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 systems1-3 (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 chains4-6. 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 domain9, 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.