Comparison of simple lattice and superlattice arrangements in the M-band. (a) Illustration of the myosin filament bare zone (the whole length between the first left and right myosin head positions) and the M-lines (M4′, M1, and M4), which form part of the M-band. Some fiber types have more lines (M6 and M6′) equally spaced outside the other lines. The bare regions lie between the M-band and the level of the first myosin head. (b and c) Schematic illustrations of the possible origins of the simple lattice and superlattice packing interactions at the M-band. The bare zone structure in panel a has the symmetry of the dihedral point group 32 (Luther et al., 1981; Al-Khayat et al., 2010). This means that it has threefold rotational symmetry around the filament long axis and three twofold rotation axes at right angles to the threefold axis and in the plane of M1. The myosin filament makes bridges in the M-band to all six surrounding myosin filaments in the hexagonal lattice, three of which must have the opposite polarity to the other three (i.e., rotated by 180° around a twofold axis). These two possibilities are shown by yellow and blue circles. The yellow and blue half-bridges represent the same molecular structure, but rotated around a twofold axis by 180°. (b) Unlike interactions automatically generate a simple lattice, as indicated by the white triangles. (c) On the other hand, if like-with-like interactions are preferred, then two out of three interactions can be satisfied for three filaments on a triangle, but the rotation of the third filament will always be ambivalent (at least one of the interactions must be an unlike interaction). Optimizing the like interactions inevitably produces a statistical superlattice. For details, see text.