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

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Abstract
A Ca2+-binding protein from human brain was 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 1 trimethyllysine) and demonstrates a higher isoelectric point. Both tyrosines and 3 of 8 phenylalanine residues also occur in the fragment, producing a different absorption spectrum. Though it contains 2 of calmodulin''s Ca2+-binding sites, CaM72-148 binds only one Ca2+ per molecule with a Kd of 17 .mu.M. No biological activity, 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.) CaM72-148 is thought to arise through proteolysis, and its implications for the structure and physiological role of calmodulin are discussed.

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