Biogenesis of epithelial cell polarity: intracellular sorting and vectorial exocytosis of an apical plasma membrane glycoprotein.

To study the biogenetic pathway of influenza hemagglutinin (HA), a model apical glycoprotein, in polarized epithelial MDCK cells, anti-HA antibodies were added to the basolateral surface during influenza infection. In monolayers grown on collagen gels influenza and VSV plaque development was blocked only when the antibodies were added to the respective budding surface. Addition of anti-HA antibodies to the basal medium of monolayers grown on nitrocellulose filter chambers neither resulted in HA-coupled transport of antibody nor inhibited HA migration to the apical surface. These results indicate that the bulk of HA is vectorially inserted into the apical surface of MDCK cells by polarized exocytosis. Other apical proteins in epithelia may use a similar mechanism during biogenesis.