Regulation by Caltropin

Caldesmon is a major actin binding protein associated with thin filament of smooth muscle (Sobue etal., 1981) and nonmuscle cells (Owada and Kakiuchi, 

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 

Spectroscopic techniques as well have been used in demonstrating an interaction between CaD and CaT (Mani and Kay, 1990). For fluorescence studies, the sulfhydryl groups in CaD were labeled with acry-lodan. Addition of CaT to labeled CaD in the presence of Ca + produced nearly 50% increase in fluorescence intensity and the emission maximum shifted to 492 nm from 504 nm. Labeled CaD was titrated with CaT. Analysis of the titration curve by curve fitting showed that the best fit was obtained when the molar ratio of CaT to CaD in the complex at saturation was 1 1. A value of (9 2) X 10 M was obtained from the titration. However, there was no evidence of any interaction between CaT and CaD in the absence of Ca2+. 

Caltropin, which is modulated by Ca2+, binds to CaD in a Ca2+-dependent manner with high affinity. It is also very potent in regulating caldesmon s inhibition in a calcium-dependent manner and could conceivably be the Ca + factor that regulates CaD in smooth muscle. 

Regulation of Caldesmon and Heavy Meromyosin Interaction by Caltropin 

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