Structural Basis for Calcium-Regulated Relaxation of Striated Muscles at Interaction Sites of Troponin with Actin and Tropomyosin
Muscle contraction, in general, is regulated by the intracellular calcium-ion concentration. Ca2+-regulation in skeletal or cardiac muscle of vertebrate is mediated at the level of thin filaments consisting of actin, tropomyosin, and troponin. However, pure actin filaments themselves activate contraction irrespective of calcium concentration. Troponin,1 together with tropomyosin, is required to regulate contraction.2 Troponin consists of three subunits:3 inhibitory TnI, Ca2+-binding TnC, and tropomyosin-binding TnT. Troponin has an elongated shape and forms two structural regions, which are a long tail region containing the N-terminal region of TnT (TnT1 (chicken skeletal residues 1–164 of TnT)4) and a globular head region containing TnI, TnC, and the C-terminal region of TnT [TnT2 (chicken skeletal residues 165–263 of TnT)]. The globular head region plays a central role in regulating muscle contraction.5
KeywordsTernary Complex Core Domain Myosin Head Inhibitory Region Switch Region
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