Identification of a secretory granule-binding protein as caldesmon

Stimulation of adrenal chromaffin cells results in a rise in the concentration of intracellular free calcium 1–3 which initiates catecholamine secretion by exocytosis 4,5 . An understanding of the molecular basis of exocytosis will require knowledge of the sites of action of calcium. A role for calmodulin has been implicated in secretion from chromaffin cells 6,7 , and isolated granule membranes bind both calmodulin 8 and a series of cytosolic proteins 9–12 in a calcium-dependent fashion. Here, we demonstrate that one of the cytosolic granule-binding proteins with a relative molecular mass (M r ) of 70,000 (70K) is a form of the calmodulin-regulated actin-binding protein caldesmon, first isolated from smooth muscle 13 . Cytoplasmic gels assembled from an adrenal medullary extract in the absence of Ca 2+ contained actin and the 70K protein. The association of both of these proteins with the cytoplasmic gel was inhibited by a micromolar concentration of Ca 2+ . In addition, we have demonstrated that the 70K protein is localized at the periphery of chromaffin cells. These results are consistent with the notion that 70K protein (caldesmon) has a role in regulating the organization of actin filaments of the cell periphery during the secretory process.