Recruitment of cytosolic proteins to a secretory granule membrane depends on Ca²⁺-calmodulin

Abstract

An increase in free calcium triggers catecholamine secretion from chromaffin cells and calmodulin is strongly implicated as the intracellular Ca²⁺ receptor^1–3. In our recent studies of calmodulin action in the chromaffin cell, micromolar Ca²⁺ concentrations resulted in calmodulin^4,5 and cytosolic proteins^6,7 becoming bound to the chromaffin granule membranes. We now report that calmodulin is bound with high affinity to granule membrane proteins of molecular weights (M_rs) 25,000 and 22,000 (25K and 22K) at low Ca²⁺ (<10⁻⁸ M) and to proteins with M_rs 69K and 50K at high Ca²⁺ (>1 µM). Other cytosolic components (M_rs 70K, 36K, 34K and 32K) require calmodulin for their interaction with membrane. These proteins separately bound to calmodulin-Sepharose at high Ca²⁺ concentrations. Although the functions of these adrenal proteins have not been established, the 34K and 32K M_r components co-migrate with clathrin light chains isolated from medullary coated vesicles and the M_r 34K components from both sources share the same one-dimensional peptide map. These interactions were observed at micromolar Ca²⁺ levels at ‘intracellular’ conditions of pH and ionic strength and would be expected to occur during secretion from the chromaffin cell.