Key result
Resonance energy transfer measurements indicate that the myosin II rod largely does not uncoil when bound to actin, maintaining distances of approximately 50 Å between regulatory light chains.
The myosin II subfragment 2 rod remains largely coiled when both heads bind to actin, suggesting that structural distortion occurs within the myosin heads rather than by uncoiling of the rod.
Challenges rod-uncoiling models in actomyosin cycling; leaves open head-distortion mechanisms for human cardiac validation.
Myosin, similar to many molecular motors, is a two-headed dimer held together by a coiled-coiled rod. The stability of the coiled coil has implications for head-head interactions, force generation, and possibly regulation. Here we used two different resonance energy transfer techniques to measure the distances between probes placed in the regulatory light chain of each head of a skeletal heavy meromyosin, near the head-rod junction (positions 2, 73, and 94). Our results indicate that the rod largely does not uncoil when myosin is free in solution, and at least beyond the first heptad, the subfragment 2 rod remains relatively intact even under the relatively large strain of two-headed myosin (rigor) binding to actin. We infer that uncoiling of the rod likely does not play a role in myosin II motility. To keep the head-rod junction intact, a distortion must occur within the myosin heads. This distortion may lead to different orientations of the light-chain domains within the myosin dimer when both heads are attached to actin, which would explain previously puzzling observations and require reinterpretation of others. In addition, by comparing resonance energy transfer techniques sensitive to different dynamical time scales, we find that the N terminus of the regulatory light chain is highly flexible, with possible implications for regulation. An intact rod may be a general property of molecular motors, because a similar conclusion has been reached recently for kinesin, although whether the rod remains intact will depend on the relative stiffness of the coiled coil and the head in different motors.
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Chakrabarty et al. (2002) studied this question. Resonance energy transfer (FRET/LRET) vs. Myosin without actin was evaluated on Distance between regulatory light chains (Cys-73 to Cys-73). Resonance energy transfer measurements indicate that the myosin II rod largely does not uncoil when bound to actin, maintaining distances of approximately 50 Å between regulatory light chains.
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