Which protein forms the thick filament in muscle fibers?

Myosin is the protein that forms the thick filament in muscle fibers. In skeletal muscle, myosin molecules assemble into thick filaments, which are part of the contractile apparatus responsible for muscle contraction. Myosin has a long rod-like structure with a globular head that interacts with actin, the protein that makes up the thin filaments, during the muscle contraction process. This interaction is critical, as the sliding motion between the thick and thin filaments leads to muscle shortening and force generation.

In contrast, actin is the protein that constitutes the thin filaments. Tropomyosin and troponin are regulatory proteins that interact with actin and play essential roles in muscle contraction, particularly in controlling the access of myosin to actin. However, they do not form the thick filament themselves, which is why myosin is the correct answer for the thick filament component in muscle fibers.