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Structure of cardiac muscle troponin C unexpectedly reveals a closed regulatory domain
S K Sia1, M X Li, L Spyracopoulos
1Department of Biochemistry, Medical Research Council Group in Protein Structure and Function, University of Alberta, Edmonton, Alberta T6G 2H7, Canada.
Insights
Cardiac troponin C (TnC) structure differs from skeletal muscle TnC, revealing a unique regulatory mechanism for heart muscle contraction. This finding aids understanding of calcium-sensitizing drugs.
Area of Science:
- Biochemistry
- Molecular Biology
- Cardiovascular Physiology
Background:
- Cardiac muscle contraction regulation differs from skeletal muscle, necessitating distinct molecular mechanisms.
- Cardiac troponin C (TnC) is a key regulator with unique Ca2+-binding properties, including an inactive Ca2+-binding site I.
- Understanding cardiac TnC structure is vital for elucidating heart muscle function and drug action.
Purpose of the Study:
- To determine the structure of cardiac troponin C (TnC) in its Ca2+-saturated state.
- To elucidate the regulatory mechanism of cardiac muscle contraction based on TnC structure.
- To provide insights into the action of calcium-sensitizing drugs.
Main Methods:
- Nuclear magnetic resonance (NMR) spectroscopy was employed to determine the structure of cardiac TnC.
- Structural analysis focused on the Ca2+-saturated state of cardiac TnC.
Main Results:
- The Ca2+-saturated cardiac TnC structure reveals a "closed" conformation of the regulatory domain, contrary to predictions.
- This conformation differs significantly from the calcium-induced structure observed in skeletal TnC.
- The determined structure highlights the interaction between cardiac TnC and troponin I (TnI) in regulating contraction.
Conclusions:
- Cardiac TnC's unique "closed" conformation in the Ca2+-bound state dictates cardiac muscle contraction regulation.
- This structural insight is crucial for understanding the mechanism of calcium-sensitizing drugs.
- The findings advance knowledge of cardiovascular physiology and potential therapeutic targets.
Abstract:
The regulation of cardiac muscle contraction must differ from that of skeletal muscles to effect different physiological and contractile properties. Cardiac troponin C (TnC), the key regulator of cardiac muscle contraction, possesses different functional and Ca2+-binding properties compared with skeletal TnC and features a Ca2+-binding site I, which is naturally inactive. The structure of cardiac TnC in the Ca2+-saturated state has been determined by nuclear magnetic resonance spectroscopy. The regulatory domain exists in a "closed" conformation even in the Ca2+-bound (the "on") state, in contrast to all predicted models and differing significantly from the calcium-induced structure observed in skeletal TnC. This structure in the Ca2+-bound state, and its subsequent interaction with troponin I (TnI), are crucial in determining the specific regulatory mechanism for cardiac muscle contraction. Further, it will allow for an understanding of the action of calcium-sensitizing drugs, which bind to cardiac TnC and are known to enhance the ability of cardiac TnC to activate cardiac muscle contraction.