Independent mobility of catalytic and regulatory domains of myosin heads

The recent determination of the myosin head atomic structure has led to a new model of muscle contraction, according to which mechanical torque is generated in the catalytic domain and amplified by the lever arm made of the regulatory domain [Fisher, A, J., Smith, C. A., Thoden, J,, Smith, R,, Sutoh, K,, Holden, H. M. & Rayment, I, (1995) Biochemistry 34, 8960-8972], A crucial aspect of this model is the ability of the regulatory domain to move independently of the catalytic domain, Saturation transfer-EPR measurements of mobility of these two domains in myosin filaments give strong support for this notion, The catalytic domain of the myosin head was labeled at Cys-707 with indane dione spin label; the regulatory domain was labeled at the single cysteine residue of the essential light chain and exchanged into myosin, The mobility of the regulatory domain in myosin filaments was characterized by an effective rotational correlation time (tau(R)) between 24 and 48 mu s. In contrast, the mobility of the catalytic domain was found to be tau(R) = 5-9 mu s. This difference in mobility between the two domains existed only in the filament form of myosin. In the monomeric form, or when bound to actin, the mobility of the two domains in myosin was indistinguishable, with tau(R) = 1-4 mu s and >1,000 mu s, respectively, Therefore, the observed difference in filaments cannot be ascribed to differences in local conformations of the spin-labeled sites, The most straightforward interpretation suggests a flexible hinge between the two domains, which would have to stiffen before force could be generated.