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

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Abstract
An increase in free calcium triggers catecholamine secretion from chromaffin cells and calmodulin is strongly implicated as the intracellular Ca2+ receptor. In our recent studies of calmodulin action in the chromaffin cell, micromolar Ca2+ concentrations resulted in calmodulin and cytosolic proteins becoming bound to the chromaffin granule membranes. We now report that calmodulin is bound with high affinity to granule membrane proteins of molecular weights (Mrs) 25,000 and 22,000 (25K and 22K) at low Ca2+ (less than 10(-8) M) and to proteins with Mrs 69K and 50K at high Ca2+ (greater than 1 microM). Other cytosolic components (Mrs 70K, 36K, 34K and 32K) require calmodulin for their interfraction 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 Mr components co-migrate with clathrin light chains isolated from medullary coated vesicles and the Mr 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.