Constitutive secretion of a bacterial enzyme by polarized epithelial cells.

The constitutive (or default) pathway for protein secretion was investigated in two epithelial cells, Madin-Darby canine kidney (MDCK) and human colonic adenocarcinoma (Caco-2), using a bacterial enzyme. The choice of a bacterial protein was based on the requirement to identify a protein devoid of sorting signals. The sorting of a bacterial endoglucanase derived from Clostridium thermocellum, endoglucanase E, from stably transfected MDCK and Caco-2 cells was examined. The choice of a bacterial endoglucanase for these studies has advantages of simple, sensitive and quantitative detection, while higher eukaryotic cells do not express endoglucanase activity. Both cell lines secreted a 50 kDa form of the bacterial protein, while smaller intracellular forms were also observed. In polarized layers of MDCK cells the endoglucanase was secreted into both membrane domains in the ratio 62% apical and 38% basolateral. In Caco-2 cells secretion was predominantly, 70%, through the basolateral membrane. These results define the constitutive pathway for protein secretion in these two model epithelial cells.