Alternative splicing regulates the endoplasmic reticulum localization or secretion of soluble secreted endopeptidase

A subfamily of zinc metalloproteases, represented by Neutral endopeptidase (EC 3.4.24.11) and endothelin-converting enzyme, is involved in the metabol ism of a variety of biologically active peptides. Recently, we cloned and c haracterized a novel member of this metalloprotease family termed soluble s ecreted endopeptidase (SEP), which hydrolyzes many vasoactive peptides, Her e we report that alternative splicing of the mouse SEP gene generates two p olypeptides, SEPDelta and SEP. After synthesis, both isoforms are inserted into the endoplasmic reticulum (ER) as type II membrane proteins. SEPDelta then becomes an ER resident, whereas SEP, which differs by only the presenc e of 23 residues at the beginning of its luminal domain, is proteolytically cleaved by membrane secretase(s) in the ER and transported into the extrac ellular compartment. An analysis of the chimeric proteins between SEP Delta and bovine endothelin-converting enzyme-1b (bECE-1b) demonstrated that the retention of SEPDelta in the ER is mediated by the luminal domain. In addi tion, the dissection of the chimeric bECE-1b/SEP insertion showed that its insertion domain is obviously responsible for its secretion. A series of mu tagenesis in this region revealed that the minimal requirement for cleavage was found to be a WDERTVV motif. Our results suggest that the unique subce llular localization and secretion of SEP proteins provide a novel model of protein trafficking within the secretory pathway.