A Central Role for Troponin C Amino-Terminal α-Helix in Vertebrate Thin Filament Ca²⁺-Activation

Troponin C (TnC) is the Ca2+-sensing subunit of troponin that is responsible for activating thin filaments in striated muscle, and in turn for regulating systolic and diastolic contractile function of cardiac muscle. Secondary structure of vertebrate TnC is mainly α-helices, with 9 helices named from N and A-H. The N-helix is a 12-residue-long α-helix located at the extreme amino terminus of the protein and is the only helical structure that does not participate in forming Ca2+-binding EF-hands. Evolutionarily, the N-helix is found only in TnC from mammalian species and most other vertebrates and is not present in other Ca2+-binding protein members of the calmodulin (CaM) family. Furthermore, the primary sequence of the N-helix differs between the genetic isoforms of fast skeletal TnC (sTnC) and cardiac/slow skeletal TnC (cTnC). The 3D location of N-helix within troponin complex is also distinct between skeletal and cardiac troponin. Physical chemistry and biophysical studies centered on the sTnC N-helix demonstrate that it is crucial to thermal stability and Ca2+-sensitivity of thin filament regulated MgATPase activity in solution and isometric force generation in the sarcomere. Comparable studies on cTnC N-helix have not yet been performed despite identification of cardiomyopathy-associated genetic variants that affect residues of cTnC’s N-helix. Here we review the current status of research on TnC’s N-helix and establish future directions to elucidate its functional significance.