What is the thick-filament protein in muscle fibers called?

The thick-filament protein in muscle fibers is myosin. Myosin is a motor protein that interacts with actin, the thin-filament protein, to facilitate muscle contraction. During the process of contraction, the myosin heads bind to actin filaments and pull them, which shortens the sarcomere—the basic unit of muscle tissue. This interaction is critical for muscle contraction and function.

In addition to its role as the structural component of thick filaments, myosin's function is essential in converting chemical energy from ATP hydrolysis into mechanical work, which is the primary mechanism behind muscle movement. It plays an indispensable role in the cross-bridge cycle, where myosin binds and releases actin in a regulated fashion.

The other proteins mentioned are important in the context of muscle contraction, but they have different roles. Troponin and tropomyosin are involved in the regulation of muscle contraction by controlling the interaction between myosin and actin. Actin is the primary protein of the thin filaments. Understanding these distinctions is key to grasping the complex processes that underpin muscle physiology.