ИСТИНА

It is known that contraction of striated muscles is initiated by the increase in calcium concentration within the muscle cell. The sensor of calcium concentration is a component of thin filaments, troponin, which undergoes conformational rearrangements resulting in a shift of the position of tropomyosin on F-actin. Depending on its location TM prevents or allows strong interaction of the myosin heads with actin. We have studied the molecular mechanisms by which troponin–tropomyosin regulates actin-myosin interaction in the ATPase cycle using fluorescent probes specifically bound to Cys36 or Cys190 of α- or β-tropomyosin, Cys707 of myosin subfragment-1 (S1) and Cys374 of actin or in the groove, in the region of the contacts between adjacent actin sub-units. The fluorescently labeled proteins were incorporated into a single ghost muscle fiber to measure the polarized fluorescence in the absence or presence of Ca2+, nucleotides and analogs of ATP. Analysis of the data allows us to determine the angles of orientation of emission (ΦE) and absorption (ΦA) dipoles, the mobility of the fluorescent probes (N) and the flexibility of the thin filaments (θ1/2) in each simulated state of the ATPase cycle. It was observed that during the simulation of transition from the weak- to strong-binding stages of the ATPase hydrolysis cycle in the absence of troponin the value of ΦE (or ΦA) increases for 1,5-IAEDANS-labeled actin and for 5-IAF-labeled tropomyosin, and decreases for FITC-phalloidin-labeled actin and for 1,5-IAEDANS-labeled S1. These changes in the parameters were interpreted as showing, respectively, the rotation of actin subdomain 1 from the filament axis to the periphery, the shifting of the tropomyosin strands towards the inner domain of actin (to the open position), and the tilt of actin monomers and the myosin motor domain towards the filament axis. Troponin modulates these spatial rearrangements in a Са2+-dependent manner. At high Ca2+, troponin–tropomyosin causes the additional rotation of actin subdomain 1 to the periphery of the thin filaments switching some extra actin monomers on, and increases the area of stereospecific interactions between actin and myosin. This favors the formation of the strong-binding states of actin–S1 and thus increases the efficiency of the actomyosin motor. At low Ca2+, troponin–tropomyosin complex, on the contrary, inhibits the work of actomyosin motor, restricting the myosin-induced movement of tropomyosin and switching a part of the actin monomers off. The application of the mutant tropomyosins in polarized fluorescence study provides new data about the molecular mechanisms of muscle contraction. It appeared that local changes in the surface charge of tropomyosin or altered bending of tropomyosin coiled coil can disorder the concordant conformational changes of actomyosin system. The different point mutations in tropomyosin associated with inherited forms of myopathies are able to decrease or increase the amount of the strongly bound myosin heads and switched on actin monomers by shifting the tropomyosin strands towards the open or closed position. The abnormal behavior of tropomyosin and the defective response of actomyosin system during the ATPase hydrolysis cycle is likely to underlie the muscle dysfunction in inherited skeletal and cardiac myopathies. This work was supported by the Russian Foundation for Basic Research (grants No. 14-04-00454, 16-34-00865).