The complete amino acid sequence of the Ca2+-dependent modulator protein (calmodulin) of bovine brain.

We present the data required to establish the complete amino acid sequence of bovine brain modulator protein, the multifunctional calcium-dependent regulatory protein. Bovine brain modulator protein contains 148 amino acid residues and has a molecular mass of 16,680 daltons. The protein commences with an acetylated alanyl residue in accord with the previous report that its NH2 terminus was blocked. The single residues of histidine and trimethyllysine occur at positions 107 and 115, respectively, in a region of the linear sequence implicated by other studies as important for calcium-dependent modulator protein-enzyme interactions. The sequence of bovine brain modulator protein demonstrated here is closely related to those of muscle troponin Cs, as originally suggested from considerations of the similarities in calcium binding and functional and physicochemical properties of these proteins (Watterson, D.M., Harrelson, W.G., Jr., Keller, P.M., Sharief, F., and Vanaman, T.C. (1976) J. Biol. chem. 251, 4501-4513). The linear amino acid sequence of bovine brain modulator protein is composed of four internally homologous sequences or domains, each of which contains the appropriate amino acids arranged so as to form a helix-loop-helix, calcium-binding structure. The high level of internal homology of bovine brain modulator protein and its relationship to the other members of the calcium-binding protein superfamily provide convincing evidence that 1) it arose early in the evolution of these related proteins and 2) it was formed by two successive tandem duplications of a gene encoding a small, single domain ancestral precursor. Comparison with the nearly complete sequences of the bovine uterus and rat testis modulator proteins reported by other laboratories indicates that this ubiquitous calcium-dependent regulatory protein does not occur in tissue-specific forms, commensurate with the proposed function of modulator protein as a mediator of calcium-second messenger function in eukaryotic cells.