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troponin A complex of three polypeptide chains that are found at regular intervals along the length of an actin filament. During muscle contraction, troponin binds to calcium ions, displacing tropomyosin and exposing the binding site on the actin filament. This allows the interaction of actin and myosin to occur. See sliding filament theory.

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troponin A globular protein associated in vivo with actin and tropomyosin in the thin filaments of striate muscle.