squire, 1972). Myosin filaments from many other muscles have appeared to be more easily preserved. For example, filaments from scallop striated adductor muscle (7stranded; Vibert and Craig, 1983), Limulus telson muscle, and scorpion and tarantula leg muscles (4stranded: Stewart et aZ., 1981, 1985; Crowther et al., 1985), all show helical arrays of myosin head mass on the thick filament surfaces. These have been visualized by electron microscopy of negatively stained isolated thick filaments. In all of these cases the helical order has been sufficiently good for threedimensional reconstructions of the filaments to be computed. In the case of vertebrate skeletal muscle, images have been obtained from negatively stained isolated filaments (Kensler and Stewart, 1983,1986, 1989), and from shadowed freeze fracture replicas of whole fibres (Cantino and Squire, 1986), with both studies showing the 3-stranded helix of myosin head mass, as well as some details of the arrangements of the individual heads. Three-dimensional reconstructions of negatively stained thick filaments from frog have been obtained by Stewart and Kensler (1986). In the case of the thick filaments from insect flight muscle, particularly of the asynchronous flight muscles as exemplified by those of the giant water bug Lethocerus, past studies of isolated filaments have shown only flat, featureless shelves (“crowns”) of density every 14.5 nm (Reedy and Garrett, 1977; Clarke et al., 1986; Bullard, 1983). Similar appear-