Which protein has receptor sites and plays a role in muscle contraction?

In the context of muscle contraction, actin is a protein that serves as the primary constituent of thin filaments in muscle fibers. It has specific sites that bind to myosin, the motor protein responsible for generating force during muscle contraction. During the contraction process, myosin heads attach to actin filaments to form cross-bridges, allowing the myosin to pull on the actin filaments, ultimately shortening the muscle fiber and leading to contraction.

While myosin is also essential in muscle contraction, as it interacts with actin, it is primarily the actin filaments that provide the attachment points for this interaction. Tropomyosin and troponin play regulatory roles; tropomyosin covers the binding sites on actin, while troponin responds to calcium levels to move tropomyosin away from binding sites, thus enabling actin and myosin interaction during contraction. But the question specifically asks about the protein with receptor sites that directly participates in the contraction mechanism, which is fulfilled by actin.

This is why actin is recognized as the protein that has receptor sites and plays a crucial role in the process of muscle contraction.