Abstract
Troponin reconstituted from the inhibitory component (troponin-I) and calcium binding protein (troponin-C) binds readily to actin-tropomyosin in 0.1 m m-EGTA but only poorly in 0.01 m m-CaCl 2 or 0.1 m m-Ca-EGTA. Troponin prepared by extraction of myofibrils with mersalyl, an organic mercurial, contains only these two components and also shows this calcium-sensitive binding and is deficient in its ability to bind to tropomyosin. Troponin-I + C is unable to confer calcium sensitivity on the Mg 2+ activated actomyosin ATPase in concentrations at which native troponin is fully effective and the ATPase activity remains high in the absence of calcium. Addition of the tropomyosin binding component (troponin-T) to the other two components restores their ability to remain associated with actin-tropomyosin in the presence of calcium as does native troponin; calcium sensitivity is also regained. The results of these experiments have been interpreted in terms of a two-site mechanism of troponin action.
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