Internal motions in myosin.

High-resolution proton nuclear magnetic resonance (1H NMR) measurements were made on myosin, heavy meromyosin (HMM), myosin subfragment 1 (S1), 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, S1, and LMM showed that the mobile region is located almost entirely in S1 and accounts for approximately 22% of its structure. Adenosine triphosphate (ATP) and ATP analogues had no measurable effect on the S1 spectrum. Actin, on the other hand, quenched the internal motions of S1. When S1 was titrated with actin, an association 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 subfragments substantiated the existence of two flexible regions in myosin. The highly mobile portion of myosin may be located in the "swivel" between S1 and the rest of myosin or in the actin binding site or in both. These possibilites are discussed, and a new possible mechanism for muscle cross bridge elasticity is proposed.