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Troponin complexes consist of three subunits - troponin T (also known as TnT or TNNT), troponin I (TnI or TNNI) and troponin C (TnC or TNNC). Troponin is therefore heterotrimeric, with each of the different subunits providing specific binding sites, allowing each troponin to interact with a single molecule of tropomyosin, an actin filament and a Ca2+ ion respectfully. The ability of the troponin C subunit to bind Ca2+ allows it to undergo a conformational and therefore moves the tropomyosin away from the myosin binding site on actin. This means that the two filaments can form a cross bridge[1].

Troponin is thought to be specialised form another calcium ion binding protein, calmodulin, which is found in smooth muscle. The extra subunits, T and I, mean that the myosin heads are able to move towards the actin filaments very quickly for rapid contraction, This is needed in skeletal muscle for example[2].


  1. Molecular Biology of the Cell, 4th edition Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter. 2002.
  2. Alberts, B. et al (2008) Molecular Biology of the Cell, 5th ed, Garland Science Taylor and Francis Group, New York, pp. 1028-1030
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