In cardiac muscle the interaction is controlled by the intracellular Ca 2+ concentration acting on troponin in the thin filaments and modulated by phosphorylation of key regulatory proteins including troponin I. Troponin is a hetero-trimer composed of troponin C (cTnC), the Ca 2+ -binding component, troponin I (cTnI), the inhibitory component and troponin T (cTnT), considered to be the structural component .