What protein blocks binding sites for myosin heads on actin filaments?

The correct response identifies tropomyosin as the protein that blocks binding sites for myosin heads on actin filaments. In muscle contraction, actin and myosin interact to facilitate the sliding filament model, which is essential for muscle movement. Tropomyosin functions to inhibit this interaction when the muscle is relaxed.

When the muscle is at rest, tropomyosin covers the myosin-binding sites on the actin filaments. This prevents myosin heads from attaching to actin, which would otherwise initiate contraction. When calcium ions are released into the muscle fibers, they bind to troponin, a regulatory protein associated with tropomyosin. This binding causes a conformational change in tropomyosin, which then shifts away from the binding sites, allowing myosin heads to attach to actin. This sequence of events is crucial for the regulation of muscle contraction.

Therefore, the role of tropomyosin in blocking the binding sites is pivotal in controlling muscle contraction, making it the accurate choice in this context.