Rotational dynamics of spin-labeled F-actin in the sub-millisecond time range

The rotational motions of F-actin filaments and myosin heads attached to them have been measured by saturation transfer electron paramagnetic resonance spectroscopy using spin-labels rigidly bound to actin, or to the myosin head region in intact myosin molecules, heavy meromyosin, and subfragment-1. The spin-label attached to F-actin undergoes rotational motion having an effective correlation time of the order of 10^-4 seconds. This cannot be interpreted as rotation of the entire F-actin filament or local rotation of the spin-label, but must represent an internal rotational mode of F-actin, possibly a bending or flexing motion, or a rotation of an actin monomer or a segment of it. The rate of this rotational motion is reduced approximately fourfold by myosin, HMM or S-1; HMM and S-1 are equally effective, on a molar basis, in slowing this rotation and both produce their maximal effect at a ratio of about one molecule of HMM or S-1 per ten actin monomers. With chymotryptic S-1, the effect is partially reversed at higher concentrations. With S-1 prepared with papain in the presence of Mg²⁺, the reversal is smaller, while with HMM or myosin there is no reversal at higher concentrations. Tropomyosin slightly decreases the actin rotational mobility, and the addition of HMM to the actin-tropomyosin complex produces a further slowing. The rotational correlation time for acto-HMM is the same whether the spin-label is on actin or HMM, indicating that the rotation of the head region of HMM when bound to F-actin is controlled by a mode of rotation within the F-actin filaments.