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 prote ins as regulatory components of the secretory machinery for ANF. Two m embers of the annexin family of calcium binding proteins have emerged as candidates for this role. The aim of this paper is to report the is olation, purification, and identification of another calcium dependent atrial secretory granule binding protein. Methods: A 20 kDa calcium b inding protein was isolated from rat cardiac tissue by affinity chroma tography and purified by ion exchange high pressure liquid chromatogra phy followed by reverse phase high pressure liquid chromatography. Ide ntification of the protein was based on its electrophoretic mobility, chromatographic properties, amino acid composition, and ability to sti mulate the activity of calmodulin dependent phosphodiesterase. Results : Analysis of this protein and its interaction with atrial secretory g ranules revealed that it was identical to calmodulin and that the calc ium dependent interaction with atrial secretory granules takes place a t physiological calcium concentrations. Conclusions: The study identif ies calmodulin as a calcium dependent atrial secretory binding protein . The interaction of calmodulin with atrial secretory granules at phys iological calcium concentrations suggest a possible role for calmoduli n in the regulation of ANF release from the secretory granules.