VESICULAR STOMATITIS-VIRUS GLYCOPROTEIN IS SORTED AND CONCENTRATED DURING EXPORT FROM THE ENDOPLASMIC-RETICULUM

Newly synthesized proteins are believed to move from the endoplasmic r eticulum (ER) to the Golgi by bulk flow, and sorting is assumed to occ ur exclusively in the trans-Golgi network (TGN). Using quantitative im munoelectron microscopy, we demonstrate that vesicular stomatitis viru s glycoprotein (VSV-G) is sorted from resident ER proteins and concent rated 5- to 10-fold in 40-80 nm vesicles during vesicle budding from t he ER. Accumulation of VSV-G in pre-Golgi vesicular carriers is the on ly detectable concentration step in its transport to the TGN. From the se results, it is apparent that export from the ER is not exclusively mediated by bulk flow. The ER exerts an unanticipated level of control to insure selective and efficient entry of mature protein into the se cretory pathway.