Troponin C Mutations Partially Stabilize the Active State of Regulated Actin and Fully Stabilize the Active State...

Area of Science:

• Muscle physiology and biochemistry
• Molecular mechanisms of muscle contraction
• Cardiovascular genetics and disease

Background:

• Striated muscle contraction is regulated by actin-associated proteins, tropomyosin and troponin.
• Calcium (Ca²⁺) and myosin binding influence the activation state of the actin-tropomyosin-troponin complex.
• Mutations in troponin subunits, particularly troponin C (TnC), are linked to cardiac dysfunction and altered Ca²⁺ sensitivity.

Purpose of the Study:

• To investigate how specific troponin C (TnC) mutants (A8V, C84Y, D145E) affect the distribution of regulated actin states.
• To determine the impact of these TnC mutations on Ca²⁺ sensitivity and the stabilization of the active myosin-binding (M) state.
• To explore the combined effects of TnC and troponin T (TnT) mutations on muscle activation.

Main Methods:

• Analysis of regulated actin states under varying Ca²⁺ concentrations.
• Quantification of the distribution of inactive (B) and active (M) actin states.
• Assessment of ATPase activity modulation by TnC mutants and combined TnC/TnT mutations.

Main Results:

• TnC mutants A8V, C84Y, and D145E increased the proportion of the active M state at saturating Ca²⁺ concentrations.
• A8V and C84Y TnC mutants showed a slight decrease in the inactive B state at low Ca²⁺.
• A combination of A8V TnC and Δ14 TnT mutations resulted in a fully active state solely upon Ca²⁺ binding, independent of myosin interaction.

Conclusions:

• Specific troponin C mutations alter the equilibrium of actin-tropomyosin-troponin complex states, enhancing muscle activation.
• These mutations increase the Ca²⁺ sensitivity of ATPase activity by promoting the active M state.
• The findings suggest that troponin complexes can stabilize the active M state with Ca²⁺ alone, independent of myosin binding, highlighting a key regulatory mechanism in muscle contraction.