Exit of newly synthesized membrane proteins from the trans cisterna of the Golgi complex to the plasma membrane

The intracellular location at which the G protein of vesicular stomatitis virus accumulated when transport was blocked at 20 degrees C has been studied by biochemical, cytochemical, and immunocytochemical methods. Our results indicated that the viral G protein was blocked in that cisterna of the Golgi stack which stained for acid phosphatase. At 20 degrees C this trans cisterna became structurally altered by the accumulation of G protein. This alteration was characterized by extensive areas of membrane buds which were covered by a cytoplasmic coat. These coated structures were of two kinds--those that labeled with anti-clathrin antibodies and those that did not. The clathrin- coated pits consistently did not label with anti-G antibodies. Upon warming infected cells to 32 degrees C, G protein appeared on the surface within minutes. Concomitantly, the trans cisterna lost its characteristic structural organization. Double-labeling experiments were performed in which G protein localization was combined with staining for horseradish peroxidase, which had been taken up from the extracellular medium by endocytosis. The results suggest that the trans cisterna was distinct from the endosome compartment and that the latter was not an obligatory station in the route taken by G protein to the cell surface.