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Effect of calcium-sensitizing mutations on calcium binding and exchange with troponin C in increasingly complex

Svetlana B Tikunova1, Bin Liu, Nicholas Swindle

  • 1Department of Pharmacological and Pharmaceutical Sciences, University of Houston, Houston, Texas 77204, USA.

Biochemistry
|February 5, 2010
PubMed

Insights

Mutations in troponin C (TnC) affect its calcium binding. However, these changes in TnC did not always increase the calcium sensitivity of the whole troponin complex or muscle contraction.

Area of Science:

  • Muscle physiology
  • Biochemistry
  • Molecular biology

Background:

  • Calcium ions regulate cardiac muscle contraction through interactions involving troponin C (TnC).
  • Hydrophobic residues within TnC are critical for its calcium-dependent function.

Purpose of the Study:

  • To investigate the impact of specific hydrophobic residue mutations in TnC on calcium binding and sensitivity.
  • To determine how these mutations affect actomyosin ATPase activity in various biochemical contexts.

Main Methods:

  • Site-directed mutagenesis of five hydrophobic residues in TnC to glutamine (Gln).
  • Assays to measure calcium binding and exchange kinetics of TnC.
  • Assessment of actomyosin ATPase activity in reconstituted muscle systems.

Main Results:

  • Mutations in hydrophobic residues significantly altered calcium binding and exchange dynamics of isolated TnC.
  • Sensitization of isolated TnC to calcium did not consistently translate to increased calcium sensitivity in the troponin complex or reconstituted thin filaments.
  • Calcium sensitivity of reconstituted thin filaments (without myosin S1) better predicted actomyosin ATPase activity than TnC or troponin complex sensitivity.

Conclusions:

  • Intramolecular and intermolecular interactions of hydrophobic residues are crucial for TnC's calcium response.
  • Modulation of cardiac muscle contraction involves both intrinsic TnC properties and its interactions with other contractile proteins.
  • Interactions within the troponin complex and with actin are key determinants of calcium sensitivity in muscle contraction.

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