Isolation of two isoforms of a 21,000-dalton Ca2+-binding protein of bovine brain

Using Ca2+-dependent hydrophobic interaction chromatography we have identified a novel bovine brain Ca2+-binding protein (CaBP) composed of 21 kDa and 23 kDa polypeptides. This calciprotein was further purified by heat-treatment in the presence of Ca2+ and ion-exchange chromatography. The isolated protein exhibits a number of properties in common with proteins belonging to the calmodulin family of CaBPs, including a Ca2+-dependent electrophoretic mobility shift on SDS-polyacrylamide gel electrophoresis, retention of the ability to bind 45Ca2+ after electrophoresis and Western blotting, and a high content of acidic amino acids. We have recently isolated and characterized a 21 kDa CaBP from bovine brain and conclude that the 21 kDa and 21/23 kDa CaBPs are isoforms since they have very similar U.V. absorption spectra and amino acid compositions, and polyclonal antibodies raised in rabbits against the 21 kDa CaBP cross-react to an identical degree with the 21/23 kDa CaBP as determined by the competitive enzyme-linked immunosorbent assay (ELISA). Both proteins contain carbohydrate, but they differ in the degree of glycosylation. Tissue distribution studies indicate the presence of both 21 kDa and 23 kDa Ca2+-binding polypeptides in bovine trachea, aorta, kidney, skeletal muscle and cardiac muscle, and chicken gizzard smooth muscle.