The effect of troponin C substitution on the Ca 2+-sensitive ATPase activity of vertebrate and invertebrate myofibrils by troponin Cs with various numbers of Ca 2+-binding sites

Abstract

The effect of four different classes of troponin C with different numbers of Ca 2+-binding sites was investigated on the Ca 2+-activation profiles of the ATPase of troponin C-depleted myofibrils prepared from vertebrate fast skeletal (rabbit), vertebrate cardiac (bovine) and invertebrate crustacean tail striated (crayfish, lobster) muscles. Troponin C from vertebrate sources [fast skeletal (rabbit, chicken) with four Ca 2+-binding sites, and cardiac (bovine, chicken) with three Ca 2+-binding sites] activated the Ca 2+-ATPase of troponin C-depleted myofibrils from the vertebrate skeletal or cardiac muscles, but did not activate the invertebrate troponin C-depleted crustacean myofibrils. On the other hand, two kinds of crustacean (crayfish, lobster) troponin C with two Ca 2+-binding sites activated only crustacean troponin C-depleted myofibrils. One invertebrate molluscan (Akazara scallop) troponin C with one Ca 2+-binding site did not activate the Ca 2+-ATPase of the troponin C-depleted myofibrils from either vertebrate or crustacean striated muscles. The results obtained from the four kinds of combinations of troponin C and troponin C-depleted myofibrils from vertebrate skeletal and cardiac muscles demonstrated that the characteristic cooperativity of the Ca 2+-activation profiles of both vertebrate skeletal and cardiac myofibrils was determined by the skeletal or cardiac origin of troponin C molecules, irrespective of the animal species, and the Ca 2+-affinity of the myofibrillar ATPase was related to the skeletal or cardiac origin of both the troponin C and myofibrils. These findings indicated that each of the four classes of troponin C has its own characteristic Ca 2+-activation profile for each kind of myofibril examined in the present study.