Crossbridges in the complete unit cell of rigor insect flight muscle imaged by three-dimensional reconstruction from oblique sections.

We have computed two types of 3-D reconstructions from single images of oblique transverse sections through rigor insect flight muscle (IFM) that permit simultaneous examination of all myosin crossbridges within the unit cell. One type, crystallographic serial section reconstruction (CSSR), utilizes primarily real space image manipulations of the periodic crossbridge lattice to obtain a 3-D reconstruction from a single image. The CSSRs, which do not average successive unit cells along the filament axis, reveal variations in the rigor double chevrons within the 116 nm long axial repeat and in particular show that specific crossbridges are absent. CSSRs establish that in rigor, the 116 nm period contains nine 12.9 nm repeats of attached crossbridges rather than the eight 14.5 nm repeats of myosin head origins observed in the relaxed state. This indicates that dominance of the actin repeat on myosin head form enforces axial and azimuthal changes on the crossbridge origins on the thick filament. The second type, superlattice reconstruction (SLR), is carried out entirely in Fourier space and produces an averaged reconstruction with the symmetry of the unit cell enforced. SLRs measure the 3-D transform of the complete unit cell, permitting direct comparison with X-ray diagrams from native muscle. Averaging several SLRs together has produced the highest resolution reconstruction of IFM to date. Oblique section reconstructions made by both methods confirm in greater detail the presence of two-headed lead crossbridges and single-headed rear crossbridges implying heads with differing angles and conformation. Reduced twist in the thin filament coincident with the lead crossbridge is also apparent. We have modeled several interpretations of this reduced twist in terms of structural changes in both myosin and actin at the lead bridge. In addition, these 3-D images resolve a feature just Z-ward of the rear crossbridge where antibody labeling has identified part of the large troponin complex of IFM.