Contraction of skeletal and cardiac muscles is regulated by Ca2+ through specific Ca(2+)-regulatory
proteins,
troponin and
tropomyosin, located in the thin filament.
Troponin is a Ca2+ receptive
protein consisting of three differet components,
troponins C, I, and T. The essential mechanisms of Ca(2+)-regulation are the inhibition of contractile interaction between
myosin and actin by the inhibitory action of
troponin I and the reversal of the inhibition by
troponin I through the Ca(2+)-binding to
troponin C. All three components of
troponin are required for the Ca(2+)-regulation of contraction. This article reviews following aspects of
troponin researches: 1) early studies on the structure and function of
troponin, 2) molecular mechanisms of Ca(2+)-regulation, 3) principles of
troponin-exchange in skinned fibers and properties of
troponin isoforms thereafter clarified under physiological conditions, 4) recent studies on the functional consequences of the mutations in human cardiac
troponins T and I that cause
genetic disorders,
familial hypertrophic cardiomyopathy (HCM), and
familial dilated cardiomyopathy (DCM).