Deviations in conformational rearrangements of thin filaments and myosin caused by the Ala155Thr substitution in hydrophobic core of tropomyosin

- Hydrophobic core substitution Ala155Thr decreases tropomyosin (TM) flexibility.
- Mutant TM shifts toward the inner actin domain at low Ca2 +.
- The substitution stimulates switching actin monomers on.
- TM's ability to prevent S1 strong binding to actin at low Ca2 + is inhibited.

Effects of the Ala155Thr substitution in hydrophobic core of tropomyosin Tpm1.1 on conformational rearrangements of the components of the contractile system (Tpm1.1, actin and myosin heads) were studied by polarized fluorimetry technique at different stages of the actomyosin ATPase cycle. The proteins were labelled by fluorescent probes and incorporated into ghost muscle fibres. The substitution violated the blocked and closed states of thin filaments stimulating abnormal displacement of tropomyosin to the inner domains of actin, switching actin on and increasing the relative number of the myosin heads in strong-binding state. Furthermore, the mutant tropomyosin disrupted the major function of troponin to alter the distribution of the different functional states of thin filaments. At low Ca2 + troponin did not effectively switch thin filament off and the myosin head lost the ability to drive the spatial arrangement of the mutant tropomyosin. The information about tropomyosin flexibility obtained from the fluorescent probes at Cys190 indicates that this tropomyosin is generally more rigid, that obviously prevents tropomyosin to bend and adopt the appropriate conformation required for proper regulation.