A protein attaches to tropomyosin in sarcomeres (the basic contractile unit of muscle). Part of the regulatory pathway for muscle contraction. When the muscle is activated, calcium pours into the sarcomeres and troponin binds the calcium and then causes a change in shape (allosteric change) that reveals the binding sites on the thin filaments (which were previously blocked by tropomyosin) allowing the force producing thick filaments, made up of Myosin motor proteins, to bind and begin pulling the two ends of the sarcomrere closer together.