Calcium-dependent interaction of S100b, troponin C, and calmodulin with an immobilized phenothiazine

We have purified the brain-specific protein S100b by affinity-based adsorption chromatography on phenothiazine-Sepharose conjugates and studied the interaction of this and other calcium-modulated proteins with the immobilized antipsychotic drug. Bovine brain calmodulin, rabbit skeletal muscle troponin C, and bovine brain S100b bind to phenothiazine-Sepharose in a calcium-dependent manner. These three proteins competitively inhibit the calcium-dependent binding of 125I-labeled chicken gizzard calmodulin to the immobilized drug. However, carp parvalbumin and chicken intestinal vitamin D-dependent calcium binding protein do not inhibit the phenothiazine--calmodulin interaction. These results suggest that the known amino acid sequence homology among calmodulin, troponin C, and S100b may be reflected in a similar functional domain present in these proteins but absent in parvalbumin and vitamin D-dependent protein.