The intracellular pathway followed by the influenza virus hemagglutinin (HA) to the apical surface of Madin-Darby canine kidney cells was studied by radioimmunoassay, immunofluorescence, and immunoelectron microscopy. To synchronize the migration, we used a temperature-sensitive mutant of influenza WSN, ts61, which, at the nonpermissive temperature, 39.5 degrees C, exhibits a defect in the HA that prevents its exit from the endoplasmic reticulum. Upon transfer to permissive temperature, 32 degrees C, the HA appeared in the Golgi apparatus after 10 min, and on the apical surface after 30-40 min. In the presence of cycloheximide, the expression was not inhibited, indicating that the ts defect is reversible; a wave of HA migrated to the cell surface, where it accumulated with a half time of 60 min. After passage through the Golgi apparatus the HA was detected in a population of smooth vesicles, about twice the size of coated vesicles, located in the apical half of the cytoplasm. These HA-containing vesicles did not react with anti-clathrin antibodies. Monensin (10 microM) delayed the surface appearance of HA by 2 h, but not the transport to the Golgi apparatus. Incubation at 20 degrees C retarded the migration to the Golgi apparatus by approximately 30 min and blocked the surface appearance by acting at a late stage in the intracellular pathway, presumably at the level of the post-Golgi vesicles. The initial appearance of HA on the apical surface was in the center; no preference was observed for the tight-junctional regions.

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