ИСТИНА

The molecular mechanisms of skeletal muscle dysfunction in congenital myopathies remain unclear. The present work examines the effect of a myopathy-causing mutation (Q147P) in beta-tropomyosin on the structural state of the myosin head and F-actin during different steady states of the ATP hydrolysis cycle observed by polarized fluorescence technique. In three series of experiments, F-actin, myosin S1 and recombinant tropomyosins (wild-type and with Q147P mutation) were modified by FITC-phalloidin, 1,5-IAEDANS and 5-IAF, respectively, with further incorporation into single ghost muscle fibers predominantly containing actin filaments free of troponin and tropomyosin. The Q147P tropomyosin binds to actin in muscle fibers despite its reduced affinity for actin in cosedimentation assay. However, as compared to the wild-type tropomyosin, it occupies a different position on F-actin, according to the steric model of regulation closer to the centre of the filament. The mutant tropomyosin increases the proportion of strong-binding actomyosin cross-bridges at some stages of the ATPase cycle and prevents the formation of the weak-binding state. The incomplete relaxation and excessive activation in the presence of the Q147P tropomyosin is likely to underlie the contractile abnormalities caused by this mutation. This work was supported by the FEBS Collaboration Scholarship, the Russian Foundation for Basic Research (Grants No. 14-04-00454a, 14-04-31527a) and the Muscular Dystrophy Campaign.