NMR Structural Study of Troponin C C-Terminal Domain Complexed with Troponin I Fragment from Akazara Scallop

Abstract

Scallop muscle has been demonstrated to possess both myosin-linked and actin-linked systems^1-3 (Fig. 1), even though molluscan muscles were known to be regulated only by the myosin-linked regulatory system mediated through Ca²⁺-binding to myosin light chains^4-6. Recently, the physiological significance of the coexistence of the two systems in scallop adductor muscle was investigated using CDTA-treated scallop myofibrils¹. Actin-linked (Troponin-linked) system has been well known as the regulatory system in the muscle contraction of vertebrate striated muscles⁷. It is regulated by troponin in a Ca²⁺ dependent manner. Troponin contains three distinct components, i.e., a Ca²⁺ binding component (TnC), an inhibitory component troponin I (Tnl), and a tropomyosin-binding component troponin T (TnT). TnC contains two independent Ca²⁺ binding domains, each of which consists of two EF-hand motifs⁸. Vertebrate striated muscle TnCs bind three or four Ca²⁺ ions in a molecule and act as the Ca²⁺ sensor of muscle contraction associated with the binding and release of one or two Ca²⁺ ion(s) in the N-terminal domain^{9, 10, 11}. The N-terminal domain has, thus, been called the regulatory domain and contains one or two low affinity Ca²⁺-binding sites (Sites I and II)¹². On the other hand, the C-terminal domain has been called the structural domain and contains two high-affinity sites (Sites III and IV). They also bind Mg²⁺ and are called as Ca²⁺/Mg²⁺ sites.