Calcium-dependent control of caldesmon-actin interaction by S100 protein.

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Caldesmon from chicken gizzard muscle has been examined for ability to interact with S100 protein using sedimentation, 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 5100 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 Mg2+-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 3×10-5M for S100 protein and 8×10-6M for calmodulin. The restoration by 5100 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.
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