THE DISTAL ECTODOMAIN OF ANGIOTENSIN-CONVERTING ENZYME REGULATES ITS CLEAVAGE-SECRETION FROM THE CELL-SURFACE

Angiotensin-converting enzyme (ACE) is a type I ectoprotein that is cl eaved off the cell surface by a plasma membrane bound metalloprotease. However, CD4, another type I ectoprotein does not undergo such cleava ge-secretion. In this study, we investigated the structural determinan ts of the ACE protein that regulate the cleavage-secretion process. Su bstitution and deletion mutations revealed that the cytoplasmic domain , the transmembrane domain, and the juxtamembrane region encompassing the major and the minor cleavage sites of ACE do not regulate its clea vage. Moreover, a chimeric protein containing the distal extracellular domain of CD4 and the juxtamembrane, transmembrane, and the cytoplasm ic domains of ACE, although transported to the cell surface, was not c leavage-secreted. In contrast, the distal extracellular domain of ACE was shown to be the important determinant: a protein containing the di stal extracellular domain of ACE and the juxtamembrane, transmembrane, and cytoplasmic domain of CD4 was efficiently cleaved off the cell su rface. The chimeric protein was cleaved within the CD4 sequence and th e responsible enzymatic activity was inhibited by Compound 3, a relati vely specific inhibitor of the ACE secretase activity. These results d emonstrate that, in a chimeric protein, the distal extracellular domai n of a cleavable protein, such as ACE, can induce a proteolytic cleava ge within the juxtamembrane domain of an uncleaved protein such as CD4 .