S1 Atomic Fitting to Tomogram Crossbridges

Wire-net view of one full 39 nm repeat of the actin helix carved out of the column-averaged tomogram of freeze-trapped, active indirect flight muscle that contains three single-headed crossbridges binding to actin in the target zone midway between troponins. The crystallographic structure of rigor S1 (Rayment) has been rebuilt (red) to match the 3-D envelope of each crossbridge motif. Bridges with axial angles above 90° are also compared to the unmodified Holmes model for pre-stroke S1 (cyan), which has the motor domain of myosin in the rigor conformation, while the converter and light chain domains (LCD) are at an 'anti-rigor' angle. Crossbridges angled closer to the end of the power stroke (<90°) are compared to unmodified rigor S1 (yellow). The LCD lever arms of the crossbridges are trapped in a range of axial angles from >100°-45°. Therefore, the atomic modelling supports the hypothesis that the LCD is a lever arm for the myosin power stroke. However, atomic fittings to in situ crossbridges indicate that in bridges angled >90°, which are the best candidates for weak-binding pre-stroke bridges, the motor domain position does not match the rigor position.

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