A di-acidic (DXE) code directs concentration of cargo during export from the endoplasmic reticulum

Efficient export of vesicular stomatitis virus glycoprotein (Vm-G), a type I transmembrane protein, from the endoplasmic reticulum requires a di-acidi c code (DXE) located in the cytosolic carboxyl-terminal tail (Nishimura, N, , and Balch, W, E, (1997) Science 277, 558-558), Mutation of the DXE code b y mutation to AXA did not prevent VSV-G recruitment to pre-budding complexe s formed in the presence of the activated form of the Sari and the Sec23/24 complex, components of the COPII budding machinery. However, the signal wa s required at a subsequent concentration step preceding vesicle fission. By using green fluorescence protein-tagged VSV-G to image movement in a singl e cell, we found that VSV-G lacking the DXE code fails to be concentrated i nto COPII vesicles. As a result, the normal 5-10-fold increase in the stead y-state concentration of VSV-G in downstream pre-Golgi intermediates and Go lgi compartments was lost. These results demonstrate for the first time tha t inactivation of the DXE signal uncouples early cargo selection steps from concentration into COPII. vesicles. We propose that two sequential steps a re required for efficient export from the endoplasmic reticulum.