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-terminal region. Mouse epithelial cells transfected with rabbit testicular ACE cDNA, synthesize, glycosylate, and secrete ACE by cleavage processing of its membrane-anchoring carboxyl-terminal region. Because the cleavage-secretion process is slow, the enzyme accumulates on the cell surface. We show that this process can be enhanced by treatment of cells with tumor-promoting phorbol esters leading to depletion of the cell surface enzyme. The cleavage processing occurs only after the protein has reached the cell surface and is not affected by disruption of the Golgi apparatus or the lysosomal compartments. The exact peptide bond cleaved has been identified by sequencing the amino-terminal residues of the purified COOH-terminal tail left in the cells after ACE is secreted and the carboxyl-terminal residues of secreted ACE. The cleavage occurs at a monobasic site between Arg-663 and Ser-664 generating the soluble enzyme and leaving a cell-bound protein of 74 residues. These results demonstrate the existence of cellular mechanisms that regulate the conversion of cell-bound ACE to a soluble enzyme.