Which protein in skeletal muscle primarily determines contractility?

Myosin is the protein in skeletal muscle that primarily determines contractility due to its role as a molecular motor in muscle contraction. The muscular contraction process involves the interaction between actin and myosin, where myosin heads attach to binding sites on the actin filaments to form cross-bridges. When these cross-bridges cycle through attachment and detachment, they generate the force necessary for muscle contraction.

Myosin's structure, particularly its heads, includes ATPase activity, which is crucial for energy production during contractions. The resulting muscle fiber shortening occurs through the sliding filament mechanism, where myosin pulls the actin filaments closer together, thus shortening the sarcomere and ultimately leading to muscle contraction.

In contrast, actin serves as the filament that myosin binds to, while tropomyosin is a regulatory protein that covers the binding sites on actin in a relaxed muscle, preventing contraction until calcium ions signal for the process to begin. Titin, while it plays a vital role in maintaining the structural integrity of the sarcomere and passive elasticity, does not directly initiate or drive contractility like myosin does.