Key points are not available for this paper at this time.
In the presence of sufficient ATP intact myofibrils respond to changes in the calcium concentration of the surrounding medium by contracting or relaxing. Troponin senses the level of calcium, and the actin molecules in the thin filament respond either by being "turned off," that is, by becoming inaccessible to ATP-activated myosin (Weber and Bremel, 1971) so that the actin and the myosin filaments remain dissociated from each other, or by being "turned on," that is, by permitting contraction to take place. The mechanism by which troponin in conjunction with tropomyosin controls the behavior of actin is not yet understood. We know, however, that the proteins of the regulated actin filament, i.e., containing troponin + tropomyosin, are assembled into repeating units (Ohtsuki et al., 1967; Ebashi et al., 1968a; O'Brien et al., 1971; Spudich, Huxley, and Finch, in press) consisting of seven actin monomers, one tropomyosin, and one troponin (Bremel and...
Bremel et al. (Mon,) studied this question.