INTERNAL MOTIONS IN MYOSIN

High-resolution proton nuclear magnetic resonance (1H NMR) measurements were made on myosin, heavy meromyosin (HMM), myosin subfragment 1 (SI), light meromyosin (LMM), and actin. A strong signal from amino acid side chains undergoing motions too fast to be accounted for by simple rotations of groups on a rigid backbone was obtained from myosin. Comparison of myosin, HMM, SI, and LMM showed that the mobile region is located almost entirely in SI and accounts for ~22% of its structure. Adenosine triphosphate (ATP) and ATP analogues had no measurable effect on the SI spectrum. Actin, on the other hand, quenched the internal motions of S1. When S1 was titrated with actin, an association constant was obtained which was in agreement with other measured values. The actin effect was reversed by adding magnesium pyrophosphate (MgPPi) or adenyl-5'-yl imidophosphate (MgAMPPNP). Quantitative treatment of the broad signals from myosin and its sub-fragments substantiated the existence of two flexible regions in myosin. The highly mobile portion of myosin may be located in the “swivel” between SI and the rest of myosin or in the actin binding site or in both. These possibilities are discussed, and a new possible mechanism for muscle cross bridge elasticity is proposed. © 1979, American Chemical Society. All rights reserved.