The regulatory domain of the myosin head behaves as a rigid lever

The regulatory domain of the myosin head is believed to serve as a lever ar m that amplifies force generated in the catalytic domain and transmits this strain to the thick filament. The lever arm itself either can be passive o r may have a more active role storing some of the energy created by hydroly sis of ATP. A structural correlate which might distinguish between these tw o possibilities (a passive or an active role) is the stiffness of the domai n in question, To this effect we have examined the motion of the proximal ( ELC) and distal (RLC) subdomains of the regulatory domain in reconstituted myosin filaments, Each subdomain was labeled with a spin label at a unique cysteine residue, Cys-136 of ELC or Cys-154 of mutant PLC, and its mobility was determined using saturation transfer electron paramagnetic resonance s pectroscopy. The mobility of the two domains was similar; the effective cor relation time (tau (eff)) for ELC was 17 mus and that for RLC was 22 mus. A dditionally, following a 2-fold change of the global dynamics of the myosin head, effected by decreasing the interactions with the filament surface (o r the other myosin head), the coupling of the intradomain dynamics remained unchanged. These data suggest that the regulatory domain of the myosin hea d acts as a single mechanically rigid body, consistent with the regulatory domain serving as a passive lever.