Both TnT and the TnT(1) tail fragment inhibit actin-tropomyosin-activated S1 ATPase, with TnT(1) producing a 10-fold reduction in the C- to M-state equilibrium, suggesting a modulatory role.
Troponin T has a modulatory as well as structural role in thin filament regulation, stabilizing the closed C-state.
In striated muscle the force generating acto-myosin interaction is sterically regulated by the thin filament proteins tropomyosin and troponin (Tn), with the position of tropomyosin modulated by calcium binding to troponin. Troponin itself consists of three subunits, TnI, TnC, and TnT, widely characterized as being responsible for separate aspects of the regulatory process. TnI, the inhibitory unit is released from actin upon calcium binding to TnC, while TnT performs a structural role forming a globular head region with the regulatory TnI- TnC complex with a tail anchoring it within the thin filament. We have examined the properties of TnT and the TnT(1) tail fragment (residues 1-158) upon reconstituted actin-tropomyosin filaments. Their regulatory effects have been characterized in both myosin S1 ATPase and S1 kinetic and equilibrium binding experiments. We show that both inhibit the actin-tropomyosin-activated S1 ATPase with TnT(1) producing a greater inhibitory effect. The S1 binding data show that this inhibition is not caused by the formation of the blocked B-state but by significant stabilization of the closed C-state with a 10-fold reduction in the C- to M-state equilibrium, K(T), for TnT(1). This suggests TnT has a modulatory as well as structural role, providing an explanation for its large number of alternative isoforms.
Maytum et al. (Thu,) reported a other. TnT and TnT(1) tail fragment was evaluated on S1 ATPase activity and S1 kinetic/equilibrium binding. Both TnT and the TnT(1) tail fragment inhibit actin-tropomyosin-activated S1 ATPase, with TnT(1) producing a 10-fold reduction in the C- to M-state equilibrium, suggesting a modulatory role.