What protein forms contractile filaments in muscle cells along with myosin?

The formation of contractile filaments in muscle cells primarily involves two key proteins, myosin and actin. Myosin serves as the motor protein that interacts with actin to facilitate muscle contraction. Actin is a globular protein that polymerizes to form long filaments, which are part of the sarcomere structure in skeletal and cardiac muscle fibers.

In the sliding filament theory of muscle contraction, the myosin filaments pull on the actin filaments, causing the muscle to contract. This mechanism is critical for muscle movement and function because it allows for the generation of force through the coordinated interaction of these two types of protein filaments.

The other options, such as collagen and fibronectin, are structural proteins that play roles in maintaining tissue integrity and facilitating cell adhesion, yet they are not involved in the contractile process of muscle cells. Myoglobin, on the other hand, is a protein that stores oxygen in muscle cells, supporting aerobic respiration during muscle activity, but it does not participate in muscle contraction directly.