Intradomain distances in the regulatory domain of the myosin head in prepower and postpower stroke states: Fluorescence energy transfer

The relative movement of the catalytic and regulatory domains of the myosin head (S1) is likely to be the force generating conformational change in th e 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-modulate d FRET, three distances between the catalytic domain and regulatory domains and within the regulatory domain of myosin. The donor/acceptor pairs inclu ded MHC cys707 and ELC cys177; ELC cys177 and RLC cys154; and ELC cys177 an d gizzard RLC cys108. The IAEDANS (donor) or acceptor (DABMI or IAF) labele d light chains (ELC and RLC) were exchanged into monomeric myosin and the d istances were measured in the putative prepower stroke states tin the prese nce of MgATP or ADP/AIF(4)(-)) and the postpower stroke slates (ADP and the absence of nucleotides). For each of the three distances, the donor/accept or pairs were reversed to minimize uncertainty in the distance measured, ar ising from probe orientational factors. The distances obtained from FRET we re in close agreement with the distances in the crystal structure. Importan tly, none of the measured distances varied by more than 2 Angstrom, 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 Angstrom for a swing of the reg ulatory domain with respect to the catalytic domain during the power stroke . Additionally, an upper bound of 22 Angstrom, could be contributed to the power stroke by a reorientation of ELC with respect to the ELC during the p ower stroke.