Tropomyosin actomyosin effects

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. 

When purified CaD binds to actin-tropomyosin, it inhibits actomyosin ATPase activity. CaM in the presence of Ca + can reverse caldesmon s inhibition of the actin-activated myosin ATPase. However, the affinity of CaM for CaD is rather low (Kgg = 1Q6 M i), with the result that a large molar excess of CaM ( 25-fold) at 25°C in 60 mM KCl is required to reverse caldesmon s inhibition (Pritchard and Mar-ston, 1989 Shirinsky et al, 1988). On the other hand, CaT in the presence of 0.2 mM Ca + is very effective in releasing caldesmon s inhibition at 25°C. Complete recovery in the ATPase rate is achieved when 1 mol of CaT is added per mol of CaD (Mani etal, 1992). In fact, most of the inhibition was released (—90%) by the time... 

The mode of inhibition of actomyosin ATPase by calponin differs from that of caldesmon. The effect is mainly on the Vmax with only minor effects on the Km (Nishida et al. 1990, Horiuchi and Chacko 1991). Calponin is shown to affect strongly bound myosin (EL-Mezgueldi and Marston 1996, Borovikov et al. 1996). The inhibition is not dependent on tropomyosin (Winder and Walsh 1990) and the effects of calponin are due to changes in the actin conformation (Noda et al. 1992, Borovikov et al. 1996). 

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