The relative movement of the catalytic and regulatory domains of the
myosin head (S1) is likely to be the force generating conformational change in the energy transduction of muscle [Rayment, I., Holden, H. M., Whittaker, M., Yohn, C. B., Lorenz, M., Holmes, K. C., and Milligan, R. A. (1993) Science 261, 58-65]. To test this model we have measured, using frequency-modulated FRET, three distances between the catalytic domain and regulatory domains and within the regulatory domain of
myosin. The donor/acceptor pairs included MHC cys707 and ELC cys177; ELC cys177 and RLC cys154; and ELC cys177 and gizzard RLC cys108. The IAEDANS (donor) or acceptor (
DABMI or IAF) labeled light chains (ELC and RLC) were exchanged into monomeric
myosin and the distances were measured in the putative prepower
stroke states (in the presence of
MgATP or
ADP/AlF(4-)) and the postpower
stroke states (
ADP and the absence of
nucleotides). For each of the three distances, the donor/acceptor pairs were reversed to minimize uncertainty in the distance measured, arising from probe orientational factors. The distances obtained from FRET were in close agreement with the distances in the crystal structure. Importantly, none of the measured distances varied by more than 2 A, putting a strong constraint on the extent of conformational changes within S1. The maximum axial movement of the distal part of
myosin head was modeled using FRET distance changes within the
myosin head reported here and previously. These models revealed an upper bound of 85 A for a swing of the regulatory domain with respect to the catalytic domain during the power
stroke. Additionally, an upper bound of 22 A could be contributed to the power
stroke by a reorientation of RLC with respect to the ELC during the power
stroke.