The Effect of Gly126Arg Substitution in Alpha-Tropomyosin on Interaction of Myosin with Actin in the ATP Hydrolysis Cycle

It is known that the regulation of muscle contraction is carried out by tropomyosin and calcium-sensitive protein troponin, which form thin filament with the F-actin. The noncanonical glycine residue at position 126 of the central part of the skeletal alpha-tropomyosin destabilizes the structure of this protein. The substitution of glycine residue by arginine residue stabilizes the central region of tropomyosin, displaces tropomyosin to the open position and activates the switching actin monomers on during the ATP hydrolysis cycle. To investigate how Gly126Arg substitution affects the interaction of the myosin head with F-actin in the ATP hydrolysis cycle, the myosin subfragment-1 (S1) was modified with a 1,5-IAEDANS fluorescent probe and AEDANS-S1 was incorporated into the ghost muscle fiber. Multistage changes in the mobility and spatial organization of the myosin head during simulation of different stages of the ATP hydrolysis cycle were studied by polarization fluorescence microscopy. It was shown that, in the regulated thin filaments of the ghost muscle fiber at high concentrations of Ca²⁺, Gly126Arg substitution significantly increases the number of myosin heads strongly associated with F-actin when simulating strong binding of myosin to actin, but reduces the number of such heads when imitating weak binding of myosin. Such changes in the behavior of myosin in the ATP hydrolysis cycle indicate an increase in the efficiency of myosin cross-bridges. A significant increase in the relative amount of myosin strongly bound to actin was also observed at low Са²⁺ concentrations. This indicates an increase in Са²⁺-sensitivity of a thin filament initiated by Gly126Arg substitution. The obtained data suggest that the stabilization effects of the central part of tropomyosin by Gly126Arg substitution are realized through the abnormal behavior of tropomyosin and troponin, which leads to a change in the nature of the interaction of myosin with actin and tropomyosin in the ATP hydrolysis cycle.