Internal motions in myosin

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Abstract
High-resolution 1H NMR measurements were made on [rabbit muscle] myosin, heavy meromyosin (HMM), moysin 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 .apprx. 22% of its structure. ATP and ATP analogs 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 constant was obtained which was in agreement with other measured values. The actin effect was reversed by adding MgPPi or adenyl-5''-yl imidodiphosphate. Quantitative treatment of the broad signals from myosin and its subfragments substantiated the existence of 2 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 possibilities are discussed, and a new possible mechanism for muscle cross bridge elasticity is proposed.