ИСТИНА

We have investigated how the congenital myopathy-causing mutation E139del in b-tropomyosin (TM) affects TM’s position on actin filament and the spatial arrangement of actin subunits during the ATPase cycle in the absence and presence of troponin. We labelled recombinant wild-type and mutant E139del TMs with 5-IAF and F-actin with FITC-phalloidin and incorporated them into ghost muscle fibres. The reconstructed thin filaments of the fibres were decorated by myosin S1 and polarized fluorescence was measured at different stages of the ATPase cycle. It was found that the position of wild-type TM was correlated with the conformational state of actin at all the intermediate stages of the ATPase cycle. At transition from the weak binding to the strong-binding actomyosin states actin monomers turned to the filament periphery and the TM strands moved towards the inner domains of actin subunits. The E139del mutation kept the TM strands near the inner domains of actin but decreased the number of the switched on actin monomers throughout the cycle. Troponin (± Ca2+) dramatically changed the position of the wild-type and mutant TMs during the ATPase cycle. At low Ca2+ actin monomers turned to the filament center, whereas TM strands moved towards the outer domains of actin. On the contrary, at high Ca2+, actin monomers turned to the filament periphery and the TM strands moved towards the inner domains of actin. The E139del mutation stabilized the TM strands near the inner actin domains both at low and high Ca2+ and increased the proportion of the switched on and off actin monomers at high Ca2+ and at low Ca2+, respectively. Thus troponin keeps its ability to switch actin monomers from the off to the on state in thin filaments containing the E139del mutant TM. The aberrant movement of the E139del TM causes abnormal response of the contractile system that may result in muscle weakness observed in patients with the E139del mutation. This work was supported by the Russian Foundation for Basic Research (grants No. 14-04-00454a, 14-04-31527a) and the Muscular Dystrophy Campaign.