THE EFFECT OF TROPONIN-C SUBSTITUTION ON THE CA2-SENSITIVE ATPASE ACTIVITY OF VERTEBRATE AND INVERTEBRATE MYOFIBRILS BY TROPONIN-CS WITH VARIOUS NUMBERS OF CA2+-BINDING SITES()

The effect of four different classes of troponin C with different numb ers of Ca2+-binding sites was investigated on the Ca2+-activation prof iles of the ATPase of troponin C-depleted myofibrils prepared from ver tebrate fast skeletal (rabbit), vertebrate cardiac (bovine) and invert ebrate crustacean tail striated (crayfish, lobster) muscles. Troponin C from vertebrate sources [fast skeletal (rabbit, chicken) with four C a2+-binding sites, and cardiac (bovine, chicken) with three Ca2+-bindi ng sites] activated the Ca2+-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 oth er hand, two kinds of crustacean (crayfish, lobster) troponin C with t wo Ca2+-binding sites activated only crustacean troponin C-depleted my ofibrils, One invertebrate molluscan (Akazara scallop) troponin C with one Ca2+-binding site did not activate the Ca2+-ATPase of the troponi n C-depleted myofibrils from either vertebrate or crustacean striated muscles. The results obtained from the four kinds of combinations of t roponin C and troponin C-depleted myofibrils from vertebrate skeletal and cardiac muscles demonstrated that the characteristic cooperativity of the Ca2+-activation profiles of both vertebrate skeletal and cardi ac myofibrils was determined by the skeletal or cardiac origin of trop onin C molecules, irrespective of the animal species, and the Ca2+-aff inity of the myofibrillar ATPase was related to the skeletal or cardia c origin of both the troponin C and myofibrils. These findings indicat ed that each of the four classes of troponin C has its own characteris tic Ca2+-activation profile for each kind of myofibril examined in the present study.