SECRETORY GRANULE TARGETING OF ATRIAL-NATRIURETIC-PEPTIDE CORRELATES WITH ITS CALCIUM-MEDIATED AGGREGATION

Atrial natriuretic peptide (ANP) is a 28-aa peptide hormone secreted p redominantly from atrial cardiocytes. ANP is first synthesized in the form of a 126-aa precursor (proANP) which is targeted to dense core gr anules of the regulated secretory pathway. ProANP is stored until the cell receives a signal that triggers the processing and release of the mature peptide (regulated secretion). Various models have been propos ed to explain the targeting of selected proteins to the regulated secr etory pathway, including specific ''sorting receptors'' and calcium-me diated aggregation, As potential calcium binding regions had previousl y been reported in the profragment of ANP, the current study was under taken in an effort to determine the relationship between the ability o f ANP to enter the regulated secretory pathway and its calcium-mediate d aggregation. Deletion and site-directed mutagenesis of selected regi ons of the prosegment demonstrates that acidic amino acids at position s 23 and 24 are critical for both regulated secretion of proANP from t ransfected AtT-20 cells and calcium-mediated aggregation of purified r ecombinant proANP in vitro. These results demonstrate that the ability of certain proteins to enter secretory granules is directly linked to their calcium-mediated aggregation.