According to the sources, troponin and myosin are both proteins essential to the structure and function of striated muscles (which include skeletal and cardiac muscles). Troponin is a complex protein distributed at regular intervals on the tropomyosin of actin (thin) filaments, while myosin is the primary polymerised protein of the thick filaments . Other options are incorrect because: (1) Prolactin is a pituitary hormone, not a stomach enzyme ; (2) Secretin is a peptide hormone from the GI tract, but the sources do not categorise rhodopsin as a polypeptide hormone ; and (3) while calcitonin is a thyroid hormone, thymosin is secreted by the thymus gland .