Tropomyosin is a regulatory protein associated with F-actin in many actomyosin contractile systems. If in vitro conditions are such that tropomyosin binds only slightly to F-actin, then the addition of myosin heads can induce stoichiometric binding between them. This suggests that formation of rigor bonds between actin and myosin heads may cause some change in the actin, stabilizing the appropriate binding site for tropomyosin.