Direct apical sorting of rat liver dipeptidylpeptidase IV expressed in Madin-Darby canine kidney cells

In Madin-Darby canine kidney (MDCK) cells, apical and basolateral membrane proteins are segregated from each other in the trans-Golgi network (TGN) and are transported to the appropriate membrane domain via separate vesicle populations. In hepatocytes, however, all plasma membrane proteins are delivered basolaterally. Apical proteins are then selectively retrieved and reach the apical surface by transcytosis. The sorting of apical proteins in different cell types may be the result of differences in the cellular sorting machinery, or alternatively, due to expression of cell-specific sorting signals on the proteins themselves. To test this directly, we have stably expressed cDNA encoding an apical protein from rat liver, dipeptidylpeptidase IV (DPPIV), in MDCK cells. We found that approximately 90% of the exogenous DPPIV is expressed on the apical cell surface at steady state. Furthermore, we demonstrate that this distribution is primarily due to vectorial transport from the TGN to the apical plasma membrane. The small pool of mis-sorted DPPIV that appears basolaterally is slowly endocytosed (t1/2 approximately 60 min) and is subsequently transcytosed. These data are consistent with the notion that both hepatocytes and MDCK cells are capable of correctly sorting rat liver DPPIV, but that this sorting occurs at different sites in the two cell types.