Polarized GP2 secretion in MDCK cells via GPI targeting and apical membrane-restricted proteolysis

The major zymogen granule membrane protein in the exocrine pancreas is glycoprotein 2 (GP2), a glycosyl phosphatidylinositol (GPI)-linked membrane protein. Despite its GPI anchor, GP2 is secreted into the pancreatic duct. We examined the mechanism underlying the secretion of GP2 in isolated pancreatic acini and transfected Madin-Darby canine kidney (MDCK) cells (MDCK-GP2). MDCK-GP2 cells release GP2 almost exclusively (> 95%) from the apical membrane. Using GP2 as a model, we defined a novel mechanism of polarized protein secretion in which a secretory protein is targeted via a GPI anchor to the apical plasma membrane, whereupon the mature form is released by proteolysis. Furthermore, we described two features of MDCK cells that enhance the polarized release of GP2: an apical plasma membrane-restricted distribution of the protease responsible for GP2 membrane cleavage, and a transcytotic pathway to reroute basolateral plasma membrane GP2 to the apical cell surface.