Calcium-Dependent Control of Caldesmon-Actin Interaction by S100 Protein1

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Abstract
Caldesmon from chicken gizzard muscle has been examined for ability to interact with S100 protein using sedeimentation, low-shear viscosity, and affinity chromatography. Ca2+/S100 protein, like Ca2+/calmodulin, inhibited the binding of caldesmon to F-actin in a concentration-dependent manner and the inhibition was not observed in the absence of Ca2+. Caldesmon was bound to S100 protein-Sepharose in the presence of Ca2+ and released with EGTA, indicating that there is a direct interaction between caldesmon and S100 protein. The binding of S100 protein to caldesmon also relieved actomyosin Mg-ATPase inhibition by caldesmon. The molar ratio of S100 protein to caldesmon required for half-maximal restoration was about 0.3, a value less than that in the case of calmodulin. S100 protein, however, was less effective in terms of the maximal extent of the restoration. With respect to Ca2+sensitivity, the restoration profiles were monophasic with a midpoint at 3 ×105 M for S100 protein and 8 ×10−6 M for calmodulin. The restoration by S100 protein was almost wholly inhibited by TFP, but not by W-7. Taken together, our results suggest that a Ca2+-binding protein other than calmodulin may regulate caldesmon-dependent cellular functions.