Movie S3.

Features of the intermediate state of the recovery stroke stabilized by CK-571. After ATP binding, myosin explores a series of conformations during the recovery stroke. CK-571 binds and stabilizes an intermediate of the recovery stroke, revealing how flexible the motor becomes after ATP binding. The CK-571 traps the molecule in a state in which the central beta sheet and the U50 position is closer to that observed in the PPS (black) than that found in Postrigor (blue). Interestingly, however, switch II is far away from the gamma phosphate position (unlike in the PPS state). Moreover, the lever arm is down, similarly to states close to the PR (blue). Rotation of the U50 and conformational changes in the seven-stranded central β-sheet are indicated by red arrows. The SMM/CK-571 structure reveals that flexibility within the molecule during the recovery stroke allows repriming of the lever arm by repositioning of the SH1 helix and the Relay, whereas the switch II remains unaffected during the first phase of the recovery stroke.