Troponin is one of the major proteins in the thin (actin) filaments of muscle fibres.It is a complex made up of three polypeptides with three binding sites for

Explanation

The correct answer is that troponin has binding sites for actin, tropomyosin, and calcium ions. Troponin is a regulatory protein that plays a crucial role in muscle contraction by binding to these three components. When calcium ions bind to troponin, it induces a conformational change that allows tropomyosin to move away from actin's active sites, enabling myosin to bind to actin and initiate contraction. ATP is not directly involved in the binding with troponin; rather, it provides energy for muscle contraction. Myosin is not directly bound by troponin either, as its interaction is mainly with actin. Hence, options A, B, and D are incorrect.