Some Aspects of Calcium-Regulatory Mechanisms

As mentioned in Section II,A, Ca regulation by troponin-tropomyosin consists of the depression of contractile interaction between myosin and actin by troponin-tropomyosin in the absence of Ca and the release of the inhibition by Ca + (Ebashi et al., 1968,1969) (Fig. 2). In the regulatory processes of the troponin complex, the essential mechanism is considered to consist of the Ca -dependent interaction between troponin C and troponin I and of the inhibitory interaction of troponin I with actin-tropomyosin the two interactions are complementary to each other (Potter and Gergely, 1974 Ebashi, 1974a Perry, 1979). 

The regulatory interactions and their change by Ca are shown schematically in Fig. 8. In the absence of Ca , troponin I interacts with troponin C weakly, but it binds strongly to actin-tropomyosin and inhibits the actin from interacting with myosin. When Ca acts on troponin C, the interaction of troponin C with troponin I is strengthened and the interaction of troponin I with actin weakens and consequently the contractile interaction is activated. Some aspects of the Ca -regula-tory processes are discussed in the following sections. 

The inhibitory action of troponin I on actomyosin-tropomyosin represents the depression of contractile interaction of myosin-actin by troponin-tropomyosin in the absence of Ca. In the absence of tropomyosin, the inhibitory action of troponin I is weak, whereas about 80% of the ATPase of actomyosin is inhibited by troponin I in the presence of 

The strong binding of the troponin T i region to tropomyosin should undoubtedly contribute to the steric stabilization of the position of the whole troponin complex on tropomyosin—actin. The Ca + sensitivity of actomyosin ATPase by troponin T2 is a little less cooperative than that by troponin T, and thus the depressive effect of free Mg on the Ca sensitivity of the actomyosin ATPase with troponin T2 is less remarkable. Troponin T1 may be involved in these aspects. The maximum activation of actomyosin ATPase by troponin T2 in the presence of tropomyosin-troponin I-C was a litde less than that by troponin T. Troponin Ti itself depressed the ATPase and superprecipitation of actomyosin-tropomyo-sin-troponin I-C at all Ca + concentrations. The mixture of troponin Ti and T2 also depressed the ATPase at all Ca concentrations to the same extent as troponin Ti. This suggests that the native position of troponin Ti in the thin filament is different from that of isolated troponin T1 in the reconstituted filament. 

Ca-SensMang Activities of Various Kinds of Hybrid Troponins 

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