Plasma membrane protein sorting in epithelial cells: Do secretory pathways hold the key?

The Madin-Darby canine kidney (MDCK) cell sorts newly synthesized apical and basolateral plasma membrane proteins intracellularly and then ships them directly to the correct plasma membrane domains, whereas the rat hepatocyte sorts apical and basolateral proteins after their arrival at the basolateral domain of the plasma membrane and then employs a transcytotic mechanism to deliver the apical proteins to the apical domain. This difference in plasma membrane protein sorting pathways may relate to the observation that the hepatocyte, unlike the MDCK cell, does not have an apically directed secretory pathway.