Geometrical constraints affecting crossbridge formation in insect flight muscle.

Computer-modelling studies have explored how rigor crossbridge interactions in insect flight muscle are affected by using a four-stranded helical thick filament and by restricting each myosin to forming one crossbridge with only one actin filament. Crossbridges searching over an axial range of +/- 7.2 nm, and within an azimuthal range around actin of +/- 45 degrees, can simulate the actin-labelling patterns observed in thin electron microscope sections well. However, the number of crossbridges attached between any myosin filament and an adjacent actin filament depends on their relative axial and azimuthal positions, and can vary by a factor of two. The relative position that maximized the number of attached bridges also produced the best modelling of the 'double chevron' appearance of two crossbridge pairs attaching within target zones every 38.6 nm, as seen in thin longitudinal sections, and the 'flared X' of crossbridges extending to four out of six surrounding actins at each crossbridge level seen in thin cross-sections. Micrographs show that excellent lattice register of rigor crossbridges in longitudinal sections does not depend on lateral register of thick or thin filament ends. Our modelling suggests how the crossbridge lattice may be generated by filaments becoming mutually annealed to the axial and azimuthal positions at which most crossbridges can attach, at which time the actin filaments are arranged at the diad positions on the P64 crystalline lattice. When the actin filaments are so oriented, in a P64 lattice, two crossbridges on adjacent actin filaments will slew toward the same point on the myosin filament, producing the flared X appearance of origin from a common stem and a single myosin, even if they originate from distinct points and separate molecules.