EFFECT OF MYOSIN LIGHT-CHAIN PHOSPHORYLATION AND IONIC-STRENGTH ON ACTIN-MYOSIN INTERACTION IN RELAXED SKELETAL-MUSCLE FIBER

The effects of phosphorylation of the myosin regulatory light chain (L C-2) and of ionic strength on F-actin structure in relaxed glycerol-tr eated muscle fibers were studied by fluorescence polarization. F-Actin of the muscle fiber thin filaments was specifically modified with the fluorescence probe rhodamine-phalloidin. Changes in the actin structu re were monitored by measuring the change in the emission oscillator o rientation angle Phi(E) and the amplitude of sin(2) theta. When the io nic strength of the relaxing solution is low, myosin LC-2 phosphorylat ion decreases Phi(E) and increases sin(2) theta. These changes are cha racteristic for the strong binding of actin with the myosin heads. At high (close to physiological) ionic strength the opposite effect was o bserved, i.e., the oscillator orientation angle does not change and si n(2) theta decreases, During muscle fiber relaxation, some of the cros s-bridges are suggested to retain the strong binding of F-actin. Phosp horylation of LC-2 increases their number at the low ionic strength of the relaxing solution and decreases their number in the actin filamen ts at physiological ionic strength, thus contributing to the relaxatio n of the fiber.