Molecular Mechanisms of Muscle Weakness Associated with E173A Mutation in Tpm3.12. Troponin Ca(2+) Sensitivity Inhibitor W7 Can Reduce the Damaging Effect of This Mutation

Substitution of Ala for Glu residue in position 173 of γ-tropomyosin (Tpm3.12) is associated with muscle weakness. Here we observe that this mutation increases myofilament Ca(2+)-sensitivity and inhibits in vitro actin-activated ATPase activity of myosin subfragment-1 at high Ca(2+). In order to determine the critical conformational changes in myosin, actin and tropomyosin caused by the mutation, we used the technique of polarized fluorimetry. It was found that this mutation changes the spatial arrangement of actin monomers and myosin heads, and the position of the mutant tropomyosin on the thin filaments in muscle fibres at various mimicked stages of the ATPase cycle. At low Ca(2+) the E173A mutant tropomyosin shifts towards the inner domains of actin at all stages of the cycle, and this is accompanied by an increase in the number of switched-on actin monomers and myosin heads strongly bound to F-actin even at relaxation. Contrarily, at high Ca(2+) the amount of the strongly bound myosin heads slightly decreases. These changes in the balance of the strongly bound myosin heads in the ATPase cycle may underlie the occurrence of muscle weakness. W7, an inhibitor of troponin Ca(2+)-sensitivity, restores the increase in the number of myosin heads strongly bound to F-actin at high Ca(2+) and stops their strong binding at relaxation, suggesting the possibility of using Ca(2+)-desensitizers to reduce the damaging effect of the E173A mutation on muscle fibre contractility.