REGULATED CLEAVAGE-SECRETION OF THE MEMBRANE-BOUND ANGIOTENSIN-CONVERTING ENZYME

Angiotensin-converting enzyme (ACE) is an ectoprotein anchored in the plasma membrane through a hydrophobic domain near its carboxyl-termina l region. Mouse epithelial cells transfected with rabbit testicular AC E cDNA, synthesize, glycosylate, and secrete ACE by cleavage processin g of its membrane-anchoring carboxyl-terminal region. Because the clea vage-secretion process is slow, the enzyme accumulates on the cell sur face. We show that this process can be enhanced by treatment of cells with tumor-promoting phorbol esters leading to depletion of the cell s urface enzyme. The cleavage processing occurs only after the protein h as reached the cell surface and is not affected by disruption of the G olgi apparatus or the lysosomal compartments. The exact peptide bond c leaved has been identified by sequencing the amino-terminal residues o f the purified COOH-terminal tail left in the cells after ACE is secre ted and the carboxyl-terminal residues of secreted ACE. The cleavage o ccurs at a monobasic site between Arg-663 and Ser-664 generating the s oluble enzyme and leaving a cell-bound protein of 74 residues. These r esults demonstrate the existence of cellular mechanisms that regulate the conversion of cell-bound ACE to a soluble enzyme.