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Specific degradation of troponin T and I by mu-calpain and its modulation by substrate phosphorylation

F Di Lisa1, R De Tullio, F Salamino

  • 1Dipartimento di Chimica Biologica, Università di Padova, Italy.

The Biochemical Journal
|May 15, 1995
PubMed

Insights

Calpain degrades troponin (Tn) subunits, with mu-calpain being more potent than m-calpain. Phosphorylation by PKA reduces TnI degradation, while PKC increases it, affecting Tn complex structure.

Area of Science:

  • Biochemistry
  • Molecular Biology
  • Muscle Physiology

Background:

  • Troponin (Tn) is crucial for muscle contraction.
  • Calpains are calcium-dependent proteases implicated in muscle proteolysis.
  • Understanding Tn degradation by calpain is vital for muscle health research.

Purpose of the Study:

  • To investigate the degradation of troponin subunits by different calpain isoenzymes.
  • To determine the influence of phosphorylation on troponin sensitivity to calpain.
  • To elucidate the structural changes in troponin following phosphorylation.

Main Methods:

  • Incubation of isolated cardiac troponins and myocardial cryosections with rat skeletal muscle calpain isoenzymes (mu-calpain and m-calpain).
  • Western-blot analysis using monoclonal antibodies against TnI and TnT.
  • Analysis of troponin complex dissociation using Sephadex G-100 gel filtration.

Main Results:

  • Mu-calpain demonstrated significantly higher activity than m-calpain in degrading TnI and TnT.
  • Troponin C (TnC) was resistant to degradation by both calpain forms.
  • PKA phosphorylation decreased TnI sensitivity to degradation, whereas PKC phosphorylation increased TnI degradation by twofold.
  • PKC phosphorylation led to TnI complex dissociation, indicated by a reduced apparent molecular mass.

Conclusions:

  • Calpain, particularly mu-calpain, actively degrades troponin subunits TnI and TnT.
  • Phosphorylation status critically modulates troponin's susceptibility to calpain-mediated proteolysis.
  • PKC-induced phosphorylation alters troponin structure, potentially impacting its function and stability.

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