Which protein is responsible for binding calcium to initiate muscle contraction?

Troponin is the protein that plays a critical role in muscle contraction by binding to calcium ions. When calcium ions are released during the excitation of muscle cells, they bind to troponin, which causes a conformational change in this protein. This change then leads to the removal of tropomyosin from the binding sites on actin filaments, allowing myosin heads to attach to actin. This attachment is essential for muscle contraction to occur through the sliding filament mechanism.

Tropomyosin, while also important in the regulatory complex of muscle contraction, does not bind calcium directly; instead, it covers the actin active sites, and its position is altered by the action of troponin upon calcium binding. Calmodulin is primarily involved in other cellular processes and not in the direct mechanism of muscle contraction in skeletal muscle. Actin is the thin filament that interacts with myosin during contraction but does not play a direct role in calcium binding or signaling. Therefore, troponin is specifically recognized for its role in initiating muscle contraction in response to calcium ions.