In the resting state, which protein masks the active binding sites of actin filaments?

Actin and Myosin are contractile proteins of muscle while Troponin and Tropomyosin are regulatory proteins. Myosin is a polymer of Meromysoin units and this is the main protein of Dark bands. Light bands consist of Actin, Troponin and Tropomyosin. Actin protein consists of two double stranded polypeptides while troponin is attached on these strands at intervals. Actin shows presence of Myosin binding site and when myosin attaches to this site the muscle shows contraction due to formation of cross bridges. However this cross bridge formation is not seen when muscle relaxes as this site is masked by Troponin molecule. At intervals on actin, trimeric molecule Tropomyosin is also seen. This includes TpC, TpT and Tpl.