Substitution of Ala for Thr residue in 155th position in γ-tropomyosin (Tpm3.12) is associated with muscle weakness. To understand the mechanisms of this defect, we studied the Ca ²⁺-sensitivity of thin filaments in solution and multistep changes in mobility and spatial arrangement of actin, Tpm, and myosin heads during the ATPase cycle in reconstituted muscle fibres, using the polarized fluorescence microscopy. It was shown that the Ala155Thr (A155T) mutation increased the Ca ²⁺-sensitivity of the thin filaments in solution. In the absence of the myosin heads in the muscle fibres, the mutation did not alter the ability of troponin to switch the thin filaments on and off at high and low Ca ²⁺, respectively. However, upon the binding of myosin heads to the thin filaments at low Ca ²⁺, the mutant Tpm was found to be markedly closer to the open position, than the wild-type Tpm. In the presence of the mutant Tpm, switching on of actin monomers and formation of the strong-binding state of the myosin heads were observed at low Ca ²⁺, which indicated a higher myofilament Ca ²⁺-sensitivity. The mutation decreased the amount of myosin heads bound strongly to actin at high Ca ²⁺ and increased the number of these heads at relaxation. It is suggested that direct binding of myosin to Tpm may be one оf the reasons for muscle weakness associated with the A155T mutation. The use of reagents that decrease the Ca ²⁺-sensitivity of the troponin complex may not be adequate to restore muscle function in patients with the A155T mutation.