Tetrahymena Calcium-Binding Protein Is Indeed a Calmodulin

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Abstract
A Ca2+-binding protein was previously isolated from a ciliate, Tetrahymena, and designated TCBP (Tetrahymena Ca2+-binding protein). TCBP, which has 2 high affinity Ca2+-binding sites (Kd = 4.6 .times. 10-6 M), could activate porcine brain cyclic nucleotide phosphodiesterase at a concentration of > 10-6 M free Ca2+, with the same mode of activation as that of authentic (porcine brain) calmodulin. The amino acid composition of TCBP was essentially the same as that of brain calmodulin. TCBP as an activator of Tetrahymena guanylate cyclase is therefore a calmodulin.