[Effect of phosphorylating myosin light chains and ionic strength on actin-myosin interaction in a relaxed skeletal muscle fiber]

The effect of LC-2 phosphorylation and ionic strength on actin-myosin interaction in relaxed skeletal muscle fibers have been studied using polarization fluorimetry. F-actin was chemically modified by the fluorescent dye, rhodamine-phalloidin, and the mode of myosin-actin interaction was estimated by a polarized fluorescence technique based on changes in the dye orientation (phi E) and thin filament flexibility (sin 2 theta). Phosphorylation of LC-2 at relaxation in low ionic strength induced typical for the force production ("strong" binding) state changes in the polarized fluorescence of F-actin (decreasing of phi E and increasing of sin 2 theta). In contrast, phosphorylation in high ionic strength induced changes similar to those typical for the nonforce production ("weak" binding) state (phi E did not change, while sin 2 theta decreased). It is suggested that phosphorylation of LC-2 at approximately physiological ionic strength may provide fiber relaxation by switching some of the cross-bridges to the nonforce production state at the initial stage of relaxation.