INDEPENDENT MOBILITY OF CATALYTIC AND REGULATORY DOMAINS OF MYOSIN HEADS

The recent determination of the myosin head atomic structure has led t o a new model of muscle contraction, according to which mechanical tor que is generated in the catalytic domain and amplified by the lever ar m made of the regulatory domain [Fisher, A, J., Smith, C. A., Thoden, J,, Smith, R,, Sutoh, K,, Holden, H. M. & Rayment, I, (1995) Biochemis try 34, 8960-8972], A crucial aspect of this model is the ability of t he regulatory domain to move independently of the catalytic domain, Sa turation transfer-EPR measurements of mobility of these two domains in myosin filaments give strong support for this notion, The catalytic d omain of the myosin head was labeled at Cys-707 with indane dione spin label; the regulatory domain was labeled at the single cysteine resid ue of the essential light chain and exchanged into myosin, The mobilit y 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 ta u(R) = 5-9 mu s. This difference in mobility between the two domains e xisted 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 ind istinguishable, 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 generat ed.