Abstract
Transferrin is taken up by receptor-mediated endocytosis into intracellular vesicles and tubules, and then recycles rapidly to the plasma membrane (diacytosis). We applied double-label cytochemistry to study whether the recycling structures containing transferrin fuse with the intracellular membranous structures that deliver newly synthesized membrane glycoproteins from the ER to the plasma membrane (exocytosis) or whether they remain independent. KB and Vero cells were infected with the temperature-sensitive transport mutant 0-45 of vesicular stomatitis virus (VSV). Temperature-regulated exocytosis of membrane glycoprotein "G" occurred simultaneously with diacytosis of transferrin. The exocytic "G" protein, as detected by immunoperoxidase electron microscopy, passed through the cisternal Golgi stacks and vacuolar, tubular, vesicular, and pit-like structures of the Golgi system. A transferrin-ferritin conjugate used in ultrastructural double-label experiments was detected in diacytic vesicles and tubules that accumulated in the proximal (trans-reticular) Golgi area of the cell. The ferritin-labeled vesicles/tubules were often close to and intermixed with the VSV-"G" containing membranous structures, but in most cases at early times (15-20 min) the transferrin and VSV-"G" containing vesicular structures remained distinct. At later times (30-45 min), the two labels were occasionally found in the same structures. These results indicate that rapid recycling of endocytosed materials and exocytosis of membrane glycoproteins to the cell surface usually occur in distinct vesicles, possibly along the same general morphologic exit pathway.