Which protein forms the structural basis of muscle contractions?

The correct choice, myosin, is a critical motor protein involved in muscle contractions. Myosin interacts with actin filaments to generate force and movement within muscle tissues. The contraction of muscle fibers occurs through a process known as the sliding filament theory, where myosin heads attach to binding sites on actin filaments and pull them closer together, leading to the shortening of sarcomeres, the basic units of muscle fibers.

Myosin has a head region that can bind to actin, along with ATP, which provides the energy necessary for muscle contraction. This interaction enables myosin to physically move along the actin filaments, facilitating the contraction process.

While keratin is a structural protein found in various tissues such as hair and nails, it does not play a role in muscle contraction. Tubulin is the building block of microtubules, which are involved in cellular structure and transport within cells, but they do not participate directly in muscle contraction. Although actin is also essential to muscle function, it is myosin that serves as the motor protein executing the power stroke during contraction. Therefore, myosin is the primary protein that forms the structural basis for muscle contractions.