Human brain calmodulin: isolation, characterization, and sequence of a half-molecule fragment.

A Ca2+-binding protein from human brain has been purified to homogeneity and identified as residues 72-148 of calmodulin. This half-molecule fragment (CaM72-148) contains 11 of calmodulin's 15 basic amino acids (including one trimethyllysine) and demonstrates a higher isoelectric point. Both tyrosines and three of eight phenylalanine residues also occur in the fragment, giving rise to a somewhat different absorption spectrum. Though it contains two of calmodulin's Ca2+-binding sites, CaM72-148 binds only one Ca2+ per molecule with a dissociation constant of 17 microM. No biological activity, as judged by its inability to activate cyclic nucleotide phosphodiesterase, is observed. The sequence of amino acids is identical with that of residues 72-148 of bovine brain calmodulin [Kasai, H., Kato, Y., Isobe, T., Kawasaki, H., & Okuyama, T. (1980) Biomed. Res. 1, 248-264]. CaM72-148 is thought to arise through proteolysis, and its implications for the structure and physiological role of calmodulin are discussed.