Identification of a calcium dependent atrial secretory binding protein as calmodulin.

OBJECTIVE

The role of calcium in the regulation of atrial natriuretic factor (ANF) secretion has focused attention on calcium binding proteins as regulatory components of the secretory machinery for ANF. Two members of the annexin family of calcium binding proteins have emerged as candidates for this role. The aim of this paper is to report the isolation, purification, and identification of another calcium dependent atrial secretory granule binding protein.

METHODS

A 20 kDa calcium binding protein was isolated from rat cardiac tissue by affinity chromatography and purified by ion exchange high pressure liquid chromatography followed by reverse phase high pressure liquid chromatography. Identification of the protein was based on its electrophoretic mobility, chromatographic properties, amino acid composition, and ability to stimulate the activity of calmodulin dependent phosphodiesterase.

RESULTS

Analysis of this protein and its interaction with atrial secretory granules revealed that it was identical to calmodulin and that the calcium dependent interaction with atrial secretory granules takes place at physiological calcium concentrations.

CONCLUSIONS

The study identifies calmodulin as a calcium dependent atrial secretory binding protein. The interaction of calmodulin with atrial secretory granules at physiological calcium concentrations suggest a possible role for calmodulin in the regulation of ANF release from the secretory granules.