Point mutations in genes encoding isoforms of skeletal muscle tropomyosin may cause nemaline myopathy, cap myopathy (Cap), congenital fiber-type disproportion (CFTD), and distal arthrogryposis. The molecular mechanisms of muscle dysfunction in these diseases remain unclear. We studied the effect of the E173A, R90P, E150A, and A155T myopathy-causing substitutions in γ-tropomyosin (Tpm3.12) on the position of tropomyosin in thin filaments, and the conformational state of actin monomers and myosin heads at different stages of the ATPase cycle using polarized fluorescence microscopy. The E173A, R90P, and E150A mutations produced abnormally large displacement of tropomyosin to the inner domains of actin and an increase in the number of myosin heads in strong-binding state at low and high Ca²⁺, which is characteristic of CFTD. On the contrary, the A155T mutation caused a decrease in the amount of such heads at high Ca²⁺ which is typical for mutations associated with Cap. An increase in the number of the myosin heads in strong-binding state at low Ca²⁺ was observed for all mutations associated with high Ca²⁺-sensitivity. Comparison between the typical conformational changes in mutant proteins associated with different myopathies observed with α-, β-, and γ-tropomyosins demonstrated the possibility of using such changes as tests for identifying the diseases.

Deletion of Glu139 in β-tropomyosin caused by a point mutation in TPM2 gene is associated with cap myopathy characterized by high myofilament Ca²⁺-sensitivity and muscle weakness. To reveal the mechanism of these disorders at molecular level, mobility and spatial rearrangements of actin, tropomyosin and the myosin heads at different stages of actomyosin cycle in reconstituted single ghost fibres were investigated by polarized fluorescence microscopy. The mutation did not alter tropomyosin’s affinity for actin but increased strongly the flexibility of tropomyosin and kept its strands near the inner domain of actin. The ability of troponin to switch actin monomers “on” and “off” at high and low Ca²⁺, respectively, was increased, and the movement of tropomyosin towards the blocked position at low Ca²⁺ was inhibited, presumably causing higher Ca²⁺-sensitivity. The mutation decreased also the amount of the myosin heads which bound strongly to actin at high Ca²⁺ and increased the number of these heads at relaxation; this may contribute to contractures and muscle weakness.