Final answer:
The thin filaments in muscle cells are surrounded by the protein tropomyosin, a cable-like protein that blocks actin-myosin interactions in a resting state.
Step-by-step explanation:
The thin filaments in muscle cells are primarily composed of the protein actin, which contains binding sites for myosin attachment. Surrounding the actin are strands of another protein called tropomyosin, which blocks the binding sites on actin when the muscle is at rest. Additionally, the troponin-tropomyosin complex involves the protein troponin, which has three subunits, each with specific binding sites for actin, tropomyosin, and calcium ions respectively. When calcium ions bind to troponin during muscle contraction, tropomyosin undergoes a conformational change that reveals the actin binding sites to the myosin heads, allowing contraction to occur.