Final answer:
The thin filaments in muscle cells contain polypeptide subunits called G actin, which have active sites for myosin attachment. This allows for the interaction and binding of myosin, leading to muscle contraction.
Step-by-step explanation:
The thin filaments in muscle cells are composed primarily of the protein actin. Actin contains polypeptide subunits known as G actin, which have active sites for myosin attachment. These active sites on actin allow for the interaction and binding of myosin, which is the protein that forms the thick filaments in muscle cells. When myosin attaches to actin, it initiates the process of muscle contraction.